, 
 calcium is a regulator of intracellular processes , activates proteases , and is essential for photosystem ii ( psii ) maturation and catalytic activity . in industrial 
 processes ,
calcium oxide plays a role as a heterogeneous catalyst 
 in transesterification reactions , offering the promise of environmentally 
 sustainable exploitation of biofuels .
calcium also plays an 
 important role as a promoter in transition metal - mediated heterogeneous 
 processes .
additionally , the use of calcium 
 compounds in homogeneous catalysis has received increasing attention , 
 with applications in polymerization catalysis , hydroamination , and 
 hydrosilylation .
the earth abundance of calcium , as well as its biocompatibility , 
 provides motivation for further development of ca - based catalysts . in order to understand the transformations that occur at the calcium 
 site , in both chemical and biological processes
, one would like to 
 selectively probe the calcium coordination environment . in this sense , 
 ca k - edge x - ray absorption spectroscopy ( xas ) is an ideal tool .
ca 
 k - edge xas results from the excitation of ca 1s electrons to empty 
 molecular orbitals localized on the calcium atom . as such
, ca xas 
 should provide a sensitive probe of the ca coordination environment . 

however , to our knowledge , a thorough investigation of ca k - edge xas 
 has not yet been made .
previous ca k - edge xas studies have generally 
 used a fingerprinting approach , and only limited studies exist in 
 which ca k - edge data have been correlated to theory .
the majority of the quantitative information that has been obtained 
 from ca xas data has largely relied on the extraction of metal 
however , due to the large inherent error in establishing coordination 
 numbers from exafs ( 25% ) , the ability to determine accurate 
 coordination numbers from exafs alone is limited .
however , previous 
 studies have successfully coupled k - edge analysis with exafs analysis 
 to describe previously uncharacterized metal centers in complex systems 
 such as proteins .
therefore , the development of ca k - edge xas should significantly 
 aid in the characterization of ca centers . in principle , the information 
 content of ca xas may be greatly enhanced through a quantitative analysis 
 of the pre - edge spectral region , in a manner analogous to previous 
 studies on first row transition metals .
herein , we systematically investigate the information content 
 of ca k - pre - edge xas and extend these studies to complex heterometallic 
 mnca clusters with relevance to the oxygen - evolving complex ( oec ) 
 of psii , which consists of a mn4o5ca cluster 
 with a  distorted chair 
ca k - edge xas data were obtained for a series 
 of six , seven , and eight coordinate calcium compounds ( table 1 and figure 1 ) , including 
 molecular and inorganic lattice species , in order to establish the 
 experimental changes that occur in the ca pre - edge xas region upon 
 altering the coordination environment of the ca ion . 
 experimental intensity and energy correlations
further , 
 we apply a td - dft protocol , which allows for the calculation of xas 
 pre - edges for both molecular and inorganic lattice systems , with excellent 
 reliability .
( 7 and 7 ) and  cubane - like  complexes ( 7 , 7 , 7 ) are shown . in the 7 complex , a tetrahydrofuran ligand 
 replaces the dimethylformamide seen in the 7 series .
having established the correlation 
 between experiment and theory , we then further explore the applicability 
 of this approach to understanding the changes that occur at the ca 
 center in complex heterometallic systems .
in particular , structural 
 information is extracted from the changes in the ca k - pre - edge associated 
 with a one electron reduction of the cubane mn3o4ca cluster ( figure 1 ) , which serves as a structural 
 reference point of the mn4o5ca cluster in the 
 oec .
as calcium is often suggested to play a mechanistic role in water 
 oxidation , the changes that occur at the ca site during the catalytic 
 cycle are of fundamental interest .
our results show that direct correlation 
 between the calcium environment and the observed ca k - pre - edge can 
 be made .
further , it is demonstrated that the ca site can serve as 
 a reporter for the changes that occur at the mn sites .
ca k - edge xas data was collected 
 on 13 model compounds consisting of both molecular and inorganic infinite 
 lattices . a td - dft protocol was developed and correlated to the experimental 
 data to help characterize the ca k - pre - edge .
cao ( 6 ) , caco3 ( 6 ) , and ca(oh)2 ( 6 ) were purchased 
 at the highest available purity and used without further purification . 

