The invention relates to a catalyst composition comprising a salt of a non- or weakly coordinating anion, said non- or weakly coordinating anion comprising at least one metal or metalloid ion M with valency v+, v representing an integer between 1 and 5, and at least one bidentate ligand coordinating to this metal or metalloid ion, and a catalyst that can be activated by said non- or weakly coordinating anion, characterized in that said bidentate ligand is a bidentate monoanionic ligand of formula (I): (R.sup.1.sub.qA.sup.1-X-A.sup.2R.sup.2.sub.r), wherein X represents a bridging moiety; A.sup.1 and A.sup.2 are each independently chosen from the group comprising N, O, P, S, and C; R.sup.1 and R.sup.2 are each independently chosen from the group comprising an optionally substituted linear or branched (hetero)alkyl group, an optionally substituted (hetero)aryl group, and a Si containing group; and q and r each independently represent an integer with 0.ltoreq.q, r.ltoreq.2.