A new silolene-bridged compound, racemic (1,4-butanediyl) silylene-bis(1-eta5-indenyl) dichlorozirconium (1) was synthesized by reacting ZrCl4 with C4H8Si (IndLi)2 in THF. 1 was reacted with trialkylaluminum and then with triphenylcarbenium tetrakis(penta-fluorophenyl)borate (2) to produce in situ the zirconocenium ion (1+). This "constraint geometry" catalyst is exceedingly stereoselective for propylene polymerization at low temperature (T(p) = -55-degrees-C), producing refluxing n-heptane insoluble isotactic poly(propylene) (i-PP) with a yield of 99.4%, T(m) = 164.3-degrees-C, DELTAH(f) = 20.22 cal/g and M(w)BAR. = 350 000. It has catalytic activities of 10(7)-10(8) g PP/(mol Zr.[C3H6].h) in propylene polymerization at the T(p) ranging from -55-degrees-C to 70-degrees-C, and 10(8) polymer/(mol Zr.[monomer].h) in ethylene polymerization. The stereospecificity of 1+ decreases gradually as T(p) approaches 20-degrees-C. At higher temperatures the catalytic species rapidly loses stereochemical control. Under all experimental conditions 1+ is more stereospecific than the analogous cation derived from rac-dimethylsilylenebis (1-eta5-indenyl)dichlorozirconium (4). The variations of polymerization activities in ethylene and in propylene for T(p) from -55-degrees-C to +70-degrees-C indicates a Michaelis Mention kinetics. The zirconocenium-propylene pi-complex has a larger insertion rate constant but lower thermal stability than the corresponding ethylene pi-complex. This catalyst copolymerizes ethylene and propylene with reactivity ratios of comparable magnitude r(E) approximately 4r(p). Furthermore, r(E).r(P) approximately 0.5 indicating random copolymer formation. Both 1 and 4 activated with methylaluminoxane (MAO) exhibit much slower polymerization rates, and, under certain conditions, a lower stereoselectivity than the corresponding 1+ or 4+ system.