the ligand precursor 
 h3l , where l is a 1,3,5-triarylbenzene - based ligand , was 
 prepared according to reported procedures .
the six coordinate { [ lmnmn2o(oac)3]2ca } ( 6 ) and seven coordinate molecular complexes lmn3cao4(oac)3(thf ) 
 ( 7 ) , lmn3cao4(oac)3(dmf ) ( 7 ) , [ lmnmn2cao2(oac)2(dme)(otf)](otf)2 ( 7 ) , and [ lmn3cao2(oac)2(dme)(otf)](otf ) 
 ( 7 ; figure 1 ) were synthesized as previously described .
xas samples of [ lmn2mncao4(oac)3(dmf ) ] ( 7 ) were prepared by the addition of 1 equiv of 
 cobaltocene to a solution of 7 in dmf .
the reaction mixture was then transferred to 40 l 
 xas liquid sample holders and frozen within 5 min of reductant addition . 

attempts to isolate 7 as 
 a solid in analytically pure form have been unsuccessful to date .
the ligand precursors h3tpaa ( 6,6,6-nitrilotris(methylene)tripicolinic 
 acid ) and h2dpaea ( n , n - bis[6-carboxypyridin-2-yl)methyl]ethylamine ) and compound [ ca(dpaea)(h2o)(meoh)]2 ( 8) were prepared according to reported procedures .
all other starting 
 materials including the h2bzida ligand ( n - benzyliminodiacetic 
 acid ) were commercially available .
the elemental analyses were carried 
 out with a c , h , n analyzer ( sca , cnrs ) .
an 
 aqueous solution of koh ( 0.1 m ) was added to a suspension of h3tpaa ( 40.0 mg , 94.70 mol ) in water ( 5 ml ) until ph 
 5 was reached , yielding a colorless solution .
solid cacl22h2o ( 14.0 mg , 95.23 mol ) was added 
 to the resulting colorless solution , and after a few minutes of stirring , 
 a white precipitate was formed . after stirring the mixture for 15 
 min , precipitation
was completed by cooling the mixture to 4 c 
 for 12 h. the white powder was filtered off , washed with h2o ( 2 ml ) , dried under a vacuum , and collected ( yield , 20.0 mg , 37% ) . 

calcd for c21h18can4o7h2okcl ( 571.03 ) : c , 44.17 ; h , 3.53 ; n , 9.81 . 
 found : c , 44.32 ; h , 3.50 ; n , 9.68 .
this can be cocrystallized with 
 the reported trinuclear compound [ { ca(tpaa)(oh2)}2{ca(oh2)4 } ] by slow evaporation of an aqueous 
 solution of tpaa ( ph 8) , in the presence of 1 
 equiv of cacl22h2o .
an aqueous solution of koh ( 0.1 m ) was added to an h3tpaa ( 40.0 mg , 94.70 mol ) suspension in water ( 10 ml ) until 
 ph 8 was reached , yielding a colorless solution .
solid mncl2 ( 13.3 mg , 105.7 mol ) and cacl22h2o ( 10.0 mg , 68.0 mol ) were successively added .
x - ray suitable 
 colorless single crystals of [ mn2(tpaa)2ca2(oh2)12][mn(tpaa)]225h2o were obtained upon standing for 24 h. these were filtered , 
 washed with cold water ( 1 ml ) , and air - dried for a few days ( yield , 
 29.0 mg , 50% ) .
calcd for c84h84n16o36mn4ca213h2o 
 ( 2427.76 ) : c , 41.56 ; h , 4.57 ; n , 9.23 . found : c , 41.58 ; h , 4.29 ; n , 
 9.18
. an aqueous solution of koh ( 0.1 m ) was added to a suspension 
 of h2bzida ( 62.0 mg , 277.7 mol ) in water ( 5 ml ) 
 until ph 9 was reached , yielding a colorless solution .
solid 
 ca(o3scf3)2 ( 97.0 mg , 286.8 mol ) 
 was added , and the resulting colorless solution was stirred for 10 
 min .
x - ray suitable colorless single crystals of 8 were obtained by slow evaporation of the solvent 
 at 20 c .
these were filtered , washed with cold water ( four drops ) , 
 and dried on standing for a few days ( yield , 33.0 mg , 36% ) .
calcd for c22h34ca2n2o140.1c11h13no4 ( 652.99 ) : 
 c , 42.49 ; h , 5.45 ; n , 4.50 .
single - crystal diffraction 
 data were taken using an oxford - diffraction xcalibur s kappa geometry 
 diffractometer ( mo k radiation , graphite monochromator ,  
 0.71073  ) .
an absorption correction was applied , using the abspack 
 oxford - diffraction program with transmission factors in the 0.6740.898 
 range .
the molecular structure was solved 
 by direct methods and refined on f by full matrix least - squares 
 techniques using the shelxtl package .
all non - hydrogen atoms were refined anisotropically , and hydrogen 
 atoms were found by fourier transformation and refined with individual 
 isotropic displacement parameters .
ccdc 995926 , ccdc 995927 , and ccdc 
 995923 contain the supplementary crystallographic data for 8 , 8 , and 8 , respectively .
a summary 
 of x - ray data collection and structure refinement for 8 is reported in supporting 
 information table s1.1 .
the x - ray structure of 8 , together with the cif file , is provided as supporting information .
solid samples were diluted 
 in boron nitride and mounted on ca - free polycarbonate holders .
x - ray absorption spectroscopy ( xas ) data on the seven coordinate calcium 
 species ( 7 , 7 , 7 , 7 , 7 ) and on the six coordinate calcium complex 6 were measured at the stanford synchrotron 
 radiation lightsource ( 3-gev ring ) beamline 43 equipped with 
 a si(111 ) double - crystal monochromator .
a four - element silicon drift 
 detector ( vortex - me4 , hitachi ) was used to collect the data as fluorescence 
 excitation spectra .
samples were maintained at a temperature of 30 
 k with a liquid he cryostream ( oxford ) to minimize radiation damage . 

