9.4: Isomerism
Metal complexes present a rich, interesting, and diverse structural chemistry. Major points of variation in the structural chemistry of metal complexes include
- coordination number and coordination geometry , which involve differences in how many ligands surround a central metal and their overall geometric arrangement. Examples of the latter include the tetrahedral and square planar geometries commonly observed for four-coordinate metal centers. Tetrahedral, square planar, and octahedral coordination serve as a backdrop to the discussion of isomerism on this page. Readers who are unfamiliar with these structures might consider reading the section on coordination geometries before this one.
-
structural iosmerism
involves differences in how potential ligand atoms are bound to metals in a complex. Possibilities for structural isomerism include
- linkage or ambidentate isomerism
- hydrate/solvate isomerism
- ionization isomerism
- coordination isomerism.
Of these, linkage isomerism should always be considered when working with ambidentate ligands. As classifications, the last three forms of structural isomerism are mainly of academic interest since they represent permutations of ordinary structure patterns for solvates, salts, and coordination complexes, respectively.
-
stereochemistry
, which includes
- which coordination geometry is adopted for a metal with a given set of ligands in a particular oxidation state. For instance, [Ni II Cl 4 ] 2- is tetrahedral while [Pt II Cl 4 ] 2- is square planar. Since this geometry is usually fixed by the metal and ligands it is typically not a source of stereoisomeric varierty.
- metal-centric stereoisomerism involving possible variations in where ligands are located relative to one another around the metal. The possibilities depend on the coordination number, geometry, and number of different types of ligands. For instance, square planar complexes can exhibit cis-/trans- isomerism while tetrahedral ones cannot. Tetrahedral complexes with four different ligands exhibit R and S chirality but complexes of that type are relatively rare. The more common cases include cis and trans isomerism in square planar complexes and cis & t rans -, mer & fac -, and \(\Lambda\) & \(\Delta\) isomerism in octahedral ones, although some multidentate ligands present additional possibilities for stereochemical variation
-
Stereoisomerism centered on the ligands bound to a metal center. Possibilities include
- stereoisomerism inherent to a ligand. This would include cases where the ligand itself is chiral or, as in the case of amines, exists as a rapidly interconverting mixture of isomers. Thus this sort of stereoisomerism is an extension of the isomerism encountered in ordinary organic and other main group compounds. The main new issues introduced by binding of such ligands to a metal center involve the creation of new possibilities for diastereoisomerism based on the stereochemistry of the metal center and the freezing out of stereocenter inversion on binding to a metal. The latter is particularly important for amines, which as free amines racemize rapidly by nitrogen inversion.
- or conformational isomerism involving five-membered chelate rings created when a multidentate ligand binds to a metal. This isomerism is often called chelate ring twist since individual rings can exhibit one of two conformations depending on how they twist on binding.
A summary of these forms of isomerism is given in Figure \(\sf{\PageIndex{1}}\).
Structural isomers
Structural isomerism involves different topological linkages of atoms. Differences in atomic linkages distinctive to coordination chemistry involve the linkages between metals and ligands. The main variations are:
1. Hydrate/solvate isomerism
Solvate isomers differ in terms of whether a molecule acts as a ligand or whether it acts as a solvate by occupying a lattice site in the crystal. Among solvate isomers, the case in which water is the ligand or solvate is the best known. The resulting isomers are called hydrate isomers after the term for water acting as a solvate, hydrate. A well-known example that illustrates how solvate isomerism works is the series [CrCl x (H 2 O) 6-x ]Cl 3 - x ·xH 2 O, for which x = 0-2. The structures of the complex ions involved in this series are shown in Figure \(\sf{\PageIndex{2}}\). 1
As can be seen from the compounds in Figure \(\sf{\PageIndex{2}}\), hydrate isomers differ both in terms of whether water acts as a ligand or hydrate and in terms of whether a potential counterion acts as a counterion or ligand. Thus, in trans- [CrCl 2 (H 2 O) 4 ]Cl·2H 2 O two chlorides act as chloro ligands and four waters as aqua ligands while in [CrCl(H 2 O) 5 ]Cl 2 ·H 2 O five water molecules act as aqua ligands while only one chloride acts as a chloro ligand. In this way, solvate isomers simply represent cases in which two or more of the possible permutations for metal ligand binding among a set of solvent and counterion molecules are stable.
2. Ionization isomerism
In ionization isomerism there are two or more potential ions that can act as ligands. The ionization isomers differ in terms of which of these ions act as counterions and which act as ligands. Consider, for instance, the complexes shown in Figure \(\sf{\PageIndex{3}}\). These complexes differ in terms of whether the chloride or sulfate acts as a ligand, with the other acting as a counterion.
3. Coordination isomerism
Coordination isomers exist in compounds containing two or more complexes, each of which possesses a different set of ligands that can in principle be swapped with a ligand of the other complex. An example involves [Co III (NH 3 ) 6 ][Cr III (ox) 3 ], depicted in Figure \(\sf{\PageIndex{4A}}\). 1 Coordination isomers of this complex involve swapping the ammine ligands around the Co 3 + center for oxalato ligands surrounding the Cr 3 + center. For instance, swapping all the ammine and oxalato ligands between the metal centers gives the coordination isomer [Cr III (NH 3 ) 6 ][Co III (ox) 3 ] shown in Figure \(\sf{\PageIndex{4B}}\).
As classifications, hydrate/solvate, ionization, and coordination isomerism represent permutations of ordinary structure patterns for solvates, salts, and coordination complexes, respectively. As a result these forms of isomerism are rarely used as independent conceptual frameworks when thinking and talking about the structure of coordination compounds. Not so the final type of structural isomerism, linkage isomerism. That is because linkage isomerism has to do with the special capacity of ambidentate ligands to bind metals in multiple ways.
4. Linkage or ambidentate isomerism
Linkage or ambidentate isomers differ in how one or more ambidentate ligands bind to metal centers in a complex. The classic example dating from the work of Jørgensen and Werner is given in Scheme \(\sf{\PageIndex{I}}\).
Just as alkenes exist as E and Z isomers, compounds possessing ambidentate ligands exist as one among the possible linkage isomers. As such, when working with ambidentate ligands the particular linkage isomer formed should be determined experimentally and considered when interpreting the complex's chemical and physical behavior.
The main ambidentate ligands which give rise to linkage isomerism are
- cyanide, CN -
- thiocyanate, SCN - , and the O and Se analogues, OCN - and SeCN -
- nitrite, NO 2 -
- sulfite, SO 3 -
- nitrosyl, NO
Although ambidentate ligands can bind metals in multiple ways, most exhibit a preferred binding mode ( i.e., prefer to bind metal centers in one of the possible ways). For instance, cyanide almost always binds through its carbon atom and thiocyanate almost always binds through its nitrogen (\(\mu\)- N ). However, the other binding mode can sometimes be formed kinetically or by using conditions that particularly favor its formation. Thus thiocyanate forms M-SCN - linkages in the presence of exceptionally soft metal centers or in hard polar solvents (in which Lewis acid groups can stablize the terminal nitrogen of a bound thiocyanate ligand).
Ambidentate ligands are of significant research interest because many don't just bind to metal centers in two ways; the linkage isomers sometimes have significantly different physical properties. In addition, the possibility of two binding modes introduces the possibility of exploiting linkage isomerism to take advantage of some of these ligands:
- different structural chemistry in different coordination modes. In mononuclear complexes the coordination mode influences which side of the ligand faces away from the complex and might be susceptible to stabilization by interaction with solvent. Additionally, the orientation of the ligand relative to the metal center might differ between one coordination mode and another. For instance, as predicted from the minor contributor to its resonance structure, thiocyanate binds nonlinearly via its S atom and linearly via its N (Figure \(\sf{\PageIndex{5}}\)). Because of this the apparent steric bulk of the ligand around the metal center might differ between forms.
- bind two different metal centers at the same time, linking them together. Perhaps the best-known examples involve thiocyanate and cyanide. The latter forms linkages of the type M-C≡N-M'. In the dye Prussian blue these take the form Fe II -C≡N-Fe III . An example from the author's graduate research involved tetrahedral clusters containing four metal atoms linked by six cyano ligands, as shown in Figure \(\sf{\PageIndex{6}}\).
- can be induced to change from one binding mode to another in response to a stimulus. The classic example involves the light-driven transformation of the thermodynamically more stable yellow nitro complex of pentamminecobalt(III), [Co(NH 3 ) 5 NO 2 ] 2+ to the less stable red O -nitrito complex, [Co(NH 3 ) 5 ONO] 2+ . Less stable cyanometallate coordination networks like those in the red K{Fe II [Cr III (CN) 6 ]} transform into the more stable green K{Fe II [Cr III (NC) 6 ]} form on heating, as shown in Figure \(\sf{\PageIndex{7}}\). 3
The hard-soft acid-base principle helps explain the preference of bridging ambidentate ligands for particular binding modes. How might the greater stability of the green K{Fe II [Cr III (NC) 6 ]} over red K{Fe II [Cr III (CN) 6 ]} be explained in terms of the hard and soft acid-base concept?
- Answer
-
The greater stability of K{Fe II [Cr III (NC) 6 ]} over K{Fe II [Cr III (CN) 6 ]} reflects the greater stability of the Fe II -CN-Cr III linkages in the former over the Fe II -NC-Cr III linkages in the latter . This is consistent with the preference for hard-hard and soft-soft Lewis acid-base interactions of the Hard-Soft Acid-Base Principle.
The Fe II -CN-Cr III linkages are more stable because they possess bonds between
- the softer Lewis acid (Fe II ) and Lewis base (the C end of CN - )
- the harder Lewis acid (Cr III ) and Lewis base (the N end of CN - )
In contrast, the Fe II -NC-Cr III linkages in the less stable linkage isomer involve bonds between
- the softer base (the C end of CN - ) and harder acid (Cr III )
- the harder base (the N end of CN - ) and softer acid (Fe II )
Stereoisomerism
Optical Isomerism/Chirality
Molecules of D n, C n , or C 1 symmetry with only proper rotation axes (including E = C 1 ) are chiral and exhibit optical isomerism. As described in Figure \(\sf{\PageIndex{1}}\), the main sources of such optical isomerism in coordination chemistry are:
-
Chirality inherent to an organic or main group ligand.
This type of isomerism is just an extension of the sort described in undergraduate organic texts and consequently does not merit separate discussion here, other than to note that some forms of optical isomerism which are of little importance in organic chemistry lead to optical activity in coordination compounds. In particular, the nitrogen inversion process which serves to rapidly racemize chiral amines is frozen out on formation of a metal-ligand bond. Because of this chiral amine, ligands bound to a metal form non-interconvertible R and S enantiomers, as shown in Figure \(\sf{\PageIndex{8}}\).
-
Chirality arising from the symmetry of ligands about a metal center.
