Mixed-metal cluster chemistry. 13. Syntheses and isomer distribution studies of Cp2W2Ir2(mu-Co)(3)(CO)(7-n)(PR3)(n) (n = 1, 2; R = Ph, Me): X-ray crystal structure of Cp2W2Ir2(mu-CO)(3)(CO)(6)(PPh3)

Reactions of Cp2W2Ir2(CO)(10) (1) with n equivalents of PPh3 and PMe3 (n = 1, 2) proceed in dichloromethane at room temperature to afford the clusters Cp2W2Ir2(mu-CO)(3)(CO)(7-n)(PPh3)(n) (n = 1(2), 2 (3)) and Cp2W2Ir2(mu-CO) (3)(CO)(7-n)(PMe3)(n) (n = 1 (4), 2(5)), respectively, in reasonable yields (38-63%). These products exhibit ligand fluxionality in solution, resolvab le at low temperature into the constituent interconverting isomers. A struc tural study of one isomer of 2, namely 2a, reveals that the three edges-of a WIr2 face of the tetrahedral core are spanned by bridging carbonyls and t hat the iridium-bound PPh3 ligates axially and the tungsten-bound cyclopent adienyl ligands coordinate epically and axially with respect to the plane o f bridging carbonyls. Information from this crystal structure and C-13 and P-31 NMR spectral data have been employed to assign coordination geometries for the isomers of 2-5. NMR studies included development and use of 2D tri ple-resonance experiments with carbonyl cluster complexes; these H-1-P-31-C -13 correlation experiments, designed to elucidate P-C coupling networks in volving phosphorus and carbonyl ligands (a "HPCO experiment"), benefit from the sensitivity enhancement gained from using proton detection. All the is omers of clusters 2-5 contain a carbonyl-bridged WIr2 basal plane and an ap ical tungsten atom; ligands can be approximately coplanar (radial) to the b asal plane or below the plane (axial). The configuration of 2a (axial phosp hine, axial Cp, apical Cp), as assigned by NMR spectroscopy, is consistent with the structural determination. A second configuration (2b, 4b: axial ph osphine, axial Cp, apical Cp) is the only one identified (within the temper ature range 183-293 K) for the PMe3-substituted cluster 4. Two isomers, bot h with radial phosphine, axial phosphine, axial Cp, apical Cp configuration s, have been identified for 5, namely 5a and 5b; it is likely that isomers of 3 adopt the same configurations, The WIr2-bridged form is seen exclusive ly across the isomers, indicating that the presence of two electropositive tungsten atoms may be polarizing the electron distribution toward the iridi ums, although steric factors may also be involved.