ca k - edge data were monitored throughout the course of data collection 
 to ensure that no x - ray induced damage occurred .
further , to test 
 for nonlocal damage , the mn k - edge was monitored before and after 
 collection of ca xas data on each sample .
all other data were 
 collected at beamline a1 ( doris light source at desy , 4.5 gev ring ) 
 equipped with a si(111 ) double crystal monochromator and a vacuum 
 sample chamber .
solid samples were diluted in boron nitride and finely 
 dispersed on kapton tape ( k104 psa polyimide film with silicone adhesive ) . 

spectra were 
 calibrated to the maxima of the rising edge feature in calcium acetate 
 monohydrate set at 4050.0 ev .
the athena software program , using the 
 autobk algorithm , was used for data reduction and normalization .
a linear pre - edge function and a quadratic polynomial 
 for the postedge were used for background subtraction , followed by 
 normalization of the edge jump . in order to extract intensities and 
 energy positions of pre - edge features ,
lorentzian sum peaks and a cumulative gaussian  lorentzian 
 function for the edge jump .
all dft calculations were carried out using the orca program . in general , for molecular complexes , crystal 
 structures with the full ligands were used to generate the starting 
 coordinates for geometry optimization calculations .
sham method 
 using the tpss functional and a def2-tzvp 
 basis set with a def2-tzvp / j auxiliary basis set , as well as a dft - d3bj dispersion correction .
a dense integration grid ( orca grid4 = lebedev 302 points ) was used 
 for all the atoms except ca and mn where a grid6 ( lebedev 590 points ) 
 was applied .
the geometry - optimized structures were later used for 
 the ca k - pre - edge xas spectra calculations .
the 8 polymer was truncated to a [ ca2(bzida)2(oh2)6 ] repeating unit , before geometry optimization , 
 as described in supporting information s2 .
additionally , the model for the 7 complex was based on the crystal structure of the previously 
 published sc analog .
the sc atom was 
 replaced with ca , and the structure was optimized , while maintaining 
 mn - ligand bond distances of the original complex .
lastly , the model for the xas spectral calculation of 6 was truncated after geometry optimization 
 to encompass the ca center and acetate ligands , with proton capped 
 oxygens instead of mn  o bonds ( supporting 
 information s2 ) .
the truncation of the model was needed due 
 to the large size of the molecular complex . in models for cao 
 ( 6 ) ,
caco3 ( 6 ) , and ca(oh)2 ( 6 ) , inorganic lattices were built 
 using the effective core potential embedding approach . in the most 
 popular variant of this approach
, the quantum cluster 
 ( qc ) is embedded in an extended point charge field ( pc ) .
furthermore , 
 in order to avoid electron attraction or electron  flow from 
 the quantum cluster region toward the positive charges at the pc region , 
 a third boundary region ( br ) is introduced between qc and pc , which 
 is constructed from repulsive capped effective core potentials ( c - ecps ) . 
 in all the cases ,
the qc is constructed around the central calcium 
 center spanning a 5  radius region around it .
the respective 
 atom centers were exchanged by capped effective core potentials ( with 
 ecp basis sets sd(10,mwb ) for ca and 
 sd(2,mwb ) for o and c ) .
finally , the 
 pc region follows the br , extending 15  from the central calcium . 

