Reaction of [P2N2]Ta=CH2(Me) with ethylene: Synthesis of [P2N2]Ta(C2H4)Et, a neutral species with a beta-agostic ethyl group in equilibrium with an alpha-agostic ethyl group ([P2N2] = PhP(CH2SiMe2NSiMe2CH2)(2)PPh)

The photolysis of [P2N2]TaMe3 ([P2N2] = PhP(CH2SiMe2NSiMe2CH2)(2)PPh) produces [P2N2]Ta=CH2(Me) as the major product. The thermally unstable methylidene complex decomposes in solution in the absence of trapping agents to unidentified products. However, in the presence of ethylene [P2N2]Ta=CH2(Me) is slowly converted to [P2N2]Ta(C2H4)Et, with [P2N2]Ta(C2H4)Me observed as a minor product. A mechanistic study suggests that the formation of [P2N2]Ta(C2H4)Et results from the trapping of [P2N2]TaEt, formed by the migratory insertion of the methylene moiety into the tantalum-methyl bond. The minor product, [P2N2]Ta(C2H4)Me, forms from the decomposition of a tantalacyclobutane resulting from the addition of ethylene to [P2N2]Ta=CH2(Me) and is accompanied by the production of an equivalent of propylene. Pure [P2N2]Ta(C2H4)Et can be synthesized by hydrogenation of [P2N2]TaMe3 in the presence of PMe3, followed by the reaction bf ethylene with the resulting trihydride. Crystallographic and NMR data indicate the presence of a beta -agostic interaction between the ethyl group and tantalum center in [P2N2]Ta(C2H4)Et. Partially deuterated analogues of [P2N2]Ta(C2H4)Et show a large isotopic perturbation of resonance for both the beta -protons and the alpha -protons of the ethyl group, indicative of an equilibrium between a beta -agostic and an a-agostic interaction for the ethyl group in solution. An EXSY spectrum demonstrates that an additional fluxional process occurs that exchanges all of the H-1 environments of the ethyl and ethylene ligands. The mechanism of this exchange is believed to involve the direct transfer of the beta -agostic hydrogen atom from the ethyl group to the ethylene ligand, via the so-called beta -hydrogen transfer process.