Oxidative Additions of Coordinated Ligands at Unsaturated Molybdenum and Tungsten Diphosphine-Bridged Carbonyl Dimers. 2. Decarbonylation Reactions of [Mo2(η5-C5H4R)2(CO)4(μ-Ph2PCH2PPh2)] (R = H, Me)

Decarbonylation of the dimolybdenum complexes [Mo 2 (η 5 -C 5 H 4 R) 2 (CO) 4 (μ-dppm)] (R = H, Me, dppm = Ph 2 PCH 2 PPh 2 ) occurs readily upon heating in tetrahydrofuran or toluene solution to afford with good yield the phosphido complexes [Mo 2 (η 5 -C 5 H 4 R) 2 (μ-CH 2 PPh 2 )(μ-PPh 2 )(CO) 2 ], which arise from an irreversible P−C(sp 3 ) bond cleavage in the backbone of the dppm ligand. Other minor products in these reactions are the oxo complexes [Mo 2 (η 5 -C 5 H 4 R) 2 (μ-CH 2 PPh 2 )(O)(μ-PPh 2 )(CO)], which are formed by the action of oxygen on the former dicarbonyl compounds, and the triply-bonded complexes [Mo 2 (η 5 -C 5 H 4 R) 2 (CO) 2 (μ-dppm)], in which the dppm ligand remains intact. By contrast, photochemical decarbonylation of the parent tetracarbonyl complexes at 10 °C yields the triply-bonded dicarbonyls as major products, along with a small amount of the monocarbonyl complexes [Mo 2 (η 5 -C 5 H 4 R) 2 (μ-CH 2 PPh 2 )(μ-PPh 2 )(μ-CO)]. Separate experiments show that the latter compounds are formed from [Mo 2 (η 5 -C 5 H 4 R) 2 (μ-CH 2 PPh 2 )(μ-PPh 2 )(CO) 2 ] under photolytic conditions, this reaction being reversible. Thus it is concluded that the P−C(sp 3 ) cleavage of the dppm ligand is fairly well suppressed at ambient temperatures or below. The reactions of all the above unsaturated species with CN t Bu proceed rapidly at room temperature. In this way, the new isocyanide derivatives [Mo 2 (η 5 -C 5 H 4 Me) 2 (μ-CH 2 PPh 2 )(μ-PPh 2 )(CN t Bu)(μ-CO)(CO)], [Mo 2 (η 5 -C 5 H 5 ) 2 (μ-CH 2 PPh 2 )(μ-PPh 2 )(CN t Bu)(CO)], and [Mo 2 (η 5 -C 5 H 4 R) 2 (μ-η 1,η 2 -CN t Bu)(CO) 2 (μ-dppm)] have been prepared. All of them are formed in good yields as single isomers but have a rather low stability. Reaction of the monocarbonyl derivative with atmospheric oxygen gives the oxo complex [Mo 2 (η 5 -C 5 H 5 ) 2 (μ-CH 2 PPh 2 )(μ-O)(μ-OPPh 2 )(CN t Bu)(CO)], which is also obtained as a single isomer. In marked contrast to their ditungsten analogues, the isocyanide-bridged compounds [Mo 2 (η 5 -C 5 H 4 R) 2 (μ-η 1,η 2 -CN t Bu)(CO) 2 (μ-dppm)] do not experience C−H bond cleavages in their cyclopentadienylic rings to a significant extent.