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

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) 4 fragment at each of the external atoms of the P 3 ligand, and the central P atom of the latter displays the lowest 31 P chemical shift reported to date (δ P -721.8 ppm). The related complexes [Mo 2 M 2 Cp 2 (μ-η 3 :η 3 :κ 1 :κ 1 -P 3 )(μ-P t Bu 2 )(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 [Mo 7 Cp 4 (μ-η 3 :η 3 :κ 1 :κ 1 -P 3 ) 2 (μ-P t Bu 2 ) 2 (CO) 14 ]. The interatomic distances in the above compounds indicate that the central Mo 2 P 3 skeleton of these molecules is little modified by the attachment of 16-electron M(CO) n fragments at the external atoms of the P 3 ligand.