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

[Image: see text] 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.