section_path: Chemistry and compounds/Coordination compounds
section_name: Coordination compounds
paragraph_text: Coordination compounds
  
  Due to its electronic structure, iron has a very large coordination and organometallic chemistry.
  
  Many coordination compounds of iron are known. A typical six-coordinate anion is hexachloroferrate(III),, found in the mixed salt tetrakis(methylammonium) hexachloroferrate(III) chloride. Complexes with multiple bidentate ligands have geometric isomers. For example, the trans - chlorohydridobis(bis-1,2-(diphenylphosphino)ethane)iron(II) complex is used as a starting material for compounds with the Fe(dppe) moiety. The ferrioxalate ion with three oxalate ligands displays helical chirality with its two non-superposable geometries labelled Λ (lambda) for the left-handed screw axis and Δ (delta) for the right-handed screw axis, in line with IUPAC conventions. Potassium ferrioxalate is used in chemical actinometry and along with its sodium salt undergoes photoreduction applied in old-style photographic processes. The dihydrate of iron(II) oxalate has a polymeric structure with co-planar oxalate ions bridging between iron centres with the water of crystallisation located forming the caps of each octahedron, as illustrated below.
  
  Iron(III) complexes are quite similar to those of chromium (III) with the exception of iron(III)'s preference for O -donor instead of N -donor ligands. The latter tend to be rather more unstable than iron(II) complexes and often dissociate in water. Many Fe–O complexes show intense colors and are used as tests for phenols or enols. For example, in the ferric chloride test, used to determine the presence of phenols, iron(III) chloride reacts with a phenol to form a deep violet complex:
  
  Among the halide and pseudohalide complexes, fluoro complexes of iron(III) are the most stable, with the colorless being the most stable in aqueous solution. Chloro complexes are less stable and favor tetrahedral coordination as in; and are reduced easily to iron(II). Thiocyanate is a common test for the presence of iron(III) as it forms the blood-red. Like manganese(II), most iron(III) complexes are high-spin, the exceptions being those with ligands that are high in the spectrochemical series such as cyanide. An example of a low-spin iron(III) complex is. Iron shows a great variety of electronic spin states, including every possible spin quantum number value for a d-block element from 0 (diamagnetic) to 5 ⁄ 2 (5 unpaired electrons). This value is always half the number of unpaired electrons. Complexes with zero to two unpaired electrons are considered low-spin and those with four or five are considered high-spin.
  
  Iron(II) complexes are less stable than iron(III) complexes but the preference for O -donor ligands is less marked, so that for example is known while is not. They have a tendency to be oxidized to iron(III) but this can be moderated by low pH and the specific ligands used.