A high-yield synthetic route for the preparation of the unsaturated anion [MoCp(μ-PBu)(μ-CO)] (2) was implemented, via two-electron reduction of the chloride complex [MoCp(μ-Cl)(μ-PBu)(CO)] (1). Reaction of 2 with [NH][PF] led to the formation of the 30-electron complex [MoCp(H)(μ-PBu)(CO)] (3), in which the hydride ligand adopts an uncommon terminal disposition. DFT analysis of the electronic structure of 3 gave support to the presence of a M≡M triple bond in this complex following from a σδδ configuration, a view also supported by the high electron density accumulated at the corresponding Mo-Mo bond critical point. In contrast, reactions of 2 with IMe or ClCHPh gave the alkyl-bridged complexes [MoCp(μ-κ:η-CHR)(μ-PBu)(CO)] (R = H (4a), Ph (4b)), which in solution display agostic Mo-H-C interactions. Decarbonylation of 4a took place rapidly under photochemical conditions to give the 30-electron complex [MoCp(μ-κ:η-CH)(μ-PBu)(μ-CO)] (7), with a stronger agostic coordination of its methyl ligand. In contrast, irradiation of 4b led to the formation of the benzylidyne derivative [MoCp(μ-CPh)(μ-PBu)(μ-CO)] (9), following from fast decarbonylation and dehydrogenation of the bridging benzyl ligand. Low-temperature photochemistry allowed for the NMR characterization of an intermediate preceding the hydrogen elimination, identified as the carbene hydride [MoCp(H)(μ-CHPh)(μ-PBu)(CO)] (10), a product which evolves slowly by H elimination to the benzylidyne derivative. Analogous dehydrogenation of the methyl ligand in 7 could be accomplished upon moderate heating, to yield the corresponding methylidyne derivative [MoCp(μ-CH)(μ-PBu)(μ-CO)] (9). A complete reaction mechanism accounting for these photochemical reactions was elaborated, based on the reaction intermediates identified experimentally and on extensive DFT calculations. Surprisingly, for both systems the C-H bond activation steps are relatively easy thermal processes occurring with modest activation energies after photochemical ejection of CO, with a rate-determining step involving the formation of agostic carbenes requiring also a strong structural reorganization of the central MoPC rings of these molecules.

The high unsaturation of the title complex enabled it to react with a wide variety of molecules under mild conditions, whereby the agostic methyl ligand underwent unusual or unprecedented processes. Methane elimination occurred in the reactions with PPh H and SiPh H , this being followed in the latter case by Si-H bond oxidative addition to give the hydride silylene derivative [Mo Cp H(μ-PtBu )(μ-SiPh )(CO)]. Dehydrogenation, however, was the dominant process in the room temperature reaction with [Fe (CO) ], to give the unsaturated methylidyne cluster [Mo FeCp (μ -CH)(μ-PtBu )(CO) ] (Mo-Mo=2.6770(8) Å). In contrast, PMe elimination took place in the reaction with P , to give the unsaturated triphosphorus complex [Mo Cp (μ-η :η -P )(μ-PtBu )] (Mo-Mo=2.6221(3) Å). Yet a most remarkable reaction occurred with BH ⋅THF, involving insertion of two BH units and dehydrogenation to yield [Mo Cp (μ-B H Me)(μ-PtBu )(CO)], with the novel methyldiboranyl ligand acting as a 5-electron donor due to the presence of two 3-centre, 2-electron B-H-Mo interactions, according to spectroscopic data and DFT calculations (Mo-Mo ca. 2.65 Å).

The triphosphorus complex [Mo 2 Cp 2 (μ-η 3 :η 3 -P 3 )(μ-P t Bu 2 )] was prepared in 83% yield by reacting the methyl complex [Mo 2 Cp 2 (μ-κ 1 :η 2 -CH 3 )(μ-P t Bu 2 )(μ-CO)] with P 4 at 333 K, a process also giving small amounts of the methyldiphosphenyl complex [Mo 2 Cp 2 (μ-η 2 :η 2 -P 2 Me)(μ-P t Bu 2 )(CO) 2 ]. The latter could be better prepared by first reacting the anionic complex Na[Mo 2 Cp 2 (μ-P t Bu 2 )(μ-CO) 2 ] with P 4 to give the diphosphorus derivative Na[Mo 2 Cp 2 (μ-η 2 :η 2 -P 2 )(μ-P t Bu 2 )(CO) 2 ] and further reaction of the latter with MeI. Density functional theory calculations on the title complex revealed that its triphosphorus group can be viewed as an allylic-like P 3 – ligand acting as a six-electron donor via the external P atoms, while coordination of the internal P atom involves donation from the π orbital of the ligand and back-donation to its π* orbital, both interactions having a weakening effect on the Mo–Mo and P–P connections. The reactivity of the title compound is dominated by the electron-donor ability associated with the lone pairs located at the P atoms. Its reaction with CF 3 SO 3 Me gave [Mo 2 Cp 2 (μ-η 3 :η 3 -P 3 Me)(μ-P t Bu 2 )](CF 3 SO 3 ) as a result of methylation at an external atom of the P 3 ligand, while its reaction with [Fe 2 (CO) 9 ] enabled the addition of one, two, or three Fe(CO) 4 fragments at these P atoms, but only the diiron derivative [Mo 2 Fe 2 Cp 2 (μ-η 3 :η 3 :κ 1 :κ 1 -P 3 )(μ-P t Bu 2 )(CO) 8 ] could be isolated. This complex bears a Fe(CO) ...

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