A transition metal complex having the following Formula (A): wherein the
monovalent groups R.sup.1 and R.sup.2 are --R.sup.a, --OR.sup.b,
--NR.sup.cR.sup.d, and --NHR.sup.e: the monovalent groups R.sup.a,
R.sup.b, R.sup.c, R.sup.d and R.sup.e, and the divalent group R.sup.3 are
(i) aliphatic hydrocarbon, (ii) alicyclic hydrocarbon, (iii) aromatic
hydrocarbon, (iv) alkyl substituted aromatic hydrocarbon (v) heterocyclic
groups and (vi) heterosubstituted derivatives of said groups (i) to (v);
M is a Group (3) to (11) or lanthanide metal; E is phosphorus or arsenic;
X is an anionic group, L is a neutral donor group; n is (1) or (2), y and
z are independently zero or integers, such that the number of X and L
groups satisfy the valency and oxidation state of the metal M. n is
preferably (2) and the two resulting R.sup.1 groups are preferably
linked. The complex can be used with optional activator to polymerise
olefins ##STR00001##