A catalyst precursor composition comprising a) a source of chromium, molybdenum or tungsten; b) a first ligand having the general formula (I): (R.sup.1)(R.sup.2)P-X-P(R.sup.3)(R.sup.4) (I) where X is a bivalent organic bridging group, R.sup.1 and R.sup.3 are hydrocarbyl groups, and R.sup.2 and R.sup.4 are aromatic groups; and c) a second ligand having the general formula (II): (R.sup.1')(R.sup.2')P-X'-P(R.sup.3')(R.sup.4') (II) where X' is a bridging group containing nitrogen and R.sup.1', R.sup.2', R.sup.3' and R.sup.4' are hydrocarbyl groups. The present invention also relates to a catalyst system comprising the catalyst precursor composition of the present invention and a cocatalyst.