DFT Investigation of the Mechanism of Phosphine-Thioether Isomerization in the Triosmium Cluster Os-3(CO)(10)(Ph2PCH2CH2SMe): Migratory Preference for the Formation of an Edge-Bridged Thioether versus a Phosphine Moiety

The rearrangement of the phosphine-thioether ligand in 1,2-(P-eq,S-eq)-Os-3(CO)(10)(Ph2PCH2CH2SMe) to 1,1-(P-eq,S-ax)-Os-3(CO)(10)(Ph2PCH2CH2SMe) was investigated by electronic structure calculations. The chelated isomer lies 2.5 kcal/mol lower in energy than its bridged counterpart, and the barrier computed for the mechanism is in agreement with the results from our earlier experimental study. Phosphine-thioether isomerization occurs via three distinct steps that involve the migration of the CO and SMe groups in a plane that is perpendicular to the trimetallic core. One of the intermediates on the reaction surface corresponds to the 50e cluster Os-3(CO)(9)(mu-CO)(mu-Ph2PCH2CH2SMe), whose edge-bridging thioether moiety functions as a 4e donor ligand. Alternative mechanisms involving ligand dissociation/association and merry-go-round sequences are energetically prohibitive.