Concerning Models for Substitution Reactions of Vibrationally Equilibrated Transition Metal Excited States

Abstract

Models that have been proposed for the substitution reactions of transition metal excited states generally involve high energy electronic configurations of intermediate species and such pathways may not be viable for vibrationally equilibrated excited states. Nevertheless, simple angular overlap considerations indicate that several dissociative and associative intermediate species are likely to have energies comparable to, or less than these vibrationally equilibrated ligand field excited states. A model is proposed for estimating the intrinsic nuclear barriers to formation of trigonal bipyramidal Cr(III) intermediates and there does seem to be some basis for stereoselectivity. Among possible seven-coordinate intermediates, the capped trigonal prismatic geometry seems most likely to be consistent with observed chromium photosubstitution chemistry.