The two common situations through which such chirality arises involve:
-
Chirality at a tetrahedral metal center surrounded by four different ligand attachment points.
Such cases are often referred to as MABCD or Mabcd where M stands for the metal and a, b, c, and d the four different ligand attachment points. An example of such a complex is given in Figure \(\sf{\PageIndex{9A}}\). The chirality of such complexes is analogous to the chirality arising from a tetrahedral carbon stereocenter. There are two caveats, though. First, it is not enough to have a four-coordinate complex; the metal must possess tetrahedral symmetry since, as shown in Figure \(\sf{\PageIndex{9B}}\), square planar M complexes with four different ligands are identical to their mirror images. Second, such chirality is most easily realized using macrocyclic ligands like proteins. Metal ligand bonds involving sterically unhindered monodentate complexes like that in the example of Figure \(\sf{\PageIndex{9A}}\) are in general weaker and more flexible than analogous carbon-carbon bonds. Consequently, in such cases it is likely that the enantiomers would interconvert through a fluxional process, making their resolution difficult if not impossible.
-
\(\Lambda\) and \(\Delta\) isomerism at octahedral metal centers surrounded by two or three chelating ligands.
Specifically,
- Octahedral tris-chelates of formula M(L~L) 3 exhibit D 3 symmetry.
- cis-octahedral bis-chelates of formula M(L~L) 2 XY exhibit C 2 or C 1 symmetry: C 2 if X = Y and C 1 if X \(\neq\) Y.
As shown in Figure \(\sf{\PageIndex{10A}}\), octahedral tris-chelates like [V(ox) 3 ] 3- are chiral and can exist as nonsuperimposable \(\Lambda\) and \(\Delta\) enantiomers. The chirality of these tris-chelates is analogous to that of pinwheels. As shown in Figure \(\sf{\PageIndex{10B}}\), when looking directly at the head of any individual pinwheel from above, the blades will be angled from back to front going around the pinwheel in either the clockwise or counterclockwise direction. As shown in Figure \(\sf{\PageIndex{10C}}\), the case for which the blades are angled in the clockwise direction is analogous to the \(\Lambda\) enantiomer and the case when they are angled in the counterclockwise direction the \(\Delta\).
The \(\Lambda\) and \(\Delta\) chirality in octahedral bis-chelates is analogous to that in the tris-chelates except this time the third chelating ligand is replaced by an arbitrary pair of ligands. This lowers the symmetry of the complex to either C 2 or C 1 , as shown for the C 2 complex [CoCl 2 (en) 2 ] + in Figure \(\sf{\PageIndex{11A}}\). In either case the result is that such complexes exist as \(\Lambda\) and \(\Delta\) enantiomers analogous to those in the metal tris-chelates, as shown in Figure \(\sf{\PageIndex{11B}}\).
Octahedral complexes containing ligands with denticities of four or more also exhibit \(\Lambda\) and \(\Delta\) chirality; it is just that in such cases the bound ligand defines multiple rings within the complex that can be defined as existing in either \(\Lambda\) or \(\Delta\) orientations relative to each other. The procedure for making these assignments is beyond the scope of most introductory inorganic courses but the result is that such complexes are designated as \(\Lambda\Lambda\), \(\Delta\Delta\), \(\Delta\Lambda\), \(\Delta\Delta\Delta\), etc., depending on the number of relationships involving the rings defined by the bound ligand. Interested readers should consult reference 4 for more details.
Circular dichroism may be used to characterize optically active metal complexes.
At the present the most definitive way to determine the absolute configuration of an optically active coordination compound is to determine its 3D molecular structure using single crystal X-ray crystallography. 5 However, as that is not always possible or convenient, the optical activity of coordination complexes is also commonly studied using circular dichroism (CD) spectroscopy. CD spectroscopy is used because, unlike most chiral organics, optically active coordination complexes possess low-energy electronic transitions that occur in the far UV and visible range. This means that it is often convenient to use the ability of a complex to refract and absorb different circular polarizations of light to derive information about both the configuration of an optically active complex and its electronic structure. The focus of this section will be to explain how the determination of absolute configuration by circular dichroism and optical rotatory dispersion work, as well as the relationship of these techniques to the more ordinary sort of polarimetry used to measure optical rotation in organic systems. Only brief notes will be made about the instrumental and electronic structure applications of these techniques since the electronic spectra of metal complexes will be discussed in Chapter 11: Coordination Chemistry III - Electronic Spectra and the instrumental aspects of CD are well-treated elsewhere .
Both polarimetry and circular dichroism spectroscopy are grounded in the recognition that plane polarized light is equivalent to a superposition of left and right circularly polarized light, as shown in Figure \(\sf{\PageIndex{12}}\).
When plane polarized light passes through a chiral medium, its left and right circularly polarized components move at different rates ( i.e., have different indices of refraction). This causes the plane of polarization to rotate in the direction of the faster component according to the relationship
\[\sf{\alpha ~=~\dfrac{n_l~-~n_r}{\lambda}} \nonumber \]
where \(\alpha\) is the angle of rotation, \(\lambda\) the light's wavelength, and n l and n r the refractive indices experienced by left and right circularly polarized light.
An example of such a rotation is shown in Figure \(\sf{\PageIndex{13}}\).
Chemists typically call this rotation of light by a chiral medium optical rotation. In organic chemistry it is common to measure the optical rotation at the sodium D wavelength of 589.29 nm, a much longer wavelength than most organics are able to absorb. Because of this the optical rotations commonly measured by organic chemists are largely independent of absorption and mainly serve as a characteristic physical property similar to melting points or are used to establish the purity of a mixture of enantiomers. The sort of spectroscopic data used to characterize chiral coordination compounds more commonly makes use of the relationship between absorption and optical rotation.
In coordination chemistry the wavelength-dependence of optical rotation is used to discern information about compounds' electronic spectra. This measurement of the wavelength dependence of optical rotation is called optical rotatory dispersion (ORD) and the resulting plots are called optical rotatory dispersion or ORD spectra or curves. Such ORD curves are useful for characterizing the chiral environment in a coordination complex because the ORD curves exhibit a characteristic shape. This characteristic shape is because according to the Cotton Effect, the optical rotation changes sign at the absorption maximum of a chromophore in the compound. For one configuration the rotation will go from negative to positive with increasing wavelength and is called a positive cotton effect ; for the other configuration the rotation will go from positive to negative or exhibit a negative cotton effect . This behavior is summarized in the top two curves of Figure \(\sf{\PageIndex{14}}\).
A technique used even more often than ORD is circular dichroism (CD) , represented by the lowest spectrum in Figure \(\sf{\PageIndex{14}}\). Circular dichroism arises from the differential absorption of left and right circularly polarized light by a chiral compound. This gives elliptically polarized light as shown in Figure \(\sf{\PageIndex{15}}\).
In circular dichroism (CD) spectroscopy a sample is irradiated with polarized light resulting in the transmission of elliptically polarized light due to differential refraction and absorption of the polarized light's left and right circularly polarized components. For achiral molecules the handedness doesn't affect absorbance, but for chiral molecules, which chiral ground and excited states the handedness of an EM wave affects can determine how readily that wave can distort the chiral ground state into the chiral excited one. For this reason chiral molecules absorb left and right circularly polarized light differently.
In CD spectroscopy the magnitudes of the left and right circularly polarized components of light that is transmitted by the sample are measured and used to determine how much light of each was absorbed (just as in regular absorbance spectroscopy). The difference in absorption between left and right circularly polarized light constitutes the primary signal in CD spectroscopy. More specifically, circular dichroism spectra (CD spectra) show the difference in extinction coefficients for left and right circularly polarized light, \(\Delta \epsilon\), as a function of wavelength, where
\[\sf{\Delta \epsilon\ ~=~ \epsilon_l ~-~\epsilon_r} \nonumber \]
where \(\epsilon_\sf{l}\) and \(\epsilon_\sf{r}\) are the extinction coefficients observed with left and right circularly polarized light, respectively.
As with ORD spectra, the signs of the features in CD spectra like that of Δ–[Co(en) 3 ]Cl 3 given in Figure \(\sf{\PageIndex{16}}\) are enantiomer-dependent. As illustrated in the lower spectrum in Figure \(\sf{\PageIndex{14}}\) , the CD spectra of enantiomers mirror one another. Because of this difference in behavior it is possible in principle to determine the absolute configuration of a complex from its CD spectrum.
There are several ways that ORD and CD spectra can be useful for determining the absolute configuration of a chiral complex.
- First, the sign of the Cotton effect and peaks in CD spectra exhibited by a given absolute configuration is often consistent across a series of compounds. Thus if a new member of the series is isolated, its configuration can be determined based on which enantiomer has the same Cotton effect. For instance, if both the \(\Lambda\) isomer of a metal diimine complex like [Ru(bpy) 3 ] 2+ exhibit a positive Cotton effect and a newly resolved metal diimine exhibits a negative Cotton effect it might reasonably be inferred that the newly-resolved complex is in the opposite or \(\Delta\) configuration.
- Second, for some systems the expected sign of the Cotton effect and CD peaks for each enantiomer may be predicted semi-empirically based on the spatial locations of substituents relative to the chromophore (the part of the molecule that changes electronic structure during the transition), although the details are beyond the scope of the present discussion.
- Consideration of interactions between multiple chromophores in a compound can be used to infer absolute configuration. Again, the details are beyond the scope of the present discussion. Interested readers are referred to reference 7 for details.
Common patterns of Diastereomerism
Geometric isomerism
Geometric isomerism involves differences in the geometric placement of atoms in a compound. In coordination chemistry, geometric isomerism involves differences in the relative placement of a set of ligands about a metal center. There are two main types:
cis and trans isomerism
This type of isomerism has to do with how two ligands are oriented relative to one another in a square planar or octahedral complex. As shown in Figure \(\sf{\PageIndex{17}}\), ligands that are next to one another with a L-M-L bond angle of 90\(^{\circ}\) are said to be cis ; those on opposite sides of the metal with a L-M-L bond angle of 180\(^{\circ}\) are in the trans arrangement.
Simple cis and trans geometric isomers can be identified when there are two identical ligands (A) or chelating ligands with distinguishable attachment points (A~B) oriented about either a square planar or octahedral center. Examples are given in Figure \(\sf{\PageIndex{18}}\). Notice from the example given in Figure \(\sf{\PageIndex{18C}}\) that multidentate ligands have the potential to constrain complexes to adopt a particular geometry.
Slightly more involved examples involve perturbations of the simple cases above in which there are multiple distinguishable cis and/or trans relationships. Two square planar examples are given in Figure \(\sf{\PageIndex{19}}\).
More complex examples of cis and trans relationships between ligands in octahedral complexes are given in the exercises that conclude this page, which also demonstrates how a systematic approach may be used to identify isomers.
mer and fac isomerism
This type of isomerism has to do with how three ligands are oriented relative to one another in an octahedral complex. The arrangements are represented in Figure \(\sf{\PageIndex{20}}\).
