Four-Electron-Donor Hemilabile η³-PPh₃ Ligand that Binds through a CC Bond Rather than an Agostic C−H Interaction, and Displacement of the CC by Methyl Iodide or Water

Hydride transfer from Cp(CO)2(PPh3)MoH to Ph3C+BAr′4− [Ar′ = 3,5-bis(trifluoromethyl)phenyl] produces [Cp(CO)2(η3-PPh3)Mo]+[BAr′4]−. Spectroscopic and crystallographic data indicate that one CC of a Ph ring is weakly bound to the Mo, so that the PPh3 ligand is a four-electron-donor ligand. Computations (DFT/B3LYP and MP2 on [Cp(CO)2(η3-PPh3)Mo]+ and [Cp(CO)2(η3-PH2Ph)Mo]+, and DFT/B3LYP on [Cp(CO)2(η3-PHtBuPh)Mo]+ and [Cp(CO)2(η3-PH2Ph)Nb]) provide further information on the bonding and on the preference for bonding of the metal to the CC bond rather than an agostic C−H interaction found in many related complexes. The hemilabile CC bond is readily displaced by CH3I or H2O, and crystal structures are reported for [Cp(CO)2(PPh3)Mo(ICH3)]+ and [Cp(CO)2(PPh3)Mo(OH2)]+. The equilibrium constant for [Cp(CO)2(η3-PPh3)Mo]+ + ICH3 to give [Cp(CO)2(PPh3)Mo(ICH3)]+ is Keq = 5.2 × 102 M−1 in CD2Cl2 at 22 °C.