the charges for the three regions were distributed such that the net 
 charge for the model is zero and the total charges for the border 
 and point charge region taken together offset the charge on the quantum 
 cluster region .
calcium k - edge xas spectra were calculated with 
 a td - dft approach employing the tamm 
spin unrestricted calculations were carried out using a large integration 
 grid ( grid5 = lebedev 434 points ) and the bhlyp functional .
as previous studies have shown , the choice of 
 functional can impact the calculated xas spectra , we also carried 
 out a detailed study on the functional dependence .
the bhlyp functional 
 was found to offer the best results in terms of resolving the pre - edge 
 consistent with previously reported studies ( supporting 
 information s3 ) .
the rijcosx approximation was implemented to help reduce the computational 
 cost of the td - dft calculations . for 
 these calculations ,
the def2-tzvp basis set with a def2-qzvpp / jk auxiliary 
 basis set was used on all atoms except calcium where a def2-qzvp basis 
 set was employed . in order to ensure 
 saturation of the particle / hole transitions spanning the pre - edge 
 and near edge region of the ca k - edge spectra ,
moreover the calculated intensities 
 include electric dipole , magnetic dipole , and quadrupole contributions .
ca k - edge data were obtained for a series of 13 inorganic lattice 
 and molecular complexes ( figure 1 and table 1 ) .
the six coordinate complexes consist of pseudo - octahedral 
 species with increasing complexity of the oxygen - based ligands .
cao 
 ( 6 ) can be considered octahedral , 
 followed by 6 , which has only 
 small distortions from octahedral symmetry and caco3 ( 6 ) , which has local d4h symmetry .
four seven - coordinate 
 complexes , which mimic the mn4o5ca cluster motifs 
 in the oec , were investigated .
these complexes can be grouped as having 
 a  cubane - like  structure ( 7 , 7 ) and 
 a  noncubane - like  structure ( 7 ) depending on the arrangement of the metal - oxo 
 atoms .
furthermore , one - electron oxidized and reduced pairs of these 
 complexes were investigated .
the cubane - like complexes can be best 
 described as having c3v symmetry at the calcium center , while the noncubanes favor a c2v symmetry . to offer a full 
 range of accessible coordination geometries , eight coordinate
calcium 
 species ( 8 , 8 , 8 , and 8) having a mixture of amine , carboxylate , 
 and water ligands were also investigated .
calcium geometries for 8 and 8 are best described as trigonal dodecahedron ( d2d ) , while 8 and 8 are more pseudosquare 
 antiprismatic having a symmetry best characterized as intermediary 
 between d4d and d2d .
the symmetries around the 
 ca centers are approximated to the first coordination sphere ligand 
 atoms .
figure 2 shows the ca k - edge spectra for all investigated compounds . 
 in the case of molecular complexes ,
the rising edges are similar , 
 and the largest variations occur in the pre - edge region .
however , 
 in the inorganic infinite lattice series , the rising edge is dominated 
 by a convolution of multiple scattering and absorption processes resulting 
 in significant variability .
such features have previously been reproduced 
 using multiple scattering theory ; however this approach fails to capture 
 the pre - edge region , which is an important probe of coordination environment 
 at the absorbing center . for these reasons , 

