Catalyst compositions useful for the polymerization of olefins are disclosed. These compositions comprise a Group 8-10 metal complex comprising a bidentate or variable denticity ligand comprising one or two nitrogen donor atom or atoms independently substituted by an aromatic or heteroaromatic ring(s), wherein the ortho positions of said ring(s) are substituted by aryl or heteroaryl groups. Also disclosed are processes for the polymerization of olefins using the catalyst compositions.