A complex of formula (I): (I') M is Hf; each X is a sigma ligand; L is a bridge of formula -(ER 8 2 ) y -; y is 1 or 2; E is C or Si; each R 8 is independently a C 1 -C 20 -hydrocarbyl, tri(C 1 -C 20 -alkyl)silyl, C 6 -C 20 - aryl, C 7 -C 20 -arylalkyl or C 7 -C 20 -alkylaryl or L is an alkylene group such as methylene or ethylene; Ar and Ar' are each independently an aryl or heteroaryl group optionally substituted by 1 to 3 groups R 1 or R 1' respectively; R 1 and R 1' are each independently the same or can be different and are a linear or branched C 1 -C 6 -alkyl group, C 7-20 arylalkyl, C 7-20 alkylaryl group or C 6-20 aryl group with the proviso that if there are four or more R 1 and R 1' groups present in total, one or more of R 1 and R 1' is other than tert butyl; R 2 and R 2' are the same or are different and are a CH 2 -R 9 group, with R 9 being H or linear or branched C 1 -C 6 -alkyl group, C 3-8 cycloalkyl group, C 6-10 aryl group; each R 3 is a -CH 2 -, -CHRx- or C(Rx) 2 - group wherein Rx is C 1-4 alkyl and where m is 2-6; R 5 is a linear or branched C 1 -C 6 -alkyl group, C 7-20 arylalkyl, C 7-20 alkylaryl group or C 6 -C 20 -aryl group; R 6 is a C(R 10 ) 3 group, with R 10 being a linear or branched C 1 -C 6 alkyl group; and R 7 and R 7' are the same or are different and are H or a linear or branched C 1 - C 6 -alkyl group. Invention relates also to a catalyst in solid form comprising (i) a complex of formula (I) and (ii) a cocatalyst of an aluminium compound and (iii) a cocatalyst of a boron compound.