An asymmetric complex of formula (I) wherein M is zirconium or hafnium; each X is a sigma ligand; L is a divalent bridge selected from —R′2C—, —R′2C—CR′2—, —R′2Si—, —R′2Si—SiR′2—, —R′2Ge—, wherein each R′ is independently a hydrogen atom, C1-C20-alkyl, tri(C1-C20-alkyl)silyl, C6-C20-aryl, C7-C20-arylalkyl or C7-C20-alkylaryl; R2 and R2′ are each independently a C1-C20 hydrocarbyl radical; R5′ is a C1-20 hydrocarbyl group; R5 is hydrogen, or a C1-20 hydrocarbyl group; R6 is a non tertiary C1-10 alkyl group or C6-10-aryl group or C7-10 arylalkyl group or ZR3; R6′ is a tertiary C4-20 alkyl group; Z is O or S; R3 is a C1-20 hydrocarbyl group optionally substituted with halo; Ar is an aryl or heteroaryl group having up to 20 carbon atoms optionally substituted by one or more groups R1; Ar′ is an aryl or heteroaryl group having up to 20 carbon atoms optionally substituted by one or more groups R1; each R1 is a C1-20 hydrocarbyl group or two R1 groups on adjacent carbon atoms taken together can form a fused 5 or 6 membered ring with the Ar group, said ring being itself optionally substituted with one or more groups R4; and each R4 is a C1-20 hydrocarbyl group.