Fluxional processes and structural characterization of μ₃-η²,η²,η²-C ₆₀ triosmium cluster complexes, Os₃(CO)_9-n(PMe₃)_n(μ ₃-η²,η²,η²-C ₆₀) (n=1, 2, 3)

The title complex, Os₃(CO)₆(PMe₃)₃(μ ₃-η²,η²,η²-C ₆₀) (3), has been prepared by decarbonylation of Os₃(CO)₉(μ₃-η²,η ²,η²-C₆₀) with three equivalents of Me₃NO in the presence of excess PMe₃ ligand. The solid-state structures of Os₃(CO)₇(PMe₃)₂(μ ₃-η²,η²,η²-C ₆₀) (2) and 3 have been determined by single-crystal X-ray diffraction studies. Compound 2 has two inequivalent equatorial phosphine ligands on adjacent osmium atoms and compound 3, with a distorted C₃ symmetry, has one equivalent equatorial phosphine ligand on each osmium center. The fluxional processes of Os₃(CO)₈(PMe₃)(μ₃-η ²,η²,η²-C₆₀) (1), 2, and 3 have been examined by variable-temperature ¹³C- and ³¹P-NMR studies. Only one isomer exists in solution and a restricted ligand rotation on each phosphine-substituted osmium center appears to occur for all three compounds, 1-3. Activation barriers for the carbonyl exchange process increase with increasing phosphine substitution, presumably, due to the steric effect of the phosphine ligands.