Reactions of tantalum alkylidene complexes with silanes. Synthesis and characterization of novel metallasilacyclodiene complexes and kinetic and mechanistic studies of their formation

The reactions of tantalum alkylidene complexes (RCH2)(3)Ta(L)=CHR (1) and R CH2Ta(L)(2)-[=CHR](2) (3) (R = SiMe3, L = PMe3) with phenylsilanes H2SiR ' Ph (R ' = Me, Ph) and (PhSiH2)(2)-CH2 were found to produce bis(silyl)-subs tituted alkylidene complexes (RCH2)(3)Ta=CR-(SiHR ' Ph) (4) and novel metal lasilacyclobutadiene and metalladisilacydohexadiene complexes. Reaction of the mixed-ligand trimethylsilylmethyl neopentylidene complex RCH2Ta(L)(2)[= CHBut](2) (6) with H2SiMePh also yielded a metallasiiacyclobutadiene comple x, but reaction of the neopenyl neopentylidene complex (BuCH2Ta)-C-t(L)(2)[ =CHBut](2) (2) with H2SiMePh yielded unidentified products. Deuterium-label ing and kinetic studies of the conversion of 1 --> 4 were found to be consi stent with a dissociative mechanism. The structures of the novel metallacyc lic complexes 5a, 5b, 7, and 8 were determined by X-ray crystallography.