Since it can help to visualize the mer and fac geometry from multiple points of view, several representations are given in Figure \(\sf{\PageIndex{21}}\). As shown in Figure \(\sf{\PageIndex{21}}\), in fac or facial arrangements , the ligands occupy the same "face" of the octahedral coordination sphere, while in the mer or meridional geometry the ligands form a T shape in the same plane or its "meridian." 8
Octahedral complexes with three identical ligands oriented about an octahedral center can only exist in mer and fac arrangements. Consequently, such complexes can be designated as mer and fac isomers. Examples are given in Figure \(\sf{\PageIndex{22}}\).
Diastereomerism arising from multiple ligand-associated chiral centers
Diastereomerism can also arise when two or more chiral ligands bind to a metal center. Such cases typically give rise to a complex mixture of isomers, as shown by the example in Figure \(\sf{\PageIndex{23}}\). As may be seen from Figure \(\sf{\PageIndex{23}}\), the differences between these isomers can arise from both changes in the stereochemical configuration of the ligand and the relationship of particular ligand centers relative to one another. For instance, the leftmost two structures in Figure \(\sf{\PageIndex{23}}\) possess one R and S nitrogen center each but differ in whether the centers are oriented so that the R and S centers on the two ligands are trans or cis to one another.
Diastereomerism arising from \(\lambda\) and \(\delta\) ring conformation isomerism.
Chelate rings are formed when a chelating ligand binds a metal
As may also be seen from the structures in Figure \(\sf{\PageIndex{23}}\), when a multidentate ligand coordinates a metal, the ligand and metal center comprise one or more chelate rings . Examples of such chelate rings are shown in Figure \(\sf{\PageIndex{24}}\).
As may be inferred from the examples in Figure \(\sf{\PageIndex{24}}\), chelate rings may be of different sizes, although four, six, and especially five-membered rings are particularly common. Exactly which ring sizes will be more stable for a given system depends on the coordination geometry and, due to differences in M-L bond lengths, to a lesser degree on the metal. As a result, for octahedral and square planar complexes with 90\(^{\circ}\) L-M-L bond angles between cis ligands, five-membered rings tend to be especially stable (Figure \(\sf{\PageIndex{25}}\)).
Not all metal chelates prefer to form five-membered rings. The steric requirements of chelate rings depend on both the preferred coordination geometry of the metal center and the stereochemistry of the ligand. Both of these effects are typically considered in terms of preferred L-M-L bond angles. From the perspective of the metal center, the preferred bond angle is determined by the coordination geometry. Octahedral and square planar metals prefer 90\(^{\circ}\) L-M-L bond amgles, trigonal bipyramidal systems 90 and 120\(^{\circ}\) L-M-L bond angles, and tetrahedral complexes 109.5\(^{\circ}\) L-M-L angles. The larger preferred bond angles in tetrahedral and trigonal bipyramidal systems often require the formation of larger six or seven-membered chelate rings for maximum stability. The size of the chelate ring actually formed between a metal and ligand is determined by the ligand's structure. As the contributor of all the atoms in the chelate ring but one, the ligand directly determines the chelate ring size. More subtly, ligands naturally prefer to coordinate metals at a particular L-M-L angle, called the bite angle , as shown in Figure \(\sf{\PageIndex{26A}}\). Ligands with bite angles corresponding to the ideal L-M-L angle for a metal's preferred geometry tend to form more stable complexes, although in turn ligand bite angles can cause metals with a weak coordination geometry preference to adopt the ligand's preferred geometry instead. 10 As may be seen from the values in Figure \(\sf{\PageIndex{26B}}\), bite angles roughly increase with the size of the chelate ring formed but are also influenced by the types of structures used to connect the ligand's Lewis base sites. This facilitates the use of diphosphine ligands to tailor the structure and reactivity of phosphine-containing organometallic catalysts.
\(\lambda\) and \(\delta\) isomerism involves differences in the conformation of nonplanar five-membered chelate rings
As shown in Figure \(\sf{\PageIndex{27}}\), some chelate ring systems are planar while others are not.
Among the nonplanar chelate ring systems are ligands like en and dppe, which contain tetrahedral C, N, O, P, and S atoms. Because these atoms create kinks in the chelate ring they introduce additional opportunities for diastereomerism due to differences in the chelate ring conformation, called ring twist . Ring twist in nonplanar systems has been most extensively explored for five-membered chelates like those formed by dppe and by en and other chelating amines. Unlike the more rigid four-membered chelate rings, five-membered chelates tend to be conformationally flexible, but not so conformationally flexible that their conformers are rapidly interconverting at room temperature (at least not when the rings are substituted to introduce additional steric strain). This balance between flexibility and rigidity enables some five-membered chelate rings to exist as mixtures of distinguishable conformational isomers at room temperature.
To understand the two most stable conformers that five-membered chelates rings tend to form, it is helpful to think of five-membered chelate rings as involving two components (Figure \(\sf{\PageIndex{28}}\)):
- a planar L-M-L group, where L are the atoms directly attached to the metal, M, and
- the remaining two ring atoms, E, which form a rigid E-E bar across the back of the chelate ring.
As can be seen from Figure \(\sf{\PageIndex{28}}\) , rotation about the M-L and L-E bonds causes twisting of the E-E bar relative to the L-M-L plane. The two most stable conformers produced by these motions are the \(\lambda\) or \(\delta\) conformations as shown in Figures \(\sf{\PageIndex{29}}\) and \(\sf{\PageIndex{29}}\).
The relative stability of the \(\lambda\) and \(\delta\) conformers will depend on the configuration of any stereocenters present and steric factors. As with organic ring systems, steric effects tend to favor conformers in which bulky groups are placed in the less sterically strained equatorial positions while the configuration of the stereocenters present can serve to restrict the allowable ring twist conformations, as illustrated in Figure \(\sf{\PageIndex{30}}\). A detailed treatment of such systems is beyond the scope of this page. Interested readers are referred to reference 12 for more details.
In closing, it should be noted that it might seem to be making much of small effects in pointing out that one source of diastereomerism in metal complexes involves the freezing out of chelate ring conformations. However, that would be to mistake the impacts that ring conformations can have on the steric accessibility of a coordination site and shaping the course of processes that might occur there. As illustrated in Figure \(\sf{\PageIndex{31}}\), a simple shift in the conformation of one ring in a square planar, pyramidal, or octahedral complex containing coplanar ethylene diamine ligands can exert a significant effect on the steric profile of the complex perpendicular to the MN 4 square plane. Because of effects like these, ring conformation isomerism plays a role in the design of stereoselective transition metal catalysts.
Exercises
What type of isomers are
- [CoCl(NH 3 ) 5 ](NO 3 ) and [Co(NH 3 ) 5 ( N- NO 2 )]Cl?
- [Co(NH 3 ) 5 -CN-Ru(NH 3 ) 5 ] 4+ and [Co(NH 3 ) 5 -NC-Ru(NH 3 ) 5 ] 4+
- [Cr(CN) 5 -CN-Co(NH 3 ) 5 ] and [Co(CN) 5 -CN-Cr(NH 3 ) 5 ]
- cis -[Mn(en) 2 (CN) 2 ] and trans -[Mn(en) 2 (CN) 2 ]
- Answer a.
-
Ionization isomers.
- Answer b.
-
Linkage isomers.
- Answer c.
-
Coordination isomers.
- Answer d.
-
Geometric isomers, specifically cis / trans isomers.
Many properties of transition metal complexes depend on the metal's oxidation state. For instance,
- octahedral complexes of Co II lose and gain ligands rapidly
- octahedral complexes of Co III lose and gain ligands very slowly
- four-coordinate complexes are generally tetrahedral
- EXCEPT four-coordinate complexes of metals like Pt II , Pd II , Rh I , and Ir I , among others, are square planar
For this reason it is important to be able to estimate the formal oxidation state of a metal in a complex. Fortunately, this is easy to do if you remember
- The sum of all atoms' oxidation states will equal the overall charge on the complex
- When determining the metal's oxidation state, the ligands can be treated as having an oxidation state equal to their charge - i.e., the charge they possess in the form in which they coordinate the metal - so if they need to lose a proton to bind, don't forget to account for that.
Given the above, estimate the oxidation state of the metal in the following real and hypothetical complexes.
- Na 4 [Fe(CN) 6 ]
- [Cu(phen) 2 ]BF 4
- [PtF 4 (NH 3 ) 2 ]
- [Ni(en) 2 ]SO 4
- Co(acac) 3
- [MnCl(O)(salen)]
- Answer for Na 4 [Fe(CN) 6 ] .
-
This contains [Fe(CN) 6 ] 4- ; so O.S. Fe + 6 x (-1) (for CN - ) = -4 (the complex's charge) so O.S. Fe = +2 or Fe 2 + .
- Answer for [Cu(phen) 2 ]BF 4 .
-
Since tetrafluoroborate is a monoanion, the complex is [Cu(phen) 2 ] + so O.S. Cu + 0 x 2 (for phen) = +1 (the complex's charge) so O.S. Cu = +1 or Cu + .
- Answer for [PtF 4 (NH 3 ) 2 ] .
-
O.S. Pt + 4 x (-1) (for F - ) + 0 x 2 (for NH 3 ) = +0 (the complex's charge) so O.S. Pt = +4 or Pt 4 + .
- Answer [Ni(en) 2 ]SO 4 .
-
The complex is [Ni(en) 2 ] + so O.S. Ni + 0 x 2 (for en) = +2 (the complex's charge) so O.S. Ni = +2 or Ni 2 + .
- Answer Co(acac) 3 .
-
O.S. Co + 3 x (-1) (for acac; see table 9.2.2 ) = +0 (the complex's charge) so O.S. Co = +3 or Co 3 + .
- Answer [MnCl(O)(salen)] .
-
O.S. Mn + 1 x (-1) (for Cl - ) + 1 x (-2)(for oxo) + 1 x (-2) (for salen; see table 9.2.2 ) = +0 (the complex's charge) so O.S. Mn = +5 or Mn 5 + .
Draw structures that match the descriptions given, assuming that
- complexes in which the metal has a coordination number of six are octahedral
- complexes in which the metal has a coordination number of five are trigonal bipyramidal
- complexes in which Pt II , Pd II , or Rh I , or Ir I have a coordination number of four are square planar
- other complexes in which the metal has a coordination number of four will be tetrahedral
- mer -triammineaqua- trans- dichlorocobalt(III) ion
- \(\Delta\)-diaminebis(oxalato)manganate(III)
- [CoCl 4 ] 2-
- trans -diamminebis(ethylenediamine)Nickel(2+) tetracyanopalladate(2-)
- \(\Lambda\)-bis(ethylenediamine)cobalt(III)- μ-amido μ-hydroxo-\(\Delta\)-bis(ethylenediamine)cobalt(III)
- Answer a. mer -triammineaqua- trans- dichlorocobalt(III) ion .
