 is bonded to metal Ln, wherein Cp0 and Cp0 comprise a substituted or unsubstituted cyclopentadienyl or fluorenyl group, bridge P corresponds to the formula MR0, in which M is silicon or another element of column IVA of Mendeleev""s periodic classification and R is an alkyl group having 0 to 00 atoms of carbon; and a co-catalyst selected from the group consisting of a magnesium alkyl, a lithium alkyl, an aluminum alkyl, a Grignard""s reagent and mixtures thereof. In a preferred embodiment