Single site nitrogen and oxygen metallacyclic catalysts comprising at least two polymerization-stable anionic ancillary ligands and a Group 4-10 metal are disclosed wherein the nitrogen or oxygen heterocyclic catalyst is produced by dimerizing an alkylamido or trimerizing a metal containing oxane. The polymerization-stable anionic ancillary ligands are preferably either cyclopentadienyl, borataaryl, pyrrolyl, azaboralinyl, pyridinyl, or a quinolinyl ring. In a preferred embodiment, the invention relates to catalysts of the formula: ##STR1## wherein M is a transition metal of Groups 3-10; L is a polymerization-stable anionic ancillary ligand; R is a hydrocarbyl group containing up to about 12 carbon atoms optionally substituted with halogen; and E is R or halogen, alkoxy from C.sub.1 to C.sub.20, siloxy from C.sub.1 to C.sub.20, N(R).sub.2, hydrogen or another univalent anionic ligand, or mixtures thereof.