A complex of formula (I) wherein M is zirconium or hafnium; each X independently 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 or a C1-C20-hydrocarbyl group optionally containing one or more silicon atoms or heteroatoms of Group 14-16 of the periodic table or fluorine atoms, and optionally two R' groups taken together can form a ring; R2 and R2' are each independently a C1-C20 hydrocarbyl group, -OC1- hydrocarbyl group or -SC1-20 hydrocarbyl group; R5 is a -OC1-20 hydrocarbyl group or -SC1-20 hydrocarbyl group, said R5 group being optionally substituted by one or more halo groups; R5' is hydrogen or a C1-20 hydrocarbyl group; -OC1-20 hydrocarbyl group or -SC1-20 hydrocarbyl group; said C1-20 hydrocarbyl group being optionally substituted by one or more halo groups; R6 and R6' are each independently a C1-20 hydrocarbyl group; -OC1-20 hydrocarbyl group or -SC1-20 hydrocarbyl group; each R1 and R1' are independently -CH2Rx wherein Rx are each independently H, or a C1-20 hydrocarbyl group, optionally containing heteroatoms.