Migration and Insertion Processes in the Reactions of the Hydrocarbyl-Bridged Unsaturated Complexes [Mo25-C5H5)2(μ-R)(μ-PCy2)(CO)2] (R = Me, CH2Ph, Ph) with CO and NO

The alkyl-bridged unsaturated complexes [Mo2Cp2(μ-R)(μ-PCy2)(CO)2] (R = Me, CH2Ph) 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 [Mo2Cp(η5-C5H4CH2Ph)(μ-H)(μ-PCy2)(CO)4] [Mo−Mo = 3.217(1) Å], whereas the methyl complex gives first the acetyl-bridged complex [Mo2Cp2{μ-κ12-C(O)Me}(μ-PCy2)(CO)3], which then evolves to give up to three different products depending on the reaction conditions: the hydrides [Mo2Cp(η5-C5H4R′)(μ-H)(μ-PCy2)(CO)4] (R′ = Me, C(O)Me) and the heptacarbonyl complex [Mo2Cp(μ-PCy2)(CO)7] (Mo−Mo = 3.2120(3) Å), the latter requiring the displacement of a Cp ligand. The phenyl-bridged complex [Mo2Cp2(μ-Ph)(μ-PCy2)(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 [Mo2Cp(η5-C5H4Ph)(μ-H)(μ-PCy2)(CO)4] and the above heptacarbonyl complex. The title complexes also react readily with NO (2000 ppm in N2) 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 ([Mo2Cp2(Me)(μ-PCy2)(CO)(NO)2], Mo−Mo = 3.074(1) Å) or bridging acetyl ([Mo2Cp2{μ-κ12-C(O)Me}(μ-PCy2)(NO)2], Mo−Mo = 2.9931(6) Å) ligands. The benzyl complex gives the analogous benzyl derivative ([Mo2Cp2(CH2Ph)(μ-PCy2)(CO)(NO)2], Mo−Mo = 3.0993(4) Å) as major product, along with small amounts of the dicarbonyl [Mo2Cp2(CH2Ph)(μ-PCy2)(CO)2(NO)2] and the trinitrosyl complex [Mo2Cp2(μ-PCy2)(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 [Mo2Cp2(Ph)(μ-PCy2)(CO)(NO)2], but small amounts of dinitrosyl derivatives having terminal benzoyl, [Mo2Cp2{C(O)Ph}(μ-PCy2)(CO)(NO)2], bridging benzoyl, [Mo2Cp2{μ-κ12-C(O)Ph}(μ-PCy2)(NO)2], and bridging phenyl ([Mo2Cp2{μ-κ12-Ph}(μ-PCy2)(NO)2], Mo−Mo = 2.9753(5) Å)) 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 [Mo2Cp2(R)(μ-PCy2)(CO)(NO)2] rearrange thermally at ca. 343 K into the corresponding acyl isomers [Mo2Cp2{μ-κ12-C(O)R}(μ-PCy2)(NO)2].