EPR of low spin d7 Fe+, Co2+, and Ni3+ cyanide complexes in NaCl and KCl

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Abstract
Low spin d7 complexes of type [M(CN)4⋅XY]n−, where M is Fe+, Co2+, or Ni3+, the axial ligands X, Y being Cl−, CN−, or NC−, have been produced and stabilized in NaCl and/or KCl crystals doped with diamagnetic Fe(CN)64− (d6), Co(CN)63− (d6) or Ni(CN)42− (d8), by a suitable combination of irradiation and heat treatment. These complexes were identified by EPR on the basis of the g factors which are characteristic of unpaired electrons in the dz2 orbital pointing towards the ligands X and Y, and the super hyperfine structure from the nuclei of these ligands observed in the EPR spectra. Systematic variations are noted in the EPR parameters, which are discussed in terms of changes in the nature of binding in these complexes in relation to the charge state of the metal ion, the host lattice spacings, and the nature of the axial ligands. In addition to showing that CN− and NC− are much stronger ligands than Cl−, the study pointed out the importance of back bonding in CN−‐ and NC−‐type ligands.