Intermolecular Methyl Group Exchange and Reversible P–Me Bond Cleavage at Cobalt(III) Dimethyl Halide Species

The cobalt­(III) dimethyl halide complexes cis,mer-(PMe₃)₃Co­(CH₃)₂X (X = Cl, I) were found to undergo a degenerate cobalt-to-cobalt transfer of the methyl ligands during isotopic labeling experiments. Extensive mechanistic studies exclude radical, methyl iodide elimination, and disproportionation/comproportionation pathways for exchange of the methyl groups between metals. A related cobalt­(III) dimethyl complex supported by the tridentate phosphine ligand MeP­(CH₂CH₂PMe₂)₂ showed dramatically slower methyl ligand transfer, indicative of a mechanism for intermetallic exchange with a requisite phosphine dissociation. Crossover experiments between cobalt­(III) dimethyl halide complexes supported by PMe₃ and MeP­(CH₂CH₂PMe₂)₂ are consistent with a dicobalt transition structure in which only one cobalt center requires phosphine dissociation prior to methyl transfer. An additional methyl group scrambling process between cis,mer-(PMe₃)₃Co­(CH₃)₂I and free PMe₃ was also identified during the investigation and originates from reversible P–CH₃ bond cleavage.