Mixed-metal cluster chemistry VII: Some phosphine and alkyne chemistry of Cp₂Mo₂Ir₂(CO)₁₀; X-ray crystal structures of Cp₂Mo₂Ir₂ (μ-CO)₃(CO)₆(PMe₃)

Reactions of Cp₂Mo₂Ir₂(CO)₁₀ (1) with stoichiometric amounts of phosphines afforded the substitution products Cp₂Mo₂Ir₂(CO)_10-xL_x(L = PPh₃, x = 1 (5), 2 (6); L = PMe₃, x = 1 (7), 2 (8)), in fair to excellent yields (36-78%), shown by low temperature ³¹P NMR to consist of mixtures of interconverting isomers. An X-ray structural study of Cp₂Mo₂Ir₂(μ-CO)₃(CO)₆(PMe₃) (7a), one isomer of 7, revealed that the PMe₃ ligand occupies the electronically-preferred axial site (with respect to the plane of the bridging carbonyls). Geometries of all other isomers of 5-8 have been postulated from a combination of NMR data and results from the analogous Cp₂W₂Ir₂(CO)₁₀ system. Reactions of 1 with a range of alkynes afforded Cp₂Mo₂Ir₂(μ₄-η²-RC₂R′)(CO)₈ (R = R′ = Ph (9), H (14); R = H, R′ = Ph (10), 4-C₆H₄NO₂ (11), 4,4′-C₆H₄C≡CC₆H₄NO₂ (12), CH₂Br (13)) in fair to good yields (34-80%). An X-ray structural study of 10 revealed that the alkynes have formally inserted into the Mo-Mo bond of 1, to afford clusters with a pseudooctahedral core geometry. Qualitative analysis of reaction rates for the syntheses of 9-14 revealed the trends acetylene > terminal alkyne > internal alkyne and 4-nitrophenylacetylene > phenylacetylene, assigned to a combination of electronic and steric effects.