The unsaturated complexes [W2Cp2(mu-PR2)(mu-PR'(2))(CO)(2)] (Cp = eta(5)-C5H5; R = R' = Ph, Et; R = Et, R' = Ph) react with HBF4 center dot OEt2 at 243 K in dichloromethane solution to give the corresponding complexes [W2Cp2(H)(mu-PR2)(mu-PR'(2))(CO)(2)] BF4, which contain a terminal hydride ligand. The latter rearrange at room temperature to give [W2Cp2(mu-H)(mu-PR2)(mu-PR'(2))(CO)(2)]BF4, which display a bridging hydride and carbonyl ligands arranged parallel to each other (W-W = 2.7589(8) angstrom when R = R' = Ph). This explains why the removal of a proton from the latter gives first the unstable isomer cis- [W2Cp2(mu-PPh2)(2)(CO)(2)]. The molybdenum complex [Mo2Cp2(mu-PPh2)(2)(CO)(2)] behaves similarly, and thus the thermally unstable new complexes [Mo2Cp2(H)(mu-PPh2)(2)(CO)(2)]BF4 and cis-[Mo2Cp2(mu-PPh2)(2)(CO)(2)] could be characterized. In contrast, related dimolybdenum complexes having electron- rich phosphide ligands behave differently. Thus, the complexes [Mo2Cp2(mu-PR2)(2)(CO)(2)] (R = Cy, Et) react with HBF4 center dot OEt2 to give first the agostic type phosphine- bridged complexes [Mo2Cp2(mu-PR2)(mu-kappa(2)-HPR2)(CO)(2)]BF4 (Mo-Mo = 2.748(4) angstrom 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(mu-PCy2)(mu-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(mu-PCy2)(mu-kappa(2)-HPCy2)(CO)(2)]BF4 with CNtBu gave mono- or d isubstituted hydride derivatives [Mo2Cp2(mu-H)(mu-PCy2)(2)(CO)(2-x)((CNBu)-Bu-t)(x)]BF4 (Mo-Mo = 2.7901(7) angstrom 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)(mu-PCy2)(2)(CO)]BF4 (Mo-Mo = 2.537(2) angstrom). Protonation of [Mo2Cp2(mu-PCy2)(2)(mu-CO)] gives the hydroxycarbyne derivative [Mo2Cp2(mu-COH)(mu-PCy2)(2)]BF4, which does not transform into its hydride isomer.