-
- Answer b. \(\Delta\)-diaminebis(oxalato)manganate(III) .
-
- Answer c. [CoCl 4 ] 2- .
-
- Answer d. trans -diamminebis(ethylenediamine)Nickel(2+) tetracyanopalladate(2-) .
-
Note that since the Pd in [Pd(CN) 4 ] 2- is Pd 2 + it will be square planar.
- Answer e. \(\Lambda\)-bis(ethylenediamine)cobalt(III)- μ-amido μ-hydroxo-\(\Delta\)-bis(ethylenediamine)cobalt(III) .
-
.
Draw all the stereoisomers of the following real and hypothetical complexes. You may assume that
- complexes in which the metal has a coordination number of six are octahedral
- complexes in which the metal has a coordination number of five are trigonal bipyramidal
- complexes in which Pt II , Pd II , or Rh I , or Ir I have a coordination number of four are square planar
- other complexes in which the metal has a coordination number of four will be tetrahedral
- [IrCl(CO)(PPh 3 ) 2 ]
- [CoCl 3 (NH 3 ) 3 ]
- [CoCl 2 (H 2 O) 2 (NH 3 ) 2 ] +
- [CoBrCl(H 2 O) 2 (NH 3 ) 2 ] +
- [CoBrClI(NH 3 ) 3 ]
- Answer a. [IrCl(CO)(PPh 3 ) 2 ]
-
This is a 4-coordinate Ir I complex and, as such will be square planar. Since two of the ligands are identical, it will have cis and trans isomers. The trans isomer is famous for its ability to form adducts and is called Vaska's complex.
- Answer b. [CoCl 3 (NH 3 ) 3 ]
-
This complex contains three ammine and three chloro ligands and so will have fac and mer isomers.
- Answer c. [CoCl 2 (H 2 O) 2 (NH 3 ) 2 ] +
-
This is a case of an MA 2 B 2 C 2 system (A = Cl - , B = H 2 O, C = NH 3 ) involving multiple cis and trans relationships. The six stereoisomers are shown below.
-
Since it may not be obvious how to arrive at a set of isomers like the ones above, it is worth considering how one might work through the possibilities for a system of ligands like this one. Several systems may be used to systematically identify isomers. Once approach is the Macbdef system described in Note \(\sf{\PageIndex{1}}\) at the end of these exercises. The solutions presented here and in subsequent problems employ a variant of that approach.
-
Start by fixing one set of ligands. In this case the chloro ligands were first fixed as
cis
to one another. Then the remaining ligands might be
cis
or
trans
to one another, although since the
chloro
ligands are already
cis,
the ammine and aqua ligands cannot both be
trans
at the same time. Thus for
cis
chloro ligands the possibilities for the others are:
- cis -H 2 O , cis -NH 3 ,
- cis- -H 2 O, trans -NH 3
- trans -H 2 O, cis- NH 3
-
Swap the configuration of the ligand set you started with. In this case it means placing the chloro ligands
trans
to one another. From that configuration, the possibilities for the ammine and aqua ligands are
- cis -H 2 O , cis -NH 3 ,
- trans -H 2 O, trans -NH 3
- Check for enantiomers and create mirror images of any chiral complexes you drew. The easiest way to do this is to assign point groups. However, it turns out that the octahedral all- cis case is D 3 and formally equivalent to the symmetry of a tris chelate, as shown below.
- finally, check to make sure you didn't include the same complex twice. Humans do make mistakes after all.
-
Start by fixing one set of ligands. In this case the chloro ligands were first fixed as
cis
to one another. Then the remaining ligands might be
cis
or
trans
to one another, although since the
chloro
ligands are already
cis,
the ammine and aqua ligands cannot both be
trans
at the same time. Thus for
cis
chloro ligands the possibilities for the others are:
- Answer d. [CoBrCl(H 2 O) 2 (NH 3 ) 2 ] +
-
This problem is analogous to the one above except that this complex contains a bromo and chloro ligand in place of the two chloro ligands in [CoCl 2 (H 2 O) 2 (NH 3 ) 2 ] + . Consequently, when the bromo and chloro ligands are cis to one another, additional possibilities for isomers arise based on whether ammine or aqua ligands are trans to the chloro and bromo. The result is eight isomers.
- Answer e. [CoBrClI(NH 3 ) 3 ]
-
There are two possibilities for this complex - a fac -(NH 3 ) 3 and a mer- (NH 3 ) 3 arrangement. These may be taken as starting points for examining the possible permutations for the Cl - , Br - , and I - ligands. The top row presents the three possibilities within the mer- (NH 3 ) 3 arrangement; each corresponds to a different ligand trans to an ammine ligand. The bottom row presents the two possible isomers with a fac -(NH 3 ) 3 arrangement. In each, the Cl - , Br - , and I - ligands are trans to ammine ligands. However, the complexes possess C 1 symmetry and so are chiral and exist as enantiomers.
Ignoring ring twist , draw all the stereoisomers of the following real and hypothetical octahedral complexes.
- [CoBr 2 Cl 2 (en)] -
- [CoBrCl(en) 2 ] +
- [CoBrCl(gly) 2 ] -
- [Co(en) 3 ] 3+
- [Co(gly) 3 ]
- Answer a. [CoBr 2 Cl 2 (en)] -
-
This complex is analogous to [CoCl 2 (H 2 O) 2 (NH 3 ) 2 ] + in possessing pairs of identical ligating groups. The main difference is that in [CoBr 2 Cl 2 (en)] - the two amine ligating group of the en ligand are restricted to a cis arrangement. Under the MABCDEF system explained in Note \(\PageIndex{1}\) , [CoCl 2 (H 2 O) 2 (NH 3 ) 2 ] + may be MA 2 B 2 C 2 , but this complex, [CoBr 2 Cl 2 (en)] - , is M(AA)B 2 C 2 . The isomers are:
- Answer b. [CoBrCl(en) 2 ] +
-
The key to problems like this is to focus on the unique ligands, in this case Br - and Cl - . These can exist in either a cis or trans arrangement. The trans form is achiral, but in the case where Br - and Cl - are cis, \(\Lambda\) and \(\Delta\) enantiomers are possible. The result is three isomers.
- Answer c. [CoBrCl(gly) 2 ] -
-
This case is analogous to the one above, but this time the bidentate ligand, gly, possesses distinguishable binding sites. Thus there are additional permutations involving how the gly ligands are bound relative to one another and/or whether the O or N end of the gly is bound trans to Br or Cl. The result is 11 different isomers, which are given below.
- Answer d. [Co(en) 3 ] 3+
-
As a tris-chelate of symmetric bidentate ligands, ignoring ring twist, [Co(en) 3 ] 3+ will only exhibit \(\Lambda\) and \(\Delta\) isomerism.
- Answer e. [Co(gly) 3 ]
-
Since there are three gly ligands, each of which have carboxy and amine ends, the allowable arrangements involve whether the resulting three carboxy and three amine ends are oriented in a mer or fac arrangement. This gives two possibilities, each of which can exist as either the \(\Lambda\) or \(\Delta\) enantiomer, for a total of four isomers, which are given below.
Ignoring ring twist , draw all chemically reasonable stereoisomers for the following real and hypothetical complexes. You may assume that
- complexes in which the metal has a coordination number of six are octahedral
- complexes in which the metal has a coordination number of five are trigonal bipyramidal
- complexes in which Pt II , Pd II , or Rh I , or Ir I have a coordination number of four are square planar
- other complexes in which the metal has a coordination number of four will be tetrahedral
- [Pt(en)(SCN) 2 ]
- [Pt(NH 3 ) 2 (SCN) 2 ]
- [Co(en) 2 (SCN) 2 ] +
- Answer a. [Pt(en)(SCN) 2 ]
-
The Pt in this neutral complex must be Pt 2 + to balance the negative charges of the two SCN - ligands. Complexes of Pt 2 + are square planar and four coordinate, consistent with the bidentate en and two thiocyanato ligands. Of the ligands, the en ligand must bind in a cis arrangement. Consequently, the two SCN - ligands will be in a cis arrangement as well. The isomers will therefore only differ in whether the two SCN - bind \(\kappa\) N or \(\kappa\) S . The possibilities are:
- Answer b. [Pt(NH 3 ) 2 (SCN) 2 ]
-
As with the preceding example, this will be a square planar Pt 2 + complex. The main difference is that this time the coordinated nitrogens are not constrained to adopt a cis arrangement so there will be both cis and trans isomers.
- Answer c. [Co(en) 2 (SCN) 2 ] +
-
Ignoring ring twist , draw all chemically reasonable stereoisomers for the following real and hypothetical complexes. You may assume that
- complexes in which the metal has a coordination number of six are octahedral
- complexes in which the metal has a coordination number of five are trigonal bipyramidal
- complexes in which Pt II , Pd II , or Rh I , or Ir I have a coordination number of four are square planar
- other complexes in which the metal has a coordination number of four will be tetrahedral
- [Ru(bpy)(phen)(dppe)] 2+
- [CoClF(PPh 3 )(py)] -
- [Ni(en) 2 (NO 2 ) 2 ]
- [Fe(H 2 O) 3 (SCN) 3 ]
- [PtClF(PPh 3 )(py)]
- [CoBr 2 Cl(NH 3 ) 3 ]
- Answer a. [Ru(bpy)(phen)(dppe)] 2+
-
This complex is an octahedral tris-chelate containing symmetric ligands. As such it will exhibit \(\Lambda\) and \(\Delta\) isomers:
- Answer b. [CoClF(PPh 3 )(py)] -
-
This complex has a coordination number of 4 and contains a Co 2 + ion with a d 7 electron configuration, so a tetrahedral geometry is expected. Tetrahedral complexes like this one with four different ligands are chiral and can form R and S enantiomers.
- Answer c. [Ni(en) 2 (NO 2 ) 2 ]
-
This is an octahedral bis-chelate and will exist in \(\Lambda\) and \(\Delta\) configurations. Within each configuration the NO 2 - ligands can exist as \(\kappa\) N and \(\kappa\) O , leading to the following possibilities:
- Answer d. [Fe(H 2 O) 3 (SCN) 3 ]
-
As an octahedral complex with three identical ligands it can exist in mer and fac configurations, with the ambidentate SCN ligand providing additional possibilities for isomerism.
- Answer e. [PtClF(PPh 3 )(py)]
-
As a square planar complex with four nonidentical ligands, this complex exists as a single isomer.
- Answer f. [CoBr 2 Cl(NH 3 ) 3 ]
-
As an octahedral complex with three identical ligands it will exhibit mer and fac isomerism. In addition, it will have two mer configurations that differ in terms of the cis and trans relationships between the bromo and chloro ligands.
