HALIDE LIGAND EFFECTS ON THE OXIDATIVE ADDITION-REACTION OF METHANE AND HYDROGEN TO 2ND ROW TRANSITION-METAL COMPLEXES

The oxidative addition reactions of methane and hydrogen to second row transition metal halide complexes have been studied and compared to previous results for hydride complexes. Both halide and hydride ligands are found to have pronounced effects on the oxidative addition, and these effects are very similar for most systems. This means that the gross features of the effects of both halide and hydride ligands can be explained by promotion and exchange loss energies. The two main conditions for a low barrier for the oxidative addition are also the same for halide and hydride complexes. There must be a low-lying state with a different spin than the ground state, and the low-spin state of these must have a low population of 5s, 5p electrons. In addition to these effects there is a tendency for the halides to destabilize the complexes to the right. This effect can be understood from the higher promotion energies required for cations to reach the bonding s-state than for the neutral atoms. Another important effect in this context is that the halides tend to have higher spin than the hydrides for the systems to the right, which in some cases increases the exchange loss energies for the halides.