Optical rotatory power of co-ordination compounds. Part X. The contributions of configuration and ring conformation to the circular dichroism and molecular rotation of platinum(IV) diamine complexes

Abstract

Circular dichroism and rotatory dispersion measurements are reported for a series of tris-diamine–platinum(IV) complexes containing ethylenediamine and (–)-propylenediamine, or (+)- and (–)-propylenediamine. It is found from the dispersion–absorption relations that the molecular rotation of these complexes in the visible region derives largely from a strong circular dichroism band due to a charge-transfer transition in the far-ultraviolet region, and not from the weak near-ultraviolet circular dichroism, which is of opposite sign, due to the d→d transitions of platinum(IV). The sign and the magnitude of the charge-transfer circular dichroism are determined by additive configurational and conformational contributions, which are reflected in the optical rotation at longer wavelengths. The weak circular dichroism, due to the d→d electronic transitions, is governed mainly by the stereochemical configuration of the chelate diamine rings round the platinum(IV) ion.