we focus on the pre - edge region between 4038 ev and 4042 
 ev ( figure 2 ; right ) .
the low intensities of 
 the features compared to the rising edge are reminiscent of the 1s 
  3d dipole forbidden transitions of pre - edges in transition 
 metals ( tm ) and have previously been assigned as such based largely 
 on empirical considerations and , more recently , based on a density of 
 states analysis of cas infinite lattices .
x - ray 
 absorption spectra at the ca k - edge of six ( bottom ) , seven ( middle ) , 
 and eight ( top ) coordinate ca species : full rising edge 
 of the spectra ( left ) and the pre - edge region ( right ) .
six coordinate species are distinguished from seven 
 and eight coordinate centers by the presence of two pre - edge features 
 split by 1 ev and a lower intensity pre - edge .
the pre - edge 
 maxima for six coordinate centers occur at 4039.5 ev and 4040.6 
 ev , thus spanning the energies of the maxima for seven and eight coordinate 
 complexes ( figures 2 and 3 ) .
structures at 4039.75 ev . furthermore , a one electron chemical 
 reduction of the 7  cubane - like  
 complex [ mn3cao4 ] to yield 7 [ mn2mncao4 ] results in a 16  3% 
 lower intensity pre - edge .
this is not observed upon one electron chemical 
 reduction of the  noncubane  7 to yield 7 , both of which have pre - edges centered around 4040.17 ev , 
 similar in intensity and energy to eight coordinate calcium complexes .
best fit line ( red ) , 
 95% confidence interval ( dashed gray ) , 95% prediction band ( dotted 
 gray ) .
as k - edge xas transition intensities 
 are predominantly dipole dependent ( 1s  np ) , the variation in pre - edge intensities can in part be rationalized 
 using symmetry arguments .
six coordinate pseudo - octahedral complexes 
 are centro - symmetric with little or no p  d mixing , resulting 
 in low intensity dipole forbidden ( but quadrupole ) 1s  3d 
 transitions into the t2 g and eg orbitals . in 
 the seven and eight coordinate complexes under study , this centro - symmetry 
 is broken , p  d mixing becomes favored , and an increase in intensity 
 occurs due to an increase in p - character mixing in the d manifold . 

the pre - edges are most intense for the cubane - like structures because 
 they have pseudo c3v symmetry 
 and p mixing is symmetry allowed with all five d orbitals .
furthermore , 
 the increase in the average pre - edge energy upon increasing the coordination 
 number is most likely attributed to destabilization of the d manifold 
 upon increasing the number of ligands .
similar effects on the pre - edge 
 energies of the first row tm have previously been reported . in order 
 to obtain a deeper understanding of the observed modulations in ca 
 k - pre - edge intensities and energies , a td - dft approach was developed 
 and calibrated to the experimental spectra . owing to the limitations 
 of dft to accurately estimate the energies of the transition probabilities 
 dominating the ca k - pre - edge spectra , an empirical , element specific 
 shift should be determined .
in fact , the calculated absolute transition 
 energies carry large but highly systematic errors that arise from 
 shortcomings of the density functionals in the core region , limitations 
 of the one - particle basis set , and shortcomings in the accurate modeling 
 of spin - free relativistic effects .
given their highly systematic nature , 
 all of these factors can ( for a given basis set and density functional ) 
 be taken into account by introducing an element - dependent shift .
in fact , it has been shown that a simple linear regression is sufficient 
 to establish predictive accuracy in the calculated transition energies 
 for any given element .
this calibration needs to be carried out with 
 respect to a test set of well  known systems and has already 
 been reported for metal and ligand k - edges in the framework of scalar 
 relativistic dft methodology .
the calibration of the ca k - pre - edge 
 spectra was performed for a series of 12 structurally characterized 
 compounds . in all cases ,
good agreement between the calculated versus 
 experimental pre - edge intensities and energies was observed ( figure 3 , table 1 ) .
the intensity 
 correlation resulted in a linear relationship with an r value of 0.968 , while the energy correlation is also 
 linear with an r value of 0.876 .
deviations 
 may be attributed to errors intrinsic to data processing ( figure 3 ) and limitations of the theoretical models . for 
 instance ,
while they are likely to be small , possible vibronic contributions are not accounted for .
similarly , 
 complexes of intermediate symmetry such as the eight coordinate d2d / d4d complex 8 may 
 have a geometry optimized structure which does not fully capture the 
 interplay between the limiting symmetries again causing deviations 
 in calculated values from experiment .
however , both the intensity 
 and energy correlation values for goodness of fit are consistent with 
 those of previously reported td - dft calculated xas spectra and accurately 
 predict the experimental trends observed as illustrated below . from the calibration fits , the energy shift to be applied to the 
 calculated transition energies was found to be 50.17  0.08 ev , 
 with an intensity normalization factor of 3500  300 .
figure 4 shows 
 an overlay of representative experimental ca k - pre - edges , their corresponding 
 fits , and the td - dft calculated transitions .
difference density maps 
 were used to visualize the transitions in terms of a molecular orbital 
 picture ( figure 5 ) .
the maps consist of a difference 
 in electron density between the excited state and ground state and 
 shows the shift in electron density due to the excitation .
as the ca 1s orbital is highly localized and 
 compact , the difference density map will be dominated by the contribution 
 from the acceptor orbital . from this 
 analysis ,
it is evident that the ca k - pre - edge consists of 1s  
 d excitations ( figures 4 and 5 ) .
in contrast to the first transition series , one generally 
 does not consider a dominant 3d contribution to bonding in ca .
nevertheless , 
 the 3d orbitals are unoccupied and thus have xas transitions , which 
 may include both quadrupole ( 1s  3d ) and dipole allowed ( 1s 
  3d + np ) transitions .
it is the dipole allowed 
 contribution that is responsible for the increase in pre - edge intensities 
 over the present series .
pre - edges of ca k - edge with their associated 
 td - dft transitions and acceptor orbital assignment . from top to bottom : 7 , 8 , 7 , 6 .
spectral 
 fitting : rising edge ( dotted lines ) , pre - edges ( dashed lines ) .
( right ) breakdown of contributions 
 to the total calculated transition intensities in terms of total dipole 
 ( green ) and quadrupole ( red ) transition intensities , summed over the 
 whole pre - edge .
the calculated transition energies were shifted by 
 50.17 ev , and intensities were multiplied by a factor of 3500 .
difference density maps for 1s  3d transitions 
 in 6 . at the six coordinate pseudo - octahedral limit ( figure 4 , 6 ) , the 
 pre - edge consists of two small features due to quadrupole transitions . 

