Direct observation of (Me(3)ECH(2))(5)Ta (E=C, Si) as the precursors to (Me(3)ECH(2))(3)Ta=CHEMe(3) and (Me(3)SiCH(2))(2)Ta(mu-CSiMe(3))(2)Ta(CH(2)SiMe(3))(2). Kinetic and mechanistic studies of the formation of alkylidene and alkylidyne ligands

Pentaalkyl complexes (Me(3)ECH(2))(5)Ta (E = C (1), Si (2)) were found to convert to alkyl alkylidene compounds (Me(3)ECH(2))(3)Ta=CHEMe(3) (E = C (3), Si (4)) with the elimination of EMe(4). Np5Ta (1, Np = Me(3)-CCH2) was observed as an intermediate to 3 in both the reaction of Np3TaCl2 with 2 equiv of NpLi and the reaction of Np4TaCl (5) with 1 equiv of NpLi. In addition to 1, 5 was also observed as an intermediate in the reaction between Np3TaCl2 and 2 equiv of NpLi. Ns(5)Ta (2, Ns = Me(3)SiCH(2)) was synthesized and isolated. Its quantitative conversion to 4 follows first-order kinetics. The further conversion of 4 to a bridged-alkylidyne complex Ns(2)Ta-(mu-CSiMe(3))(2)TaNs(2) (7) follows second-order kinetics. The activation parameters are Delta H-1 double dagger = 21.6 (1.4) kcal/mol and Delta S-1 double dagger = -5(5) eu for the conversion 2 --> 4 and Delta H-2 double dagger = 6.2(0.3) kcal/mol and Delta S-2 double dagger = -61.6(0.8) eu for 4 --> 7. Based on these results we propose mechanistic pathways for the formation of the alkylidene and bridged-allkylidyne complexes.