C-H and H-H Activation in Transition Metal Complexes and on Surfaces.

Abstract : The breaking of the H-H bond in H2 and a C-H bond in CH4 on both discrete transition metal complexes and on Ni and Ti surfaces is studied, and the essential continuity and similarity of the physical and chemical processes in two cases is demonstrated. We begin with an orbital analysis of oxidative addition, delineating four basic interactions: H-H or C-H sigma M electron transfer the reverse M sigma* transfer (both weakening the sigma bond, forming the M-H bond), a repulsive interaction between sigma and metal filled orbitals, and a rearrangement of electron density at the metal. The molecular cases analyzed in detail are d6 ML5, d8 ML4 and CRhoM'L. Coordinative unsaturation is necessary, and consequently sigma M electron transfer dominates the early stages of the reaction. Steric effects are important for CH4 reaction. Activation in angular ML4 or CpM L is achieved through a destabilized yz MO, and d10 ML3, ML2 candidates for activation are described. For our study of the surface we develop tools such as projections of the density of states and crystal orbital overlap populations - the extended structure analogues of a population analysis. These allow a clear understanding of what happens when an H2 or a CH4 molecule approaches a surface. Because of the higher energy of the occupied metal orbitals on the surface the M sigma* interaction leads the reaction. There are great similarities and some differences between the activation acts in a discrete complex and on a surface. (Author)