The present invention is directed toward Group 4, 5, 6, 7, 8, 9, 10 or 11 transition metal compounds containing neutral, mono- or di-anionic tridentate nitrogen/oxygen based ligands that are useful, with or without activators, to polymerize olefins, particularly .alpha.-olefins, or other unsaturated monomers. For the purposes of this disclosure, ".alpha.-olefins" includes ethylene. The present invention is also directed toward Group 4, 5, 6, 7, 8, 9, 10 or 11 transition metal compounds containing neutral, bidentate nitrogen/oxygen based ligands that are useful to polymerize olefins, particularly .alpha.-olefins, or other unsaturated monomers.