The present invention relates to compounds of formula (I) wherein each of X, Y, Z is independently selected from O, S, NR1, CR2R3, N and CR4, and where optionally X—Y, Y-Z, Z-E1 and X-E2 each independently form part of a saturated or unsaturated ring
system which may be substituted or unsubstituted; m is 0 or 1; M is a
metal selected from Ti[III], Ti[IV], Fe[II], Fe[III], Co[I], Co[II], Co[III], Ni[II], Cr[III], Mn[II], Mn[III], Mn[IV], Ru[II], Ru[III], Ru[IV], Pd[II], V[II], V[III], V[IV], V[V], Cu[I], Cu[II], Rh[I], Rh[III], Mo[III], Mo[V], Re[I] and Re[II]; each of E1 and E2 is independently selected from O, S, NR5, N, P, PR6, where at least one of either E1 or E2 carries a formal
negative charge; L2 is a one
electron donor ligand; n is zero or an integer such that the compound has an overall charge of zero or +1; L1 is NR7RB, PR7RB, OR7, SR7, O, S or NR16, imidazolyl, pyridinyl, benzimidazolyl or quinolinyl; each of R1-8 and R16 is independently H or a hydrocarbyl group; Q is a
linker group; and r is 0 or 1. Further aspects of the invention relate to catalyst compositions comprising compounds of formula (I), and their use in the polymerisation of olefinically unsaturated monomers.