Studies of spin-pairing in some six co-ordinate iron (II) complexes

Hexa-co-ordinated complexes of formula MQ3X2, M=Fe(II), Co(II) and Ni(II), Q=2-(2'-pyridyl)-benzimidazole, 2-(2'-pyridyl)-imidazole and 2-(2'-pyridyl)-im'dazoline have been prepared and studied by the following techniques: magnetic susceptibility measurements (80-350 K), Mossbauer spectroscopy (4-300 K), electronic spectroscopy, infrared spectroscopy (especially the low energy region) and X-ray powder diffraction patterns. Spin-transitions were found to occur in the Fe(II) complexes and their properties have been shown to depend on the ligand, on the anion and on the method of preparation of samples. The models for the quantitative interpretation of the magnetic properties of Fe(II) complexes near the "cross-over" have been reviewed and discussed. They have been shown to be different versions of a model based on van Vleck theory in which the separation between high- and low-spin term energies changes empirically with the temperature, It is suggested that the effects of mixing of high- low- and intermediate-spin terms by spin-orbit coupling under distortions should be included in the model in order to explain the diversity of experimental behaviour; and that in the case of most Fe(II) spin transitions the so called low-spin state may be an intermediate spin state.