the calculated and experimental spectra both show a 1 ev split 
 with transitions roughly grouped into the t2 g and eg set of acceptor orbitals .
this is due to destabilization 
 of the dz and dx  y orbitals upon interaction with the ligands ( figure 5 ) .
for the seven and eight coordinate complexes , there is 
 an increase in intensity concomitant with the presence of dipole character . 
 in the seven coordinate noncubane complex ( 7 ) , with an approximate c2v local symmetry at ca , p  d mixing is formally 
 allowed for all of the d orbitals except dxy , which has the lowest calculated intensity . on the other hand , 
 in the pseudo d2d
eight 
 coordinate complex 8 , p  d 
 mixing is lowest in the dz and dx  y orbitals , consistent with expectations from group 
 theory .
seven coordinate 7 species , 
 where the calcium center has a c3v local symmetry that favors p  d mixing in all the d 
 orbitals .
an in silico study of a hexa - aqua calcium complex , [ ca(h2o)6 ] , was carried out in order to further understand the 
 nature of p  d mixing responsible for intensity variations at 
 the ca k - pre - edge .
a c4v distortion was applied to a model octahedral [ ca(h2o)6 ] complex , with initial ca 
oh2 distances of 2.4  , by varying the distance of one of the ca 
both compression 
 and elongation of the ca  oh2 axial bond are expected 
 to enhance p 
d mixing , through symmetry lowering and thus increase 
 the pre - edge intensity .
when the calculated intensities for all transitions 
 in the pre - edge region are summed ( black line , figure 6 ) , one observes that the maximum pre - edge intensity occurs 
 upon the greatest compression or greatest elongation of the ca 
oh2 bond rather than 
 an elongation , which may be in part explained by covalency arguments , 
 as has been previously discussed .
however , 
 if one sums the ca p - character contributing to all acceptor molecular 
 orbitals over the pre - edge region ( green line , figure 6 ) , one finds that the percent p - character does not directly 
 correlate with the intensity increase , as one might naively expect . 

this can be rationalized in terms of the origins of the p - character . 

