The metallocene compounds of the formula (I) can be prepared by a process which represents another object of the present invention and which comprises the reaction of the corresponding bis-cyclopentadienyl ligands of the formula (III): wherein R1, R2, R3, R4, R5 and R6 are defined as above, and A is a suitable leaving group, with a compound of the formula MX4, wherein M and X are defined as above.