Synthesis of the triply-bonded dimolybdenum anions [Mo(2)(eta(5)-C(5)H(5))(2)(mu-PA(2))(mu-CO)(2)](-) (A = Cy, Et, Ph, OEt): unsaturated hydride and carbyne derivatives.

Tetrahydrofuran solutions of the 30-electron anions [Mo(2)Cp(2)(mu-PA(2))(mu-CO)(2)](-) (A = Cy, Et, Ph, OEt) are conveniently prepared through a two-step approach. In the first step, [Mo(2)Cp(2)(CO)(6)] is treated with the chlorophosphines ClPR(2) (R = Cy, Et, Ph) or the chlorophosphite ClP(OEt)(2), in refluxing toluene or diglyme respectively, to give the corresponding 32-electron chloro-complexes [Mo(2)Cp(2)(mu-Cl)(mu-PA(2))(CO)(2)] as major products. In the second step, these air-sensitive intermediates are treated in tetrahydrofuran solution at room temperature with one of several reducing agents such as Li[BHEt(3)], Li(Hg), Na(Hg) or K[BH(s)Bu(3)] to give red solutions of the corresponding alkali-metal salts of the anions, which display significant ion pairing involving one or both oxygen atoms of the bridging carbonyl ligands, depending on the cation. All these triply bonded species are quite air-sensitive and could not be isolated as pure solids, but they can be easily protonated using a weak acid such as [NH(4)]PF(6) to give with good yield the corresponding unsaturated hydrides [Mo(2)Cp(2)(mu-H)(mu-PA(2))(CO)(2)], which are species of low to moderate sensitiveness to air, and also formally containing an intermetallic triple bond. The reactivity of the dicyclohexylphosphide-bridged anion (mainly as its Li(+) salt) towards different hydrocarbon halides RX was studied in detail. These reactions were found to be rather complex, critically depending on the reagent used, and generally resulting in the formation of several products, of which four types were identified: (a) the known agostic products [Mo(2)Cp(2)(mu-PCy(2))(mu-R)(CO)(2)] (R = Me, CH(2)Ph), (b) the new alkoxycarbyne products [Mo(2)Cp(2)(mu-COR)(mu-PCy(2))(mu-CO)] [R = Me, Et, C(O)Ph, (i)Pr, Cy], which could be conveniently isolated as pure solids, (c) the iodoxycarbyne complex [Mo(2)Cp(2)(mu-COI)(mu-PCy(2))(mu-CO)], a very unstable species formed in the reaction with EtI, and (d) the halide complexes [Mo(2)Cp(2)(mu-PCy(2))(mu-X)(CO)(2)] [X = Cl, Br, I], which were more conveniently prepared by the direct reaction of the anion with the pertinent halogen (X = Br, I). The analysis of the above results suggests that at least three primary reaction pathways are in operation: (a) nucleophilic attack of the anion through its dimetal centre, (b) nucleophilic attack of the anion through the oxygen atoms of its bridging carbonyls and (c) electron-transfer with the reagent, this being the main path to the halo-complexes [Mo(2)Cp(2)(mu-PCy(2))(mu-X)(CO)(2)].