depending on the length of the ca  ligand bond , in principle , 
 both 3p and 4p orbitals can contribute , with 3p orbitals having larger 
 contributions at shorter distances .
it is important to recognize , 
 however , that the 3p and 4p orbitals will have different intrinsic 
 dipole moment integrals .
a 1s  3p dipole transition is expected 
 to be significantly more intense than a 1s  4p transition , 
 as the 3p orbitals have far more contracted radial functions , resulting 
 in a larger intrinsic transition dipole moment integral .
( top ) variation in pre - edge intensity ( black ) and p - character 
 ( green ) ; ( bottom ) variation in 3p ( gray ) and 4p ( cyan ) orbital energies .
it is then reasonable to discuss 
 the changes in intensity as coming from p  d mixing with the 
 p - character due to a combination of 3p and higher energy np orbitals .
based on this hypothetical series , it is predicted that 
 as the axial ca - aqua bond distance is compressed , the 3p and 4p orbitals 
 are closer in energy , and when the bond is elongated , their energies 
 become more separated , with the 3p orbitals being largely responsible 
 for the energy difference ( figure 6 , bottom ) . 
 as the 3p and higher
np orbitals become energetically 
 closer , more 3p mixing into these orbitals is possible .
consequently 
 as 3p mixing increases , less total p character is required for an 
 increase in pre - edge intensity ( figure 6 , top ) .
thus far , the current study 
 has shown a correlation between ca k - pre - edges and ca coordination 
 geometry by comparing structurally characterized complexes with their 
 experimental spectra .
it is important , however , to be able to deduce 
 structure  function relationships from experimental spectra 
 even in the absence of a priori structural information . 

compounds 7 and 7 together with its one electron reduced 
 analog 7 are good structural 
 references for the mn3o4ca  cubane - like  
 part of the core of the oec cluster .
these models offer an 
 ideal test set for exploring the correlations between spectroscopic 
 characteristics and geometric and electronic structures with relevance 
 to the heterometallic cluster of the oec .
cyclic voltammetry studies 
 show that the one electron mn centered reduction of 7 ( mn3o4ca ) to 7 ( mnmn2o4ca ) occurs as a quasireversible 
 wave at 890 mv versus ferrocene / ferrocenium in dmf .
however , while the structure 7 was characterized via crystallography , the 
 structure for 7 was not . 

therefore , a model was inferred for 7 based on the previously published analog , containing a sc 
 center instead of ca .
calculating the 
 xas spectrum , and comparing it to the experimentally determined 7 spectrum , supports this model s 
 validity and demonstrates the ability of ca k - edge xas to provide 
 structural information .
experimentally , the 7  cubane - like  complex has 
 a distinct pre - edge signature at the ca k - edge .
a one electron chemical 
 reduction leading to the 7 species results in a ca k - edge spectrum with a 16  
 3% lower pre - edge intensity ( table 1 , figure 7 ) . a similar change in intensity
is not observed 
 in the noncubane complexes 7 and 7 , which have similar 
 pre - edge intensities .
this implies that factors other than electronegativity 
 are the driving force for the changes in intensity .
complex 7 has a favorable geometry in 
 terms of p  d mixing ; however this is expected to change upon 
 one electron reduction .
indeed , a jahn  teller like distortion 
 ( jt ) is predicted to occur at the mn(iii ) center in the 7 model , resulting in subtle changes 
 in both ca bond distances and bond angles .
the result is a calculated 
 ca k - pre - edge with a 23% lower intensity in the reduced species 7 than in its oxidized counterpart 
 ( 7 ; figure 7 ) .
furthermore , the effect of total spin at the mn centers 
 on the calculated ca k - pre - edge was explored .
the xas spectra for 
 a high - spin ( s = 5 ) system , as well as mn(iii)mn(iv ) 
 ( s = 1 ) and mn(iv)mn(iv ) ( s = 2 ) antiferromagnetically coupled systems were calculated , and 
 the spin state was found not to have any significant impact on the 
 ca k - pre - edge ( figure 7 ) .
similarly , previous 
 work by pantazis et al . used an in silico truncated 
 model of 7 to investigate 
 the possible reduction of each of the mn(iv ) centers to mn(iii ) and 
 the resulting preferred jt distortion .
three lowest energy structural models for the reduced 7 were proposed , of which two were high - spin 
 ( s = 5 ) and one was of intermediate spin ( s = 2 ) .
calculation of ca k - edge spectra for these models 
 all resulted in a 25% decrease in intensity for the pre - edge , 
 independent of spin state , comparable to the result from using the 7 model presented herein ( supporting information figure s5 ) .
experimental 
 spectra ( top ) . calculated spectra for 7 and 7 ( bottom ) . 

