cis-iso-Specific Polymerization of Norbornenes by a Unique Combination of Cp* and 1,3-Butadiene Ligands on Tantalum: Crystal Structures of Cp*(η4-C4H6)Ta(CH2Ph)2 and Cp*(η4-C4H6)Ta(CHPh)(PMe3)

cis - iso -Specific homogeneous ring-opening metathesis polymerization of norbornenes is achieved by using the catalyst precursor Cp*(η 4 -C 4 H 6 )Ta(CH 2 Ph) 2 ( 2 ). Thermolysis of 2 generates a benzylidene species, which is trapped by PMe 3 to give a benzylidene complex Cp*(η 4 -C 4 H 6 )Ta( CHPh)(PMe 3 ) ( 4 ). The crystal structures of 2 and 4 are determined by X-ray analysis.