Spin-exchange processes in transition-metal complexes

Abstract

Rates of spin exchange (spin crossover) of some complexes of iron(II), iron(III) and cobalt(II) have been analysed by an intersecting-state model. Similarities in parameters with those observed in elementary gas-phase reactions suggests that the main energy barrier involves the internal reorganization of the coordination shell of the complex. Whilst the expected transition-state bond orders of one are observed with Fe^II and most Co^II complexes, bond orders greater than one are observed in bis(terpyridyl)cobalt(II) and in Fe^III complexes. This is explained in terms of involvement of non-bonding electrons in the transition state. In contrast to previous suggestions, nuclear tunnelling does not appear to be an important mechanism in spin exchange.