MECHANISMS FOR THE REACTIONS BETWEEN METHANE AND THE NEUTRAL TRANSITION-METAL ATOMS FROM YTTRIUM TO PALLADIUM

Calculations including electron correlations have been performed for the oxidative addition reactions between methane and the whole sequence of second row transition metal atoms from yttrium to palladium. The lowest barrier for the C-H insertion reaction is found for the rhodium atom. Palladium has the lowest methane elimination barrier. The barrier height is governed by two factors. In the reactant channel low repulsion favors a low barrier, and in the product channel strong bond formation is important. The atomic state with lowest repulsion toward methane is the d(n)+2 state and the strongest bonds are formed to the d(n+1) s state. For rhodium both these states are energetically low lying. Only palladium has a bound eta-2 precursor state on the ground state potential surface. Another interesting result is that the potential surface for the reaction between methane and the rhodium atom is remarkably similar to the potential surface for the reaction between methane and ClRhL2, which has been studied experimentally.