Stereochemistry and Steric Effects in the Photosubstitutions of Chromium(III) Complexes, and Their Utility as Probes of Mechanism

Abstract

A feature of chromium(III) photoaquation reactions which is unparalleled in the photochemistry of any other transition metal ions is the high degree of stereo-chemical change that occurs concomitant with most of these photosubstitutions. This stands in stark contrast to the nature of the ground state thermal substitutions, which, in aqueous solution, are amongst the most stereoretentive of transition metal kinetic processes. Some of the evidence behind these claims is discussed, some theoretical rationalizations presented, some problems of experimental verification and interpretation outlined and some ideas for further study are offered. The significance of these results for the mechanistic pathways of Cr(III) photochemistry is discussed, related to other photophysical results and a working hypothesis to guide future work is offered.

Key Words:

• stereochemical change
• photosubstitution
• photoaquation
• photoanation
• associative
• dissociative
• excited state
• intersystem crossing
• chromium
• complex