Solvent dependence of the circular dichroism spectra of some cis-dianionobis(ethylenediamene)cobalt(III) complexes.
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The circular dichroism spectra of some <I>cis</I>-bis(ethylenediamine)cobalt(III) complexes have been measured in several solvents. Among these complexes, the diisothiocyanato complex was found to exhibit a marked solvent dependence; the dominant circular dichroism band in the lowest energy spin-allowed transition region changed its sign upon going from, <I>e.g.</I>, water, dimethyl sulfoxide, methanol, or acetone to pyridine or its analogues. Coupled with the proton NMR spectra and a similar solvent dependence of the circular dichroism spectra of nitroisothiocyanatobis(ethylenediamine)cobalt(III) and diisothiocyanatobis(ethylenediamine)chromium(III) complexes, we ascribed the origin of the sign inversion of the diisothiocyanato complex to the stereoselective hydrogen bonding of <I>cis</I> N–H<I><SUB>B</SUB></I> with pyridine-like molecules, where <I>cis</I> N–H<SUB>B</SUB> is the hydrogen bonded to the nitrogen atom <I>cis</I> to NCS<SUP>−</SUP> and directed off the molecular twofold axis.
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