OXIDATIVE ADDITION OF METHANE TO RHODIUM(I) AND RUTHENIUM(II) COMPLEXES OF PARTICULAR INTEREST

The oxidative addition reaction of methane to some model Rh(I) and Ru(II) complexes has been studied in order to suggest directions for new experiments. The Rh(I) model complexes are of the general form RhXL where X is either a hydrogen or a chlorine and where L stands for a lone-pair ligand. Lone-pair ligands with a varying amount of pi-bonding were investigated. It is suggested that a combination of hydride and lone-pair ligands with a minimum of pi-bonding should be an optimal combination for the methane reaction. Of the systems studied here, RhH(NH3) thus gives the most stable products and the lowest transition state energy. The trends of the results are analyzed and compared to the results for the corresponding Ru(I) complexes. In a previous study it was found that RuH2 has a very low barrier for the methane reaction. In order to approach more realistic systems the reactant RuH2 is modified in various ways in the present study. First, the hydrides are substituted by methyl groups and halides. Secondly, lone-pair ligands are added. The conclusions are that it should be advantageous for the methane reaction to have methyl (or more general R groups) and lone-pair ligands with as small a covalent bonding as possible. To make a Ru(II) complex activate methane, which has not yet been done experimentally, halide ligands and lone-pair ligands with strong pi-bonding should be avoided.