A complex of formula (I): (I′) M is Hf; each X is a sigma ligand; L is a bridge of formula -(ER82)y—; y is 1 or 2; E is C or Si; each R8 is independently a C1-C20-hydrocarbyl, tri(C1-C20-alkyl)silyl, C6-C20- aryl, C7-C20-arylalkyl or C7-C20-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 R1 or R1′ respectively; R1 and R1′ are each independently the same or can be different and are a linear or branched C1-C6-alkyl group, C7-20 arylalkyl, C7-20 alkylaryl group or C6-20 aryl group with the proviso that if there are four or more R1 and R1′ groups present in total, one or more of R1 and R1′ is other than tert butyl; R2 and R2′ are the same or are different and are a CH2—R9 group, with R9 being H or linear or branched C1-C6-alkyl group, C3-8 cycloalkyl group, C6-10 aryl group; each R is a —CH2—, —CHRx- or C(Rx)2-group wherein Rx is C1-4 alkyl and where m is 2-6; R5 is a linear or branched C1-C6-alkyl group, C7-20 arylalkyl, C7-20 alkylaryl group or C6-C20-aryl group; R6 is a C(R10)3 group, with R10 being a linear or branched C1-C6 alkyl group; and R6 and R7′ are the same or are different and are H or a linear or branched C1-C6-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.