Complexes of formula Ru(TPP)py 2 have been prepared, in which TPP is tetraphenylporphyrin, which binds metals in the form given below. Draw all isomers of Ru(TPP)py 2 .
- Answer
-
There is only one isomer. While normally complexes containing two identical ligands (in this case py) can exhibit cis and trans isomerism, in this case the planar tetraphenylporphyrin ring ligates the Ru 2 + ion in a square planar arrangement, as shown at left in the image below. This leaves only a pair of trans coordination sites for the chloro ligands to occupy, giving the isomer shown at right below.
Draw the diastereomers formed due to the chirality of the amine nitrogen atoms in [PtCl 2 ( N , N '-dimethylethane-1,2-diamine)] which has the atomic connectivity represented below
- Answer
-
The diastereomers will differ in whether the N atoms adopt an R or S configuration at the nitrogen atoms. The possible permutations are ( R,R ), (S ,S ), ( R,S ), and (S ,R ). However, as may be seen from the image below, the ( R,S ) and (S ,R ) configurations are identical (the two projections shown can be interconverted through a C 2 rotation along an axis bisecting the Cl-Pt-Cl unit). As a result there are only three unique isomers. Of these, the ( R,R ) and (S ,S ) configurations are enantiomers (they are mirror images by reflection in the PtCl 2 N 2 plane). Note that in solving problems like this one it can be helpful to keep track of possible isomers by assigning the stereochemistry at each chiral center as R or S using the Cahn-Ingold-Prelog-convention , though it is not strictly necessary to do so.
Label the conformations of all chelate rings in the structure below.
- Answer
-
.
Appendix: The MABCDEF bookkeeping system for identifying isomers
The Mabcdef or MABCDEF system is one method for identifying the number of isomers in an octahedral complex, although since it is really just a bookkeeping and organizational system it can easily be extended to other geometries as well. The M in Mabcdef stands for metal and the other letters are used to stand in for ligands. The basic approach involves
- classifying the ligands as A, B, C, D, E, or F in order of multiplicity. Thus for
[Cr(NH 3 ) 6 ] 3+ A = NH 3 ; there are no B, C, D,E, or F ligands; and the complex is classified as MA 6
[CoCl(NH 3 ) 5 ] 2+ A = NH 3 , B = Cl - , and the complex is MA 5 B
[CrCl 2 (H 2 O) 2 (NH 3 ) 2 ] + A = Cl - , B = H 2 O, C = NH 3 , and the complex is MA 2 B 2 C 2
- in the classification above, multidentate ligands are typically classified before monodentate ones and are designated AA, AB, ABC, ABA, etc. based on the symmetry of the attachment points. Thus
en has identical attachment amine points and is AA
gly has a carboxy and amine attachment points and is AB
trien is ABA
CoCl 2 (en) 2 + is M(AA) 2 B 2
CoCl 2 (gly) 2 - is M(AB) 2 C 2
CoCl 2 (en)(gly) is M(AA)(AB)C 2
-
Systematically list out the possible
trans
arrangements of ligands by
- assigning one pair of ligands to be trans to one another.
- Then systematically list out the other possible trans pairs by permuting the remaining trans arrangements. It can help to organize the permutations in a table, such as that shown below for an MABCDEF complex where A and B are assigned trans . In looking at the table notice how the second set of trans permutations is systematically varied. This helps ensure that no possibility is skipped.
- Go through the list of isomers and remove any duplicates you generated so far. For instance, notice in the table below that the last three stereoisomers are identical with the first three ( e.g., stereoisomer 4 is identical to stereoisomer 3, 5 with 2, and 6 with 1).
| Stereoisomer 1 | Stereoisomer 2 | Stereoisomer 3 |
Stereoisomer 4 (same as isomer 3) |
Stereoisomer 5 (same as isomer 2) |
Stereoisomer 6 (same as isomer 1) |
| trans AB (fixed) | trans AB (fixed) | trans AB (fixed) | trans AB (fixed) | trans AB (fixed) | trans AB (fixed) |
| trans CD | trans CE | trans CF | trans DE | trans DF | trans EF |
| trans EF | trans DF | trans DE | trans CF | trans CE | trans CD |
- If the complex possesses multidentate ligands that demand that certain groups exist and cis pairs, then also remove any configurations which do not agree with the known binding capability of the ligand ( e.g., the two amine groups of ethylenediamine cannot be trans to one another, so if you have an en ligand, remove configurations like that from the list).
- Next, swap or permute the original trans pair and repeat the process you just followed. In the case of Mabcdef this gives the following results:
| Isomer | "fixed" trans pair | Additional trans pairs | |
| 1 | AB | CD | EF |
| 2 | AB | CE | DF |
| 3 | AB | CF | DE |
| 4 | AC | BD | EF |
| 5 | AC | BE | BF |
| 6 | AC | BF | DE |
| 7 | AD | BC | EF |
| 8 | AD | BE | CF |
| 9 | AD | BF | CE |
| 10 | AE | BC | DF |
| 11 | AE | BD | CF |
| 12 | AE | BF | CD |
| 13 | AF | BC | DE |
| 14 | AF | BD | CE |
| 15 | AF | BE | CD |
- Since the procedure explained above only identifies isomers based on unique trans pairings, it does not identify when a configuration is chiral and corresponds to a pair of enantiomers. Any enantiomers can be identified by drawing out the complexes and either classifying their point groups or by drawing their mirror images and checking if they are superimposable. In the MABCDEF case - i.e., where all the ligands are different - all of the isomers identified have C 1 symmetry so are chiral. This means there will be a pair of enantiomers for each, giving 15 x 2 = 30 different stereoisomers.
References
1. These examples are taken from http://wwwchem.uwimona.edu.jm/courses/inorgnom.html
2. Contakes S. M.; Rauchfuss, T.B. Chem . Commun . 2001, 553-554.
3. Shriver, D. F.; Shriver, S. A.; Anderson, S. E., Inorg. Chem. 1965, 4(5) , 725-730.
4. Zelewsky, A. v., Stereochemistry of Coordination Compounds Wiley, 1996.
5. In the past it was difficult to determine absolute configurations from X-ray data alone, but recent advances have made it easier to do so.
6. Pavan M. V. Raja & Andrew R. Barron "Circular Dichroism Spectroscopy and its Application for Determination of Secondary Structure of Optically Active Species" in Physical Methods in Chemistry and Nano Science https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Book%3A_Physical_Methods_in_Chemistry_and_Nano_Science_(Barron)/07%3A_Molecular_and_Solid_State_Structure/7.07%3A_Circular_Dichroism_Spectroscopy_and_its_Application_for_Determination_of_Secondary_Structure_of_Optically_Active_Species
7. Berova, N.; Bari, L. D.; Pescitelli, G., Chemical Society Reviews 2007, 36 (6), 914-931.
8. Of course, if one set of ligands occupies a meridional plane, then the other three ligands will be oriented in an equatorial one. The reason they may both be considered meridional is because if the complex were rotated, the equatorial plane would be meridional and vice versa. From that point of view both might be considered meridional planes - albeit not at the same time.
9. Bite angles are taken from Mansell, S. M., Catalytic applications of small bite-angle diphosphorus ligands with single-atom linkers. Dalton Transactions 2017, 46 (44), 15157-15174.
10. Hancock, R. D., The pyridyl group in ligand design for selective metal ion complexation and sensing. Chemical Society Reviews 2013, 42 (4), 1500-1524.
11. Sasi, D.; Ramkumar, V.; Murthy, N. N., Bite-Angle-Regulated Coordination Geometries: Tetrahedral and Trigonal Bipyramidal in Ni(II) with Biphenyl-Appended (2-Pyridyl)alkylamine N,N'-Bidentate Ligands. ACS Omega 2017, 2 (6), 2474-2481.
12. Ehnbom, A.; Ghosh, S. K.; Lewis, K. G.; Gladysz, J. A., Octahedral Werner complexes with substituted ethylenediamine ligands: a stereochemical primer for a historic series of compounds now emerging as a modern family of catalysts. Chemical Society Reviews 2016, 45 (24), 6799-6811.


15.8: Should students have differentiated assessments based on their needs and abilities?
-
- Last updated
- Save as PDF
Carissa Eckelman
Learning Objectives
- Identify what Differentiated assessment is
- Learn different types of differentiated assessments
- Understand the different types of learners
Introduction
There are many different types of students that teachers will encounter throughout their years of teaching. Each child has different gifts, and they also have different struggles. Teachers need to be able to identify the different types of learners that they will encounter, and how to successfully teach and give assessment to these students. The first type of students that a teacher will encounter is referred to as the "normative students." These students as referred to as by Webster is, "of, relating to, or determining norms or standards." Which in teaching means that these students can complete most tasks or assessments that are given to them. Normative students are given what are called Norm-Referenced Tests or (NRTs). These tests are defined as a type of test, assessment, or evaluation which yields an estimate of the position of the tested individual in a predefined population, with respect to the trait being measured. (Wikipedia) In this you compare one of the students to another with the results. The next type of students that a teacher could be presented with is a disabled learner. The definition of a disabled person is any person who has a physical or mental impairment which substantially limits one or more of such person's major life activities, has a record of such impairment, or is regarded as having such impairment. (Administration 2004.) As teacher this means that you will be responsible for coming up with different ways of giving assessments to these students, because they can't take the NRTs.
Different Assessments
Since you will be responsible for coming up with different ways to test students with disabilities it is important to know some ways that could really help you. First let's start with the basics. The definition of differentiated assessment is an ongoing process through which teachers gather data before, during and after instruction from multiple sources to identify learners' needs and strengths. Students are differentiated in their knowledge and skills. (Chapman and King 2004.) Since you know the definition of what differentiated assessment is you can use it towards your teaching. For instance you don't have to necessarily test students on a subject, but rather on how they are improving. Types of things to assess could be completion of tasks, ability to work with and listen to others, participation levels, respects self and others, ability to discuss, explain, make connections, debate, support opinions, infer, reason, re-tell, describe, report, predict. (About, 2009). Another way of differentiating assessment is when you have students who can't read as well as others, and you are testing say social studies, read the questions to the students. You aren't giving them a reading test, so by reading them the different questions they can actually have a chance of figuring out the correct questions.
Note
Here are some GREAT tips on how to differentiate assessment and instruction www.frsd.k12.nj.us/rfmslibrarylab/di/differentiated_instruction.htm
Two key types of different assessments are performance based assessment and portfolio assessment (Eric, 1993.). In performance based assessment you ask the students to perform tasks to evaluate certain skills. For portfolio assessment portfolios are an effective way to bring assessment into harmony with instructional goals. Portfolios can be thought of as a form of "embedded assessment"; that is, the assessment tasks are a part of instruction. Teachers determine important instructional goals and how they might be achieved. Through observation during instruction and collecting some of the artifacts of instruction, assessment flows directly from the instruction. (Shavleson 1992.)
