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

The triphosphorus complex [Mo₂Cp₂(μ-η³:η³-P₃)(μ-PBu₂)] was prepared in 83% yield by reacting the methyl complex [Mo₂Cp₂(μ-κ¹:η²-CH₃)(μ-PBu₂)(μ-CO)] with P₄ at 333 K, a process also giving small amounts of the methyldiphosphenyl complex [Mo₂Cp₂(μ-η²:η²-P₂Me)(μ-PBu₂)(CO)₂]. The latter could be better prepared by first reacting the anionic complex Na[Mo₂Cp₂(μ-PBu₂)(μ-CO)₂] with P₄ to give the diphosphorus derivative Na[Mo₂Cp₂(μ-η²:η²-P₂)(μ-PBu₂)(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-PBu₂)](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-PBu₂)(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 (δ_P -721.8 ppm). The related complexes [Mo₂M₂Cp₂(μ-η³:η³:κ¹:κ¹-P₃)(μ-PBu₂)(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-PBu₂)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) fragments at the external atoms of the P₃ ligand.