Trapping Intermediates in Phosphavinyl Coupling Reactions:  Synthesis and Structural Characterization of a Novel Bis(phosphinoalkylidene)−Tantalum Complex, [{CpTaCl2[C(But)−P(Cy)−]}2]

The reactions of the stable phosphavinyl Grignard reagent (Z)-[MgCl(OEt2){C(But)PCy}] with [Cp*TaCl4] or [Cp*NbCl4] have been investigated. In both cases phosphavinyl coupling reactions are the ultimate outcome to give the known 2,4-diphosphabicyclo[1.1.0]butane, Cy2P2C2But 2, in the former and a mixture of this compound and its 1,2-dihydro-1,2-diphosphete valence isomer in the latter. A novel bis(phosphinoalkylidene)−tantalum complex, [{Cp*TaCl2[C(But)−P(Cy)−]}2], has been isolated and structurally characterized as an intermediate in the tantalum reaction.