A metallocene compound is represented by the formula (1): A.sub.2Y.sub.yMX.sub.n-2 wherein: M is a transition metal atom having a coordination number of n selected from Group 3, 4, 5 or 6 of the Periodic Table of Elements, or a lanthanide metal atom, or actinide metal atom; each A is independently a substituted polycyclic arenyl ligand pi-bonded to M, each A ligand includes at least one halogen substituent directly bonded to an sp.sup.2 carbon at a bondable ring position and, when each A is a substituted indenyl ligand and y is equal to one and the ligand includes at least one chloro, bromo or iodo substituent at the 4, 5, or 6 position of the indenyl ligand, then A also includes at least one other substituent in the indenyl ligand selected from hydrocarbyl, substituted hydrocarbyl, halogen, halocarbyl, substituted halocarbyl, silylcarbyl, substituted silylcarbyl, germylcarbyl, substituted germylcarbyl, or other heteroatom substituents wherein the heteroatom is bonded directly to a ring carbon of the ring structure ligand and is boron, a Group 15 atom, or a Group 16 atom, preferably boron, nitrogen, oxygen, phosphorus, or sulfur; Y is an optional bridging group containing at least one Group 13, 14, 15, or 16 element and is bonded to any single position of the ring structure of A, y is zero or 1, indicating the absence (y=0) or presence (y=1) of Y; and the or each X is a univalent anionic ligand, or two X are joined and bound to the metal atom to form a metallocycle ring, or two X are joined to form a chelating ligand, a diene ligand, or an alkylidene ligand; provided that when y is equal to 1, then A is not 5,6-dichloroindenyl or an indenyl group having a fluoro- or chloro substituent at the 7 position and a hydrocarbyl or substituted hydrocarbyl substituent at the 4 position, and provided further that when y is equal to 0, then A is not 5,6-difluoroindenyl.