Moessbauer emission spectroscopic study of some spin crossover cobalt(II) complexes

The Moessbauer emission spectra of [57Co(terpy)2]X2 (X = Cl and ClO4), [57Co(snn)2](ClO4)2, and [57Co(snnme)2](ClO4)2 (terpy = 2,2' : 6'2''-terpyridine, snn = N-(2-pyridylmethylene)-2-(methylthio)aniline, and snnme = N-(6-methyl-2-pyridylmethylene)-2-(methylthio)aniline) were examined. These cobalt complexes show thermally induced spin crossover behavior and the temperature dependence of their magnetic moment varies with the counter anion, although the corresponding iron complexes are in a low-spin state. The high-spin iron-(II) species ascribed to a metastable form are observed in the emission spectra of [57Co(terpy)2](ClO4)2.2H2O and [57Co(snnme)2](ClO4)2, and their spectral areas increase with decreasing source temperature. The effective vibrating mass for the iron complex observed in the emission spectra of [57Co(snnme)2](ClO4)2 was calculated from the temperature dependence of the isomer shift value, and was used to distinguish whether the iron complex formed by EC-decay of a cobalt-57 atom is stable or metastable. (author)