<i>cis</i>-<i>iso</i>-Specific Polymerization of Norbornenes by a Unique Combination of Cp* and 1,3-Butadiene Ligands on Tantalum: Crystal Structures of Cp*<b>(</b>η<sup>4</sup>-C<sub>4</sub>H<sub>6</sub>)Ta(CH<sub>2</sub>Ph)<sub>2</sub> and Cp*(η<sup>4</sup>-C<sub>4</sub>H<sub>6</sub>)Ta(CHPh)(PMe<sub>3</sub>)
cis-iso-Specific homogeneous ring-opening metathesis polymerization of norbornenes is achieved by using the catalyst precursor Cp*(eta(4)-C4H6)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*(eta(4)-C4H6)Ta(=CHPh)(PMe(3)) (4). The crystal structures of 2 and 4 are determined by X-ray analysis.