The invention is directed to a compound having the general formula (I): ##STR00001## wherein each of L.sub.a and L.sub.b is an independently selected from a linking moiety comprising 0 to 10 main chain atoms, optionally substituted; each of Z.sub.a and Z.sub.b is an independently selected complexing moiety comprising at least one nitrogen atom; either both or one of Z.sub.a and/or Z.sub.b is coordinatively bonded to a respective transition metal complex M.sub.aV.sub.a and M.sub.bV.sub.b through said nitrogen atom, wherein each of M.sub.a and M.sub.b is an independently selected transition metal, and each of V.sub.a and V.sub.b is an independently selected valence group.