Conclusion and Author's Opinion
So there are a lot of different ways that teachers can learn to test and instruct students with different needs and abilities. Students don't have to be given a standard 100 question multiple choice test to really prove what they know. They could be given an experiment to do or a paper to write. I personally think that every teacher should practice differentiated assessment. Since, inclusion is a huge part in almost all public schools teachers will come across students that will need differentiated assessment. From what I have seem, through observation and from other teachers, you can assess students in lots of different ways. For example, my mom, who is a second grade teacher uses differentiated assessment a lot. She reads test questions to students who wouldn't be able to read them. She also assesses them in other ways than just tests. One teacher I observed used projects to assess students. She would give them each a project after each unit. Sometimes it would be a book to make, a poster, or a brochure. By using these different ways to assess, each students get a fair chance at being able to learn and succeed.
Exercise \(\PageIndex{1}\)
1. All of the following are examples of types of differentiated assessment EXCEPT:
a. Performance Based Assessment
b. N.R.T.s
c. Portfolio Assessment
d. Debating a subject
2. What is Differentiated Assessment?
a. Figuring out different needs and skills of students and giving tests based on them.
b. Choosing which students get to take a test
c. Instead of testing, giving the students all A's.
d. Making the students all take the same test
3. You are a new teacher and want to use different assessments for every level of students. What is an example of a problem you might occur?
a. Since they are all in the same class they don't need differentiated assessment.
b. No problem
c. The students will all rebel and could hurt you.
d. You will have a long time grading them, and therefore it might not be worth making different tests for all the levels.
4.  What is an example of an assessment.
a. Having the students draw a picture explaining the lesson
b. Having the students take a multiple choice test.
c. Having the students do an experiment
d. Both a and d.
References
Administrative Office of the Trial Court. (2004.)Affirmative Action Plan of the Massachusetts Trial Court. Retrieved on March 21, 2009. Website: http://www.mass.gov/courts/admin/hr/affirm7.html ( Wikipedia. Norm-Referenced Test. Retrieved on March 21, 2009. Website: http://en.Wikipedia.org/wiki/Norm-referenced_test
Chapman, C and King R. Differentiated Assessment Strategies: One Tool Doesn't Fit All. 2004. Differentiated Assessment. Retrieved on March 21, 2009. Website: http://books.google.com/books?id=BWKe0dlY7dcC&dq=definition+of+differentiated+assessment&source=gbs_summary_s&cad=0
About: Differentiated Instruction and Assessment. 2009. Differentiated Assessment. Retrieved on March 21, 2009. Website: http://specialed.about.com/od/teacherstrategies/a/differentiated.htm
ERIC Clearinghouse on Information and Technology Syracuse NY. (Dec 1993.) Alternative Assessment and Technology. Retrieved on March 21, 2009. Website: http://www.ericdigests.org/1994/technology.htm
Shavelson, R.J. (1992). What we've learned about assessing hands-on science. Educational Leadership, 49, 20-25. Retrieved on March 21, 2009. Website: www.eduplace.com//res/literacy/assessr.html
Reader Response Tbandy001 (talk) 21:05, 16 April 2009 (UTC) I think that this article was very important in interested because there needs to be different wasys to asses the student because as you said every single student is different. Having the same assessment for everyone through out the year will only show favor to a certain constant group of people. As you said before inclusionis a big part of going to school now and because of this I agree with you in that teachers need to take this into consideration and allow for different assesments. You made that very clear in your conclusion we all aren't so called normal so why would everyone be required to take normative assesments.


7.6: Hybrid Forms of Ownership
What you’ll learn to do: discuss the advantages and disadvantages of hybrid forms of business ownership
The concept of “limited liability” has given rise to hybrid forms of business ownership such as LLCs and LLPs. In this section you’ll learn what these forms are and the pros and cons of each.
Learning Objectives
- Discuss the advantages and disadvantages of LLPs
- Discuss the advantages and disadvantages of LLCs
Limited Liability Partnership (LLP)
A limited liability partnership (LLP) is a partnership in which some or all partners (depending on the jurisdiction) have limited liabilities. It therefore exhibits elements of partnerships and corporations. In an LLP, one partner is not responsible or liable for another partner’s misconduct or negligence.
In an LLP, some partners have a form of limited liability similar to that of the shareholders of a corporation. Some states require one partner to be a “general partner” with unlimited liability, meaning he/she is ultimately responsible for the debts of the business and for any lawsuits such as personal injury or breach of contract. Unlike corporate shareholders, the partners have the right to manage the business directly. In contrast, corporate shareholders have to elect a board of directors under the laws of various state charters. The board organizes itself (also under the laws of the various state charters) and hires corporate officers who, as “corporate” individuals, then have the legal responsibility to manage the corporation in the corporation’s best interest. An LLP also has a different level of tax liability compared with that of a corporation.
As in a partnership or limited liability company (LLC), the profits of an LLP are allocated among the partners for tax purposes, avoiding the problem of “double taxation” often found in corporations.
Forming an LLP
Verify Eligibility Status . In the United States, each individual state has its own law governing the formation of LLPs. Although found in many business fields, the LLP is an especially popular form of organization among professionals such as lawyers, accountants, and architects. In California, New York, Oregon, and Nevada, LLPs can only be formed for such professional uses.
Choose a Business Name. When selecting a name for the LLP, generally the name (1) must be different from an existing LLP in your state, and (2) most states require the inclusion of “Limited Liability Partnership,” “LLP,” or another related abbreviation at the end of your business name.
Draft a Limited Liability Partnership Agreement . Although not required in every state, this agreement is strongly recommended. A limited liability partnership agreement should define each partner’s role and responsibilities. It should clearly define the partners’ assets and liability limitations. The agreement should also outline capital contributions, distribution of profits and losses, buyout agreements, expulsion or addition of partners, etc.
File a Certificate of Limited Liability Partnership. The drafting of an LLP agreement is optional; however all LLPs must file a certificate of limited liability partnership (sometimes called a certificate of registration as a limited liability partnership). The certificate of limited liability partnership is more general than the limited liability partnership agreement, as it does not detail responsibilities, capital contributions, buyouts, etc. The certificate requires the listing of your business’s name and address, the names and contact information of the partners, and information on the registered agent of the LLP.
Obtain Licenses and Permits. Once your business is registered, you must obtain business licenses and permits. Regulations vary by industry, state and locality.
Announce Your Business. Some states, including Arizona and New York, require the extra step of publishing a statement in your local newspaper about your LLP formation.
LLP Taxes
The tax treatment for LLPs is similar to general partnerships, as discussed earlier. Profits and losses are passed through to the partners so the partners reflect them on their individual tax return.
Advantages of an LLP
- Single Taxation . The credits and deductions of the company are passed through to partners to file on their individual tax returns. Credits and deductions are divided by the percentage of individual interest each partner has in the company.
- Limited Liability. The LLP structure protects individual limited partners from personal liability for negligent acts of other partners or employees not under their direct control. In addition, individual partners are not personally responsible for company debts or other obligations.
- Flexibility . LLPs provide the partners flexibility in business ownership. Partners have the ability to decide how they will individually contribute to business operations, both financially and physically. Management duties can be divided equally or unequally based on the experience of each partner. Partners who have a financial interest in the company can elect not to have any authority over business decisions but still maintain ownership rights based on their percentage interest in the company.
Disadvantages of an LLP
- Duration . The business life of a LLP is unstable because the partnership can be dissolved by agreement of the partners or upon the death or withdrawal of a partner. A limited liability partnership agreement can prevent dissolution if a partner dies or withdraws.
- Limitation of Formation : Unlike general partnerships, limited liability partnerships are not recognized as legal business structures in every state. Some states limit the creation of a limited liability partnership to professionals such as doctors or lawyers.
- Partner Control. If an LLP is formed without a limited liability partnership agreement, individual partners are not obligated to consult with other participants in certain business agreements. The fact that a partner can make business decisions without consulting the other partners can be problematic, to say the least.
Limited Liability Company (LLC)
A limited liability company (LLC) is a hybrid business structure allowed by state statute. LLCs are attractive to small business owners because they provide the limited liability features of a corporation and the tax efficiencies and operational flexibility of a partnership. Each state may use different regulations , and you should check with your state if you are interested in starting a limited liability company.
Owners of an LLC are called members. Most states do not restrict ownership, and so members may include individuals, corporations, other LLCs and foreign entities. There is no maximum number of members. Most states also permit “single-member” LLCs, those having only one owner.
Unlike shareholders in a corporation, LLCs are not taxed as a separate business entity. Instead, all profits and losses are “passed through” the business to each member of the LLC. LLC members report profits and losses on their personal federal tax returns, just like the owners of a partnership would.
Forming an LLC
While each state has slight variations on forming an LLC, they all adhere to some general principles:
Choose a Business Name . There are three rules that your LLC name needs to follow: (1) it must be different from an existing LLC in your state, (2) it must indicate that it’s an LLC (such as “LLC” or Limited Company”) and (3) it must not include words restricted by your state (such as “bank” and “insurance”). Your business name is automatically registered with your state when you register your business, so you do not have to go through a separate process.
File the Articles of Organization. The “articles of organization” is a simple document that legitimizes your LLC and includes information like your business name, address, and the names of its members. For most states, you file with the Secretary of State. However, other states may require that you file with a different office such as the State Corporation Commission, Department of Commerce and Consumer Affairs, Department of Consumer and Regulatory Affairs, or the Division of Corporations & Commercial Code.
Create an Operating Agreement. Most states do not require operating agreements. However, an operating agreement is highly recommended for multimember LLCs because it structures your LLC’s finances and organization, and provides rules and regulations for smooth operation. The operating agreement usually includes percentage of interests, allocation of profits and losses, member’s rights and responsibilities, and other provisions.
Obtain Licenses and Permits . Once your business is registered, you must obtain business licenses and permits. Regulations vary by industry, state, and locality.
Announce Your Business . Some states, including Arizona and New York, require the extra step of publishing a statement in your local newspaper about your LLC formation.
LLC Taxes
In the eyes of the federal government, an LLC is not a separate tax entity, so the business itself is not taxed. Instead, all federal income taxes are passed on to the LLC’s members and are paid through their personal income tax. While the federal government does not tax income on an LLC, some states do, so check with your state’s income tax agency.
Since the federal government does not recognize LLC as a business entity for taxation purposes, all LLCs must file as a corporation, partnership, or sole proprietorship tax return. Certain LLCs are automatically classified and taxed as a corporation by federal tax law.
Advantages of an LLC
- Limited Liability . Members are protected from personal liability for business decisions or actions of the LLC. This means that if the LLC incurs debt or is sued, members’ personal assets are usually exempt. This is similar to the liability protections afforded to shareholders of a corporation. Keep in mind that limited liability means “limited” liability—members are not necessarily shielded from wrongful acts, including those of their employees.
- Less Record Keeping. An LLC’s operational ease is one of its greatest advantages. Compared to an S Corporation, there is less registration paperwork and there are smaller start-up costs. However, it is very important to keep proper and separate business financial records. If it appears that the LLC is co-mingling personal and business funds, it can be legally reclassified and end up assuming additional liability.
- Sharing of Profits. There are fewer restrictions on profit sharing within an LLC, as members distribute profits as they see fit. Members might contribute different proportions of capital and sweat equity. Consequently, it’s up to the members themselves to decide who has earned what percentage of the profits or losses.
Disadvantages of an LLC
- Possible Limited Life. When an LLC is formed, the members must decide on the duration of the LLC. If an LLC is formed in a state where perpetual life is not permitted, then the death or disassociation of a member will dissolve the LLC, and the members must fulfill all remaining legal and business obligations to close the business. For this reason, it is important for individuals seeking to use this form of ownership verify the requirements for an LLC in the state in which they intend to operate.
- Self-Employment Taxes. Members of an LLC are considered self-employed and must pay the self-employment tax contributions towards Medicare and Social Security. The entire net income of the LLC is subject to this tax.


5.1: Introduction
In this chapter, we look at the properties and mechanism of action of enzymes. These include allosteric change ( induced fit , enzyme regulation ), energetic events (changes in activation energy ), and how enzymes work in open and closed (experimental) systems. Any catalyst, by definition, accelerates a chemical reaction. But enzymes and inorganic catalysts differ in important ways (blue and red in the table below).
Enzymes are long polymers that can fold into intricate shapes. As a result, they can be more specific than inorganic catalysts in which substrates they recognize and bind to. Finally, enzymes are flexible and can be regulated in ways that rigid, inflexible, metallic inorganic catalysis cannot. The specificity of an enzyme lies in the structure and flexibility of its active site . We will see that the active site of enzymes undergo conformational change during catalysis. The flexibility of enzymes also explains the effects of enzymes to cellular metabolites that indicate the biochemical status of the cell. When such metabolites bind to an enzyme, they force a conformational change in the enzyme that change the catalytic rate of the reaction, a phenomenon called allosteric regulation . As you might imagine, changing the rate of a biochemical reaction can change the rate of an entire biochemical pathway…, and ultimately the steady state concentrations of products and reactants in the pathway.
To understand the importance of allosteric regulation, we’ll look at how we measure the speed of enzyme catalysis. As we consider the classic early 20th century enzyme kinetic studies of Leonor Michaelis and Maud Menten, we’ll focus on the significance of the Km and Vmax values that they derived from their data. But, before we begin our discussion here, remember that chemical reactions are by definition, reversible. The action of catalysts, either inorganic or organic, depends on this concept of reversibility.
Finally, let’s give a nod to recent human ingenuity that enabled enzyme action to turn an extracellular profit ! You can now find enzymes in household cleaning products like detergents, where they digest and remove stains caused by fats and pigmented proteins. Enzymes added to meat tenderizers also digest (hydrolyze) animal proteins down to smaller peptides. Enzymes can even clean a clogged drain!
Learning objectives
When you have mastered the information in this chapter, you should be able to:
1. describe how the molecular flexibility of protein and RNA molecules make them ideal biological catalysts.
2. compare and contrast the properties of inorganic and organic catalysts.
3. explain why catalysts do not change equilibrium concentrations of a reaction conducted in a closed system.
4. compare the activation energies of catalyzed and un-catalyzed reactions and explain the roles of allosteric effectors in enzymatic reactions.
5. discuss how allosteric sites interact with an enzyme's active site and explain the concept of the rate limiting reaction in a biochemical pathway.
6. write simple rate equations for chemical reactions.
7. write the possible rate equations for equations for catalyzed reactions.
8. distinguish between Vmax and Km in the Michaelis-Menten kinetics equation.
9. state what Vmax and Km say about the progress of an enzyme catalyzed reaction.
10. interpret enzyme kinetic data and the progress of an enzyme-catalyzed reaction from this data.
11. more accurately identify Leonor Michaelis and Maud Menten!


20.3B: Horizontal Gene Transfer
- Explain how horizontal gene transfer can make resolution of phylogenies difficult
Horizontal gene transfer (HGT) is the introduction of genetic material from one species to another species by mechanisms other than the vertical transmission from parent(s) to offspring. These transfers allow even distantly-related species (using standard phylogeny) to share genes, influencing their phenotypes. It is thought that HGT is more prevalent in prokaryotes, but that only about 2% of the prokaryotic genome may be transferred by this process. Some researchers believe these estimates are premature; the actual importance of HGT to evolutionary processes must be viewed as a work in progress. As the phenomenon is investigated more thoroughly, it may be revealed to be more common. Many evolutionists postulate a major role for this process in evolution, thus complicating the simple tree model. A number of scientists believe that HGT and mutation appear to be (especially in prokaryotes) a significant source of genetic variation, which is the raw material for the process of natural selection. These transfers may occur between any two species that share an intimate relationship, thus adding a layer of complexity to the understanding or resolution of phylogenetic relationships.
HGT in Prokaryotes
The mechanism of HGT has been shown to be quite common in the prokaryotic domains of Bacteria and Archaea, significantly changing the way their evolution is viewed. These gene transfers between species are the major mechanism whereby bacteria acquire resistance to antibiotics. Classically, this type of transfer was thought to occur by three different mechanisms:
- Transformation: naked DNA is taken up by a bacteria.
- Transduction: genes are transferred using a virus.
- Conjugation: the use a hollow tube called a pilus to transfer genes between organisms.
More recently, a fourth mechanism of gene transfer between prokaryotes has been discovered. Small, virus-like particles called gene transfer agents (GTAs) transfer random genomic segments from one species of prokaryote to another. GTAs have been shown to be responsible for genetic changes, sometimes at a very high frequency compared to other evolutionary processes. The first GTA was characterized in 1974 using purple, non-sulfur bacteria. These GTAs, which are thought to be bacteriophages that lost the ability to reproduce on their own, carry random pieces of DNA from one organism to another. The ability of GTAs to act with high frequency has been demonstrated in controlled studies using marine bacteria. Gene transfer events in marine prokaryotes, either by GTAs or by viruses, have been estimated to be as high as 10 13 per year in the Mediterranean Sea alone. GTAs and viruses are thought to be efficient HGT vehicles with a major impact on prokaryotic evolution.
HGT in Eukaryotes
Although it is easy to see how prokaryotes exchange genetic material by HGT, it was initially thought that this process was absent in eukaryotes. After all, prokaryotes are only single cells exposed directly to their environment, whereas the sex cells of multicellular organisms are usually sequestered in protected parts of the body. It follows from this idea that the gene transfers between multicellular eukaryotes should be more difficult. Indeed, it is thought that this process is rarer in eukaryotes and has a much smaller evolutionary impact than in prokaryotes. In spite of this fact, HGT between distantly-related organisms has been demonstrated in several eukaryotic species. It is possible that more examples will be discovered in the future.
In plants, gene transfer has been observed in species that cannot cross-pollinate by normal means. Transposons or “jumping genes” have been shown to transfer between rice and millet plant species. Furthermore, fungal species feeding on yew trees, from which the anti-cancer drug TAXOL® is derived from the bark, have acquired the ability to make taxol themselves; a clear example of gene transfer.
In animals, a particularly interesting example of HGT occurs within the aphid species. Aphids are insects that vary in color based on carotenoid content. Carotenoids are pigments made by a variety of plants, fungi, and microbes, which serve a variety of functions in animals who obtain these chemicals from their food. Humans require carotenoids to synthesize vitamin A and we obtain them by eating orange fruits and vegetables: carrots, apricots, mangoes, and sweet potatoes. On the other hand, aphids have acquired the ability to make the carotenoids on their own. According to DNA analysis, this ability is due to the transfer of fungal genes into the insect by HGT, presumably as the insect consumed fungi for food. A carotenoid enzyme called a desaturase is responsible for the red coloration seen in certain aphids. Furthermore, it has been shown that when this gene is inactivated by mutation, the aphids revert back to their more common green color.
Key Points
- It is thought that HGT is more prevalent in prokaryotes than eukaryotes, but that only about 2% of the prokaryotic genome may be transferred by this process.
- Many scientists believe that HGT and mutation appear to be (especially in prokaryotes) a significant source of genetic variation, which is the raw material for the process of natural selection.
- HGT in prokaryotes occurs by four different mechanisms: transformation, transduction, conjugation, and via gene transfer agents.
- HGT occurs in plants through transposons (jumping genes), which transfer between different species of plants.
- An example of HGT in animals is the transfer (through consumption) of fungal genes into insects called aphids, which allows the aphids the ability to make carotenoids on their own.
Key Terms
- transformation : the alteration of a bacterial cell caused by the transfer of DNA from another, especially if pathogenic
- transduction : horizontal gene transfer mechanism in prokaryotes where genes are transferred using a virus
- conjugation : the temporary fusion of organisms, especially as part of sexual reproduction


9.2: Maternal and Infant Health
Many elements impacting an individual's health may be at play prior to their birth. This includes the health of biological parents prior to conception, genetic material combined at conception, maternal and fetal health during pregnancy, and the events of labor and delivery. Genetic components and the condition of gametes (sperm and ovum) affect the probability of conception and successful implantation in the uterus, as well as the potential for miscarriage. Environmental factors during pregnancy also have an effect on fetal development and future health; everything from the availability of nutrients, to exposure to toxins, and even levels of maternal stress can increase or decrease the risk of congenital defects or future diseases. Additionally, a variety of factors have an impact on both physical and mental health of the pregnant person during and after pregnancy.


6.3: Assignment- Organizational Structures
Preparation
This module introduced common organizational structures, including the four classic organizational structures and various management and team organizational structures, including the situations where they are particularly well suited. This exercise is an opportunity to check your understanding of this material, matching structures with characteristics and identifying a company or industry example for each structure.
To prepare for your written assignment:
- Read the Organizational Structures module with a specific focus on the Common Organizational Structures and Factors Impacting Organization Design sections.
- Take notes on the companies and industries cited as examples of specific organizational/management structure. Some example answers will require supplemental Internet research.
Your Task
Download one of the following formats:
Read the structure descriptions in the left column and the answer choices in the answer choices pool. Complete the table by inserting the correct answer choices in the two columns on the right. Each table entry has only one correct answer and answers are not duplicated. Submit your completed table for grading.
Grading
Your assignment will be graded on the basis of number of correct matches out of a total of 28.
