om hydrogen; optionally R6 and R7 can form a ring comprising 4 to 8 carbon atoms, which can bear substituents; and when Z is a moiety of formula (II), Y and Z can be the same or different from each other; L is a divalent bridging group; M is an atom of a transition metal selected from those belonging to group 3, 4, 5, 6 or to the lanthanide or actinide groups in the Periodic Table of the Elements