a 50.17 ev energy shift 
 was applied , and intensities were multiplied by 3500 units .
a 
 series of six , seven , and eight coordinate calcium compounds were 
 systematically investigated using ca k - edge xas .
the spectra of six 
 coordinate complexes are distinguished by a low intensity pre - edge 
 split by 1 ev corresponding to excitations from the 1s orbital 
 to the t2 g and eg set of 3d orbitals .
seven 
 and eight coordinate species consist of a single well - resolved 1s 
  3d feature in the pre - edge with seven coordinate mn3o4ca  cubane - like  ( 7 , 7 ) clusters 
 having the most intense pre - edges .
a td - dft based protocol was 
 developed to calculate and analyze the ca k - pre - edges .
it was found 
 that the determining factor for the pre - edge is the calcium coordination 
 number and geometry .
the intensities of the transitions are dependent 
 on p  d mixing , which is least favored in six coordinate centro - symmetric 
 species and becomes more favored as the symmetry is lowered in seven 
 and eight coordinate complexes . at the c3v limit , which best characterizes the  cubane - like  
 complexes , the highest pre - edge intensities are observed .
the distribution 
 of the transition energies correlates with an energetically destabilizing 
 interaction with the ligands , where 3d orbitals interacting with the 
 ligands are higher in energy .
furthermore , we have investigated 
 the potential applicability of ca k - edge xas to probe changes in the 
 mn4o5ca cluster of the oec as it progresses 
 through the kok cycle .
such investigations could provide important 
 complementary information to existing mn xas data , while offering 
 a more streamlined analysis due to the presence of only one ca in 
 the cluster , as opposed to four mn ions .
it is already evident that 
 ca xas is sensitive to the ligand coordination environment directly 
 surrounding the ca center in terms of coordination number and ligand 
 geometry .
since calcium is proposed to play a direct role in the catalytic 
 cycle by binding one substrate water molecule , ca k - edge xas could 
 potentially make an important contribution in further understanding 
 the mechanism of water oxidation .
however , the question arises if 
 xas at the ca center of the oec is sensitive to  global changes  
 of the cluster .
using the oec closed cubane cluster mimics , 7 and its one electron reduced 
 analog 7 , the sensitivity 
 of ca xas to a one electron reduction at one of the three mn centers 
 was investigated .
it was found that reduction of one mn(iv ) to mn(iii ) 
 produces a jt distortion at the mn , which can be detected through 
 changes in the ca k - pre - edge xas data .
these studies thus establish 
 ca xas as a potentially powerful probe for the understanding of the 
 geometric and electronic structural changes that occur at the oec . 

for example , ca k - pre - edge xas could be used to differentiate between 
 the proposed oec s2 state conformations that consist of 
 the  open - cubane  form with an epr signal centered at g  2.0 and a  closed - cubane  form 
 with an epr signal centered at g  4.1 .
the  closed - cubane  s2 state model has 
 a  cubane - like  mn3o4ca structural 
 motif , analogous to the  cubane - like  model series presented 
 herein , connected to the remaining  dangling  mn by 
 a mn  o  mn oxo bridge .
 
 conformation on the other hand has a structurally distorted cubane 
 motif where the calcium forms an oxo bridge with the  dangling  

mn . the current xas study strongly suggests 
 that there should be distinct differences in the ca k - pre - edges of 
 the two models , and therefore the application of these methods to 
 the s states of the oec are the subject of ongoing research in our 
 laboratories .
 herein , ca k - edge x - ray absorption 
 spectroscopy ( xas ) is developed as a means to characterize the local 
 environment of calcium centers . 
 the spectra for six , seven , and eight 
 coordinate inorganic and molecular calcium complexes were analyzed 
 and determined to be primarily influenced by the coordination environment 
 and site symmetry at the calcium center . 
 the experimental results 
 are closely correlated to time - dependent density functional theory 
 ( td - dft ) calculations of the xas spectra . 
 the applicability of this 
 methodology to complex systems was investigated using structural mimics 
 of the oxygen - evolving complex ( oec ) of psii . 
 it was found that ca 
 k - edge xas is a sensitive probe for structural changes occurring in 
 the cubane heterometallic cluster due to mn oxidation . 
 future applications 
 to the oec are discussed . 
Introduction
Experimental Section
Results
Discussion
Conclusions