Bridging Methylene and Methyl Complexes of Rhodium/Osmium:  Influence of the Ancillary Ligands on the Methyl Binding Mode

Reaction of [RhOs(CO)<sub>4</sub>(μ-CH<sub>2</sub>)(dppm)<sub>2</sub>][CF<sub>3</sub>SO<sub>3</sub>] (<b>1</b>) with PMe<sub>3</sub>, PEt<sub>3</sub>, PMePh<sub>2</sub>, and P(OMe)<sub>3</sub> occurs by substitution of a carbonyl to yield the products [RhOsL(CO)<sub>3</sub>(μ-CH<sub>2</sub>)(dppm)<sub>2</sub>][CF<sub>3</sub>SO<sub>3</sub>], in which the phosphine or phosphite ligand (L) is bound to Rh adjacent to the bridging methylene group. Identical products are obtained by the reaction of these ligands with the related tricarbonyl species [RhOs(CO)<sub>3</sub>(μ-CH<sub>2</sub>)(dppm)<sub>2</sub>][CF<sub>3</sub>SO<sub>3</sub>] (<b>2</b>). The P(OMe)<sub>3</sub> adduct slowly isomerizes to a product in which the Rh-bound carbonyl and phosphite ligands have exchanged positions, such that the phosphite ligand is opposite the Rh−CH<sub>2</sub> bond. Although P(OPh)<sub>3</sub> does not react with <b>1</b>, it does react with <b>2</b> to yield a product analogous to the second P(OMe)<sub>3</sub> isomer. Protonation of the three phosphine adducts generates methyl complexes in which this group is σ-bound to Os but has a bridged agostic interaction with Rh. In these three products the phosphine ligands are coordinated to Rh. The PMe<sub>3</sub> compound isomerizes to the thermodynamic product in which migration of the PMe<sub>3</sub> group to Os is accompanied by migration of a carbonyl to Rh, whereas the products containing the PEt<sub>3</sub> and PMePh<sub>2</sub> ligands dissociate these phosphines, generating [RhOs(CH<sub>3</sub>)(OSO<sub>2</sub>CF<sub>3</sub>)(CO)<sub>3</sub>(dppm)<sub>2</sub>][CF<sub>3</sub>SO<sub>3</sub>], in which the methyl group is terminally bound to Rh. Protonation of the first P(OMe)<sub>3</sub> isomer yields a methyl-bridged product analogous to those noted above for the phosphine adducts. This product slowly isomerizes to one having the methyl ligand terminally bound to Rh and the phosphite ligand bound to Os. Protonation of the second P(OMe)<sub>3</sub> isomer yields two products; the first is that noted above having the methyl group on Rh, while the second has the opposite arrangement of methyl and phosphite ligands, with the methyl group on Os and the phosphite on Rh. This latter product slowly isomerizes to the former. Protonation of the methylene-bridged triphenyl phosphite adduct yields a product in which the phosphite ligand is coordinated to Rh while the methyl group is terminally bound to Os. The X-ray crystal structures of a number of species are reported, and a rationalization for the products observed and the rearrangements occurring is presented.