The title complex, Os-3(CO)(6)(PMe3)(3)(mu(3)-eta(2),eta(2),eta(2)-C-60) (3), has been prepared by decarbonylation of Os-3(CO)(9)(mu(3)-eta(2),eta(2),eta(2)-C-60) with three equivalents of Me3NO in the presence of excess PMe3 ligand. The solid-state structures of Os-3(CO)(7)(PMe3)(2)(mu(3)-eta(2),eta(2),eta(2)-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 C-3 symmetry, has one equivalent equatorial phosphine ligand on each osmium center. The fluxional processes of Os-3(CO)(8)(PMe3)(mu(3)-eta(2),eta(2),eta(2)-C-60) (1), 2 and 3 have been examined by variable-temperature C-13- and P-31-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. (C) 2000 Elsevier Science S.A. All rights reserved.