Electronic Structure and Donor Ability of an Unsaturated Triphosphorus-Bridged Dimolybdenum Complex.

The triphosphorus complex [Mo2Cp2(μ-η3:η3-P3)(μ-PtBu2)] was prepared in 83% yield by reacting the methyl complex [Mo2Cp2(μ-κ1:η2-CH3)(μ-PtBu2)(μ-CO)] with P4 at 333 K, a process also giving small amounts of the methyldiphosphenyl complex [Mo2Cp2(μ-η2:η2-P2Me)(μ-PtBu2)(CO)2]. The latter could be better prepared by first reacting the anionic complex Na[Mo2Cp2(μ-PtBu2)(μ-CO)2] with P4 to give the diphosphorus derivative Na[Mo2Cp2(μ-η2:η2-P2)(μ-PtBu2)(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 P3- 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 CF3SO3Me gave [Mo2Cp2(μ-η3:η3-P3Me)(μ-PtBu2)](CF3SO3) as a result of methylation at an external atom of the P3 ligand, while its reaction with [Fe2(CO)9] enabled the addition of one, two, or three Fe(CO)4 fragments at these P atoms, but only the diiron derivative [Mo2Fe2Cp2(μ-η3:η3:κ1:κ1-P3)(μ-PtBu2)(CO)8] could be isolated. This complex bears a Fe(CO)4 fragment at each of the external atoms of the P3 ligand, and the central P atom of the latter displays the lowest 31P chemical shift reported to date (δP -721.8 ppm). The related complexes [Mo2M2Cp2(μ-η3:η3:κ1:κ1-P3)(μ-PtBu2)(CO)10] (M = Mo, W) were prepared by reacting the title compound with the corresponding [M(CO)5(THF)] complexes in toluene, while reaction with [Mo(CO)4(THF)2] also enabled the formation of the heptanuclear derivative [Mo7Cp4(μ-η3:η3:κ1:κ1-P3)2(μ-PtBu2)2(CO)14]. The interatomic distances in the above compounds indicate that the central Mo2P3 skeleton of these molecules is little modified by the attachment of 16-electron M(CO)n fragments at the external atoms of the P3 ligand.