An analysis of the electronic spectra of bis-amine cobalt halides: a novel effect of steric hindrance
The electronic absorption spectra (5000–30,000 cm.–1)(in chloroform) of thirty-three complexes of general formula (Amine)2CoX2 where the amine is quinoline, isoquinoline, 2,6-dimethylpyrazine, pyridine, or a 2-, 3-, or 4-substituted pyridine, and X is Cl, Br, I, NCS, NCO, or NCSe, are reported. The near-infrared absorption band is split into three well defined components, whose overall width is some 1500–2000 cm.–1 greater than is observed in regular tetrahedral cobalt derivatives (such as CoX42–). These three components are assigned to the 4B2â†�4A2, 4A2â†�4A2, and 4B1â†�4A2 transitions in C2v symmetry. With the exception of one band the absorption spectra are virtually independent of the amine throughout the entire region studied. The supposed 4B2â†�4A2 component of the near-infrared absorption decreases in energy by as much as 1000 cm.–1 when the complex contains an α-substituted pyridine. A simple group-theoretical argument consistent with the occurrence of inter-ligand repulsions is suggested to explain this phenomenon. Mean values of 10Dq and B are computed and discussed.