Understanding the reactivity of transition metal complexes involving multiple spin states

In coordination chemistry. many reactions involve several electronic states, in particular states of different spin. This phenomenon of 'Multiple-State Reactivity' has been recognized for some time, both for gas-phase reactions of 'bare' metal ions, and for transition metal complexes in solution. Until recently, however, much of the discussion of these systems has remained qualitative, because standard computational methods do not allow the location of the critical points for these processes, the Minimum Energy Crossing Points (MECPs) between states of different spin. Increased computational resources and new algorithms now enable MECPs to be located for large. realistic transition metal containing systems, yielding important new insight into the mechanisms of important reactions such as oxidative addition of C-H bonds to metal centers and ligand association/dissociation processes. Several examples will be presented for inorganic, organometallic and bioinorganic reactions. (C) 2002 Elsevier Science B.V. All rights reserved.