M−P versus MM Bonds as Protonation Sites in the Organophosphide-Bridged Complexes [M₂Cp₂(μ-PR₂)(μ-PR‘₂)(CO)₂], (M = Mo, W; R, R‘ = Ph, Et, Cy)^†

The unsaturated complexes [W2Cp2(μ-PR2)(μ-PR‘2)(CO)2] (Cp = η5-C5H5; R = R‘ = Ph, Et; R = Et, R‘ = Ph) react with HBF4·OEt2 at 243 K in dichloromethane solution to give the corresponding complexes [W2Cp2(H)(μ-PR2)(μ-PR‘2)(CO)2]BF4, which contain a terminal hydride ligand. The latter rearrange at room temperature to give [W2Cp2(μ-H)(μ-PR2)(μ-PR‘2)(CO)2]BF4, which display a bridging hydride and carbonyl ligands arranged parallel to each other (W−W = 2.7589(8) Å when R = R‘ = Ph). This explains why the removal of a proton from the latter gives first the unstable isomer cis-[W2Cp2(μ-PPh2)2(CO)2]. The molybdenum complex [Mo2Cp2(μ-PPh2)2(CO)2] behaves similarly, and thus the thermally unstable new complexes [Mo2Cp2(H)(μ-PPh2)2(CO)2]BF4 and cis-[Mo2Cp2(μ-PPh2)2(CO)2] could be characterized. In contrast, related dimolybdenum complexes having electron-rich phosphide ligands behave differently. Thus, the complexes [Mo2Cp2(μ-PR2)2(CO)2] (R = Cy, Et) react with HBF4·OEt2 to give first the agostic type phosphine-bridged complexes [Mo2Cp2(μ-PR2)(μ-κ2-HPR2)(CO)2]BF4 (Mo−Mo = 2.748(4) Å for R = Cy). These complexes experience intramolecular exchange of the agostic H atom between the two inequivalent P positions and at room-temperature reach a proton-catalyzed equilibrium with their hydride-bridged tautomers [ratio agostic/hydride = 10 (R = Cy), 30 (R = Et)]. The mixed-phosphide complex [Mo2Cp2(μ-PCy2)(μ-PPh2)(CO)2] behaves similarly, except that protonation now occurs specifically at the dicyclohexylphosphide ligand [ratio agostic/hydride = 0.5]. The reaction of the agostic complex [Mo2Cp2(μ-PCy2)(μ-κ2-HPCy2)(CO)2]BF4 with CNtBu gave mono- or disubstituted hydride derivatives [Mo2Cp2(μ-H)(μ-PCy2)2(CO)2-x(CNtBu)x]BF4 (Mo−Mo = 2.7901(7) Å for x = 1). The photochemical removal of a CO ligand from the agostic complex also gives a hydride derivative, the triply bonded complex [Mo2Cp2(H)(μ-PCy2)2(CO)]BF4 (Mo−Mo = 2.537(2) Å). Protonation of [Mo2Cp2(μ-PCy2)2(μ-CO)] gives the hydroxycarbyne derivative [Mo2Cp2(μ-COH)(μ-PCy2)2]BF4, which does not transform into its hydride isomer.