The unsaturated complexes [W2Cp2(mu-PR2)(mu-PR'2)(CO)2] (Cp = eta5-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)(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) A 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.OEt2 to give first the agostic type phosphine-bridged complexes [Mo2Cp2(mu-PR2)(mu-kappa2-HPR2)(CO)2]BF4 (Mo-Mo = 2.748(4) A 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-kappa2-HPCy2)(CO)2]BF4 with CN(t)Bu gave mono- or disubstituted hydride derivatives [Mo2Cp2(mu-H)(mu-PCy2)2(CO)2-x(CNtBu)x]BF4 (Mo-Mo = 2.7901(7) A 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) A). 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.

The 30-electron benzylidyne complex [Mo2Cp2(μ-CPh)(μ-PCy2)(μ-CO)] (Cp = η5-C5H5) could be conveniently prepared upon photolysis of the benzyl-bridged complex [Mo2Cp2(μ-CH2Ph)(μ-PCy2)(CO)2]. It reacted with CO to give the ketenyl complex [Mo2Cp2{μ-C(Ph)CO}(μ-PCy2)(CO)2] (2.6101(2) Å), which in turn could be selectively decarbonylated at 353 K to give the 32-electron benzylidyne derivative [Mo2Cp2(μ-CPh)(μ-PCy2)(CO)2] (Mo−Mo = 2.666(1) Å). Related methylidyne complexes could be obtained from the methyl-bridged complex [Mo2Cp2(μ-CH3)(μ-PCy2)(CO)2] via its trinuclear derivative [Mo3Cp2(μ3-CH)(μ-PCy2)(CO)7]. Thus, the carbonylation of the latter cluster gave the ketenyl complex [Mo2Cp2{μ-C(H)CO}(μ-PCy2)(CO)2], whereas its reaction with P(OMe)3 gave the substituted cluster [Mo3Cp2(μ3-CH)(μ-PCy2)(CO)6{P(OMe)3}], which in turn could be thermally degraded to give selectively the 30-electron methylidyne derivative [Mo2Cp2(μ-CH)(μ-PCy2)(μ-CO)] (Mo−Mo = 2.467(1) Å). DFT calculations on the phenylketenyl complex revealed that the metal−ligand interaction is intermediate between the extreme descriptions represented by the acylium (3-electron donor) and ketenyl (1-electron donor) canonical forms of this ligand.

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The alkyl-bridged unsaturated complexes [Mo 2 Cp 2 (μ-R)(μ-PCy 2 )(CO) 2 ] (R = Me, CH 2 Ph) react readily with CO (1 atm) at room temperature or below, but the products are strongly dependent on the alkyl bridge. Thus the benzyl complex 1b is selectively carbonylated to give the hydride derivative [Mo 2 Cp(η 5 -C 5 H 4 CH 2 Ph)(μ-H)(μ-PCy 2 )(CO) 4 ] [Mo-Mo = 3.217(1) A ˚], whereas the methyl complex gives first the acetyl-bridged complex [Mo 2 Cp 2 {μ-κ 1 :η 2 -C(O)Me}(μ-PCy 2 )(CO) 3 ], which then evolves to give up to three different products depending on the reaction conditions: the hydrides [Mo 2 Cp(η 5 -C 5 H 4 R 0 )(μ-H)(μ-PCy 2 )(CO) 4 ] (R 0 = Me, C(O)Me) and the heptacarbonyl complex [Mo 2 Cp(μ-PCy 2 )(CO) 7 ] (Mo-Mo = 3.2120(3) A ˚), the latter requiring the displacement of a Cp ligand. The phenyl-bridged complex [Mo 2 Cp 2 (μ-Ph)(μ-PCy 2 )(CO) 2 ] requires higher CO pressures (ca. 3 atm) and temperatures (ca. 333 K) to be carbonylated at a reasonable rate, then yielding a mixture of the corresponding hydride [Mo 2 Cp(η 5 -C 5 H 4 Ph)(μ-H)(μ-PCy 2 )(CO) 4 ] and the above heptacarbonyl complex. The title complexes also react readily with NO (2000 ppm in N 2 ) at room temperature, but the products obtained, again, strongly depend on the starting substrate. The methyl-bridged complex gives a mixture of two dinitrosyl derivatives having terminal methyl ([Mo 2 Cp 2 (Me)(μ-PCy 2 )(CO)(NO) 2 ], Mo-Mo = 3.074(1) A ˚) or bridging acetyl ([Mo 2 Cp 2 {μ-κ 1 :η 2 -C(O)Me}(μ-PCy 2 )(NO) 2 ], Mo-Mo = 2.9931(6) A ˚) ligands. The benzyl complex gives the analogous benzyl derivative ([Mo 2 Cp 2 (CH 2 Ph)(μ-PCy 2 )(CO)(NO) 2 ], Mo-Mo = 3.0993(4) A ˚) as major product, along with small amounts of the dicarbonyl [Mo 2 Cp 2 (CH 2 Ph)(μ-PCy 2 )(CO) 2 (NO) 2 ] and the trinitrosyl complex [Mo 2 Cp 2 (μ-PCy 2 )(μ-NO)(NO) 2 ], the formation of the latter requiring the displacement of the benzyl ligand. Finally, the phenyl-bridged complex gives again the analogous phenyl derivative [Mo 2 Cp 2 (Ph)(μ-PCy 2 )(CO)(NO) 2 ], but small amounts of dinitrosyl derivatives having terminal benzoyl, [Mo 2 Cp 2 {C(O)Ph}(μ-PCy 2 )(CO)(NO) 2 ], bridging benzoyl, [Mo 2 Cp 2 {μκ 1 :η 2 -C(O)Ph}(μ-PCy 2 )(NO) 2 ], and bridging phenyl ([Mo 2 Cp 2 {μ-κ 1 :η 2 -Ph}(μ-PCy 2 )(NO) 2 ], Mo-Mo = 2.9753(5) A ˚)) ligands are now formed. The latter complex can be selectively generated through the photochemical decarbonylation of the major phenyl complex. In contrast, separate experiments revealed that all hydrocarbyl complexes of the type [Mo 2 Cp 2 (R)(μ-PCy 2 )(CO)(NO) 2 ] rearrange thermally at ca. 343 K into the corresponding acyl isomers [Mo 2 Cp 2 {μ-κ 1 :η 2 -C(O)R}(μ-PCy 2 )(NO) 2 ].

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