A compound of the formula I for preparing essentially isotactic olefin polymers ##STR11## in which M1 is a metal from group IVb, Vb or VIb of the Periodic TableR1 and R2 are identical or different and are a hydrogen atom, a C1 -C10 -alkyl group, a C1 -C10 -alkoxy group, a C6 -C10 -aryl group, a C6 -C10 -aryloxy group, a C2 -C10 -alkenyl group, a C7 -C40 -arylalkyl group, a C7 -C40 -alkylaryl group