Reactions of the unsaturated complex [Mo(2)(eta(5)-C(5)H(5))(2)(mu-PEt(2))(2)(CO)(2)] with [Au(PR(3))](+) cations: kinetic preference of the Mo-P bonds as the site of attack of the gold(I) electrophile.

The title complex reacts with [Au(PR(3))](+) cations (PF(6)(-) salts, R = p-tol, Me) in dichloromethane solutions to give first the corresponding agostic-like products [AuMo(2)Cp(2)(mu-PEt(2))(mu(3)-PEt(2))(CO)(2)(PR(3))]PF(6), which then partially rearrange to reach an equilibrium with the hydride-like isomers [AuMo(2)Cp(2)(mu-PEt(2))(2)(CO)(2)(PR(3))]PF(6), the latter being characterized through an X-ray study (R = p-tol, Mo-Mo = 2.8244(2) A). These unsaturated complexes react smoothly with CO (1 atm) to give the corresponding electron-precise derivatives [AuMo(2)Cp(2)(mu-PEt(2))(2)(CO)(3)(PR(3))](+) (Mo-Mo = 3.0438(6) A when R = Me), this implying the rearrangement of the mu(3)-PEt(2) ligand to a more common mu(2)-coordination mode. Density functional theory (DFT) calculations on the dimolybdenum complexes [Mo(2)Cp(2)(mu-PR(2))(2)(CO)(2)] (R = Cy, Et) reveal the presence of a framework M-P bonding orbital high in energy and with the right shape to act as a donor to H(+) and [Au(PR(3))](+) cations, thus explaining the formation of agostic and agostic-like products respectively in these reactions. The unusually high energy of this donor orbital can be related to the close approach of the metal centers in these unsaturated molecules. The carbyne complex [Mo(2)Cp(2)(mu-COMe)(mu-PCy(2))(CO)(2)] reacts with [Au{P(p-tol)(3)}](+) to give the tricarbonyl [AuMo(2)Cp(2)(mu-COMe)(mu-PCy(2))(CO)(3){P(p-tol)(3)}](+) (Mo-Mo = 2.986(1) A), a process most likely initiated by the binding of the gold cation to one of the Mo-P bonds in the carbyne complex.