A complex compound comprising the skeletal unit of Formula A, wherein the ring represented by C(R1)-A1-A2-(A3)x-C(R2)—C— has delocalised unsaturation and is optionally substituted via one or more of A1, A2 and A3 with atoms or groups selected from hydrogen, alkyl, aryl, halogen, or heterocyclic groups containing at least one N, S or O in a carbon ring; A1, A2 and A3 are selected from carbon, nitrogen or oxygen, R1 and R2 are each selected from chlorine, bromine or iodine; x is zero or 1, O is oxygen, E is nitrogen, phosphorus or arsenic, Q represents a divalent bridging group comprising one or more Group 14 atoms; M is a metal selected from Groups 3 to 7; X represents a monovalent atom or group covalently or ionically bonded to M; L is a mono- or bidentate molecule datively bound to M, y satisfies the valency of M and z is from 0 to 5. The complex can be used to polymerise olefins optionally with organo-A1 or -B compounds as activator.