Contributors and Attributions
- Assignment: Organizational Structures. Authored by : Nina Burokas. Provided by : Lumen Learning. License : CC BY: Attribution


2.1.1: Drug Use in History
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Learning Objectives
- Discuss the presence of drugs in ancient times.
- Summarize the use of drugs in the United States during the nineteenth century.
- Explain the racial basis for decisions to ban opium, cocaine, and marijuana in the United States.
Shakespeare once wrote that “what’s past is prologue.” This familiar phrase means that what happened in the past provides a context for, and can help to understand and predict, the future. To the extent that the past is prologue, the history of drug use provides a sobering lesson: Drug use has been common since ancient times and has been common in almost every society. As a recent book on drug policy states, “People have used chemicals to alter their state of mind since before there were written records” (Kleiman, Caulkins, & Hawken, 2011, p. xviii). If past is indeed prologue, then it is no surprise that drug use remains common in contemporary nations despite considerable efforts to reduce it.
One manifestation of the long history of drug use is that humans have used mind-altering plants since prehistoric times. “Early humans discovered that eating some plants gave a feeling of relaxation, happiness, drowsiness, or peace,” one scholar writes. “Some gave a feeling of increased energy, alertness, and stamina. And some caused strange sensations, terrifying visions, or a profoundly different awareness” (Gahlinger, 2004, p. 5).
Examples of drug use thousands of years ago abound (Escohotado, 2010; Faupel, Horowitz, & Weaver, 2010; Goodman, Sherratt, & Lovejoy, 2007).Escohotado, A. (2010). The general history of drugs (G. W. Robinette, Trans.). Valparasio, Chile: Graffiti Milante Press; Faupel, C. E., Horowitz, A. M., & Weaver., G. S. (2010). The sociology of American drug use . New York, NY: Oxford University Press; Goodman, J., Sherratt, A., & Lovejoy, P. E. (Eds.). (2007). Consuming habits: Drugs in history and anthropology (2nd ed.). New York, NY: Routledge. Mead, an alcoholic drink made from fermented honey, was first used about 8000 BCE, and beer and berry wines were first used about 6000 BCE. The ancient Sumerians used opium starting about 5000 BCE. Ancient Egypt used alcohol in 3500 BCE, while ancient China used cannabis (the source of marijuana) around 3000 BCE. Ancient people in what is now Switzerland ate poppy seeds (the source of opium) in 2500 BCE. Coca leaves (the source of cocaine) have been chewed for thousands of years. Folk medicines made from plants and herbs have also been used since ancient times. People in ancient Palestine drank wine in 350 BCE. Ancient Greeks drank poppy juice in 300 BCE. In about the same period, South American tribes used a hallucinogen called cohoba , made from mimosa beans. The Chinese and other Asians were using opium regularly by 1000 CE. Native Americans used tobacco before being discovered by Columbus in 1492. The use of various drugs has also been common in the many societies that anthropologists have studied (Durant & Thakker, 2003; Page & Singer, 2010).Durant, R., & Thakker, J. (2003). Substance use and abuse: Cultural and historical perspectives . Thousand Oaks, CA: Sage Publications; Page, B., & Singer, M. (2010). Comprehending drug use: Ethnographic Research at the social margins . New Brunswick, NJ: Rutgers University Press.
Sociologist Erich Goode (2008, p. 176)Goode, E. (2008). Deviant behavior (8th ed.). Upper Saddle River, NJ: Prentice Hall. summarizes the history of drug use as follows: “Humans have been ingesting drugs for thousands of years. And throughout recorded time, significant numbers of nearly every society on earth have used one or more drugs to achieve certain desired physical or mental states. Drug use comes close to being a universal, both worldwide and throughout history.”
Drug Use in US History
This history of drug use includes the United States, where past is again prologue. During the colonial era, tobacco was a major crop in Virginia and other colonies thanks to slave labor. After being processed, it was commonly used by colonists and also exported to Europe in great quantities (Gately, 2001).Gately, I. (2001). Tobacco: The story of how tobacco seduced the world . New York, NY: Grove Press. From the earliest colonial days, alcohol was another drug used in great quantities, as “Americans were drinkers right from the start” (Genzlinger, 2011, p. C1).Genzlinger, N. (2011, October 1). Bellying up to the time when America went dry. New York Times , p. C1. The Mayflower, the celebrated ship that brought the first Puritans to what eventually became the United States, was filled with barrels of beer. In colonial New England, rum manufacturing was a major industry, and rum drinking was common. During the early 1770s, New England had more than 140 rum distilleries, and rum consumption in the colonies averaged 7.5 million gallons annually. This massive drinking has led one author to call rum “the real spirit of 1776” (Williams, 2006).Williams, I. (2006). Rum: a social and sociable history of the real spirit of 1776 . New York, NY: Nation Books. Rum was also a major export to Europe and elsewhere. In addition to rum, colonists routinely drank beer and hard cider.
During the nineteenth century, Americans began to use drugs other than alcohol in great quantities. One popular drug was coffee. Before the Civil War, Americans who drank coffee had to buy green (unroasted) coffee beans in bulk and roast their own coffee. Then in 1865, John Arbuckle, a Pittsburgh grocer, began selling roasted coffee inside a new invention—the paper bag.
Alcohol also remained a very popular drug, and use of this drug during the 1800s was probably greater than during colonial America. Two reasons help account for this trend (Faupel et al., 2010).Faupel, C. E., Horowitz, A. M., & Weaver., G. S. (2010). The sociology of American drug use . New York, NY: Oxford University Press. One reason was the western frontier. As the nation moved west, many of the explorers and settlers who led the way were men who were unmarried or, if married, men who had left their families behind. To put it mildly, they drank a lot, fought a lot, and gambled a lot. A second reason was that many Irish immigrants came to the United States during a great wave of immigration that began in the mid-nineteenth century. Although it might sound like a stereotype, the Irish drank a lot of alcohol back in their homeland, and they continued to do so once they reached the United States. Regardless of who was drinking, heavy alcohol use contributed greatly to poverty, to physical assaults and homicides, and to domestic violence and other family problems.
Three other popular drugs in this era were opium, cocaine, and marijuana. Use of these drugs was so common that nineteenth-century America has been called a “dope fiend’s paradise” (Brecher, 1973).Brecher, E. M. (1973). Licit and illicit drugs . Boston, MA: Little, Brown. A brief discussion of these drugs’ histories will underscore the widespread use of drugs in the American past and also racial issues that arose when laws were passed to ban these drugs (Musto, 1999).Musto, D. F. (1999). The American disease: Origins of narcotic control (3rd ed.). New York, NY: Oxford University Press.
Opium
During the decades before and after the Civil War, the use of opium was extremely common (Goode, 2012).Goode, E. (2012). Drugs in American society (8th ed.). New York, NY: McGraw-Hill. Beyond making people feel good, opium is an effective painkiller and cough suppressant. Accordingly, it was a staple in many patent medicines, elixirs and tonics, sold back then in apothecaries, general stores, and other venues. Large numbers of people from all social backgrounds used these opium-laced medicines for problems such as depression, headaches, menstrual cramps, and toothaches. It is not much of an exaggeration to say that the United States was a nation of opium users during this period; an estimated 500,000 Americans were addicted to opium by the end of the century. As anthropologist Robert B. Edgerton (1976, pp. 57–58)Edgerton, R. (1976). Deviance: A cross-cultural perspective . Menlo Park, CA: Cummings. summarizes the situation, “The use of opium was widespread in all segments of American society. Children were calmed with opium derivatives, women used many popular patent medicines which were liberally larded with opiates, and ‘opium dens’ were probably present in all cities and most towns as well.”
Opium was a common ingredient in nineteenth-century tonics and elixirs that were sold widely to the public.
© Thinkstock
Attendance at opium dens (the equivalent of today’s bar or tavern, with opium the drug of choice rather than alcohol) was a popular activity for the Chinese immigrants who began coming to the United States during the 1850s to help build the nation’s railroads and perform other jobs. White workers feared their growing numbers as a threat to their jobs, and racial prejudice against the Chinese increased. Politicians, labor unions, and other parties began to focus on the Chinese habit of smoking opium at opium dens and warned that the Chinese were kidnapping little white children, taking them to the opium dens, and turning them into “opium fiends.” This campaign had two effects: it increased prejudice against the Chinese, and it increased public concern about opium. This rising concern led San Francisco in 1875 to become the first locality to ban opium dens. Other California cities did the same, and the state itself banned opium dens in 1881. Three decades later, the federal government banned the manufacture, sale, and use of opium (except for use with a physician’s prescription) when it passed the Harrison Narcotics Act in 1914.
Cocaine
Cocaine was another drug that was very popular in the nineteenth century, beginning in the 1880s, thanks in part to enthusiastic claims by Sigmund Freud and American physicians that cocaine could help relieve asthma, depression, hay fever, sexual impotence, toothache pain, and a host of other problems. Like opium, cocaine was a popular ingredient in the many patent medicines that people bought at various stores, and the US Army Surgeon-General advocated its medical use. It was a major ingredient in a new beverage introduced in 1886, Coca-Cola, which became an instant hit because people naturally felt so good when they drank Coke! During the next two decades, however, concern grew about cocaine’s effects. Some of this concern was fueled by the absurd belief that African Americans who used cocaine became extra strong, dangerous, and even invulnerable to bullets. Cocaine was heavily taxed by the 1914 Harrison Narcotics Act and later banned.
Marijuana
A third legal drug during the late nineteenth century was marijuana. It joined opium and cocaine in being a common ingredient in patent medicines, and it was a popular drug for problems like migraine headaches, menstrual cramps, and toothache pain. After the Mexican Revolution of 1910, Mexicans moved to the United States in increased numbers and brought with them their habit of marijuana use. Whites feared that Mexicans would take their jobs, and, similar to what happened with opium and Chinese immigrants during the 1870s, began to charge that Mexicans who used marijuana would become violent and more likely to rape and murder innocent white victims. This racially prejudiced claim increased concern about marijuana and helped lead to the federal Marijuana Tax Act of 1937 that banned its use.
This brief history shows that drug use has been part of the American culture ever since the nation began. If past is prologue, it should come as no surprise that drugs remain part of the American culture today, and it should also come as no surprise that efforts to reduce or eliminate drug use often meet with much resistance and little success. As the United States continues to try to deal with drug use, these basic facts must not be forgotten.
KEY TAKEAWAYS
- Drug use has been common since ancient times.
- Alcohol was widely drunk in colonial America. During the latter nineteenth century, opium, marijuana, and cocaine were legal drugs that were also widely used.
- Racial prejudice played an important role in decisions during the late nineteenth century and early twentieth century to ban opium, marijuana, and cocaine.
FOR YOUR REVIEW
- Were you surprised to read that mind-altering drug use has been common since ancient times? Why or why not?
- Were you surprised to read that racial prejudice helped lead to bans on opium, marijuana, and cocaine? Why or why not?