A complex of transition metal Ti, Fe, Co, Ni, Cr, Mn, Ta, Rh, Y, Sc, Ru, Pd, Zr, Hf, V or Nb and a mono-, bi-, tri-, or tetra-dentate ligand, wherein at least one of the donor atoms of the ligand is a nitrogen atom N with a 5-membered heterocyclic substituent joined to the N by a carbon atom. The complex is preferably Formula (I) wherein R5—N-G-X1 is a bi-, tri, or tetra-dentate ligand, N is joined to G by an imine linkage; G is a bridging group which can contain a third or fourth donor atom; X1 is —O or —S if the X1-M bond is covalent, or if the X1-M bond is dative X1 is ═S, —PR7R8, —PR8R9, ═NR7, ═NR8, —NR7R8 or —NR8R9; R5 and R7 are 5-membered heterocyclic substituents joined to the nitrogen (or phosphorus) atoms via a carbon atom: R8 and R9 are hydrocarbyl or heterohydrocarbyl, substituted hydrocarbyl or aryl substituent; X represents an atom or group covalently or ionically bonded to the transition metal M: L is a group datively bound to M, and n is from 0 to 5; q is 1 or 2. The complexes find use as polymerisation catalysts, preferably with an activator, e.g. MAO