Novel compounds are provided which are useful as catalysts, particularly in the polymerization of addition polymerizable monomers such as olefinic or vinyl monomers. The compounds are complexes of a mid-transition metal coordinated to at least two ligands, at least one of which is an unsaturated nitrogenous ligand. Depending on ligand substitution, stereospecific catalysts can be provided, including isospecific catalysts and syndiospecific catalysts. Catalyst systems containing the novel compounds in combination with a catalyst activator are provided as well, as are methods of using the novel compounds in the preparation of polyolefins. One embodiment of the present invention is a compound having the formula L1[MQ1Q2]L2 in which M is Nb, Ta, Mo, W, Mn or Re, Q1 and Q2 are each a univalent substituent, and L1 and L2 are ligands coordinated to M, wherein L1 contains a first coordinating nitrogen atom within a C═N group, and a second coordinating atom that is either a second nitrogen atom, optionally present in a second C═N group, or an oxygen, sulfur or phosphorus atom. The L2 group contains a coordinating atom that is either a second nitrogen atom, optionally within a C═N group, or a sulfur or phosphorus atom.