A monocyclopentadienyl or substituted monocyclopentadienyl metal complex containing compound useful as a polymerization catalyst corresponding to the formula:CpMX.sub.n.sup.+ A.sup.-wherein:Cp is a single .eta..sup.5 -cyclopentadienyl or .eta..sup.5 -substituted cyclopentadienyl group optionally covalently bonded to M through a substituent;M is a metal of Group 3-10 or the Lanthanide Series of the Periodic Table bound in an .eta..sup.5 bonding mode to the cyclopentadienyl or substituted cyclopentadienyl group;X each occurrence independently is selected from the group consisting of hydride, halo, alkyl, aryl, silyl, germyl, aryloxy, alkoxy, amide, siloxy, neutral Lewis base ligands and combinations thereof having up to 20 non-hydrogen atoms, and optionally one X together with Cp forms a metallocycle with M;R is alkyl or aryl of up to 10 carbons;n is one or two depending on the valence of M; andA is a noncoordinating, compatible anion of a Bronsted acid salt.