Abstract: A catalyst precursor composition comprising: a) a source of chromium, molybdenum or tungsten; b) a first ligand having the general formula (I); (R1)(R2)P—X—P(R3)(R4)??(I) wherein: X is a bridging group of the formula —N(R5)—, wherein R5 is selected from hydrogen, a hydrocarbyl group, a substituted hydrocarbyl group, a heterohydrocarbyl group, a substituted heterohydrocarbyl group, a silyl group or derivative thereof; at least three of R1 to R4 are independently selected from optionally substituted aromatic groups, each bearing a polar substituent on at least one of the ortho-positions; and optionally one of R1 to R4 is independently selected from hydrocarbyl, substituted hydrocarbyl, heterohydrocarbyl and substituted heterohydrocarbyl groups with the proviso that when the group is aromatic it does not contain a polar substituent at any of the ortho-positions; c) a second ligand having the general formula (II); (R1?)(R2?)P—X?—P(R3?)(R4?)??(II) wherein: X? is a bridging group as defined for X of the fi