Supported catalyst compositions use for use in the gas-phase polymerization of one or more α-olefins and methods for making and using the same, the catalyst composition including A) an inert support; B) a Group 4–10 metal complex corresponding to the formula: where M is a metal from one of Groups 4 to 10 of the Periodic Table of the Elements in the +2 or +4 formal oxidation state, Cp is a π-bonded anionic ligand group, Z is divalent moiety bound to Cp and bound to M by either covalent or coordinate/covalent bonds and contains boron or a member of Group 14 of the Periodic Table of the Elements, and also nitrogen, phosphorus, sulfur or oxygen, and X is a neutral conjugated diene ligand group having up to 60 atoms, or a dianionic derivative thereof; and C) an ionic cocatalyst capable of converting the metal complex into an active polymerization catalyst represented by the formula: [L*-H]+[(C6F5)3BC6H4—O—MORCx-1Xay]−, wherein L* is a neutral Lewis base, Mo is a metal or metalloid selected from Groups 1–14 of the Periodic Table of the Elements, RC independently each occurrence is a hydrogen or a hydrocarbyl, hydrocarbylsilyl, or hydrocarbysilylhydrocarbyl group having from 1 to 80 nonhydrogen atoms; Xa is a halo-substituted hydrocarbyl, hydrocarbylamino-substituted hydrocarbyl, hydrocarbyloxy-substituted hydrocarbyl, hydrocarbylamino, di(hydrocarbyl)amino, hydrocarbyloxy or halide noninterfering group having from 1 to 100 nonhydrogen atoms; x is an integer which ranges from 1 to an integer equal to the valence of MO; y is an integer which ranges from 0 to an integer equal to 1 less than the valence of MO; and x+y equals the valence of MO.