12 alkyl groups substituted 5- or 6-membered imino ring which contains zero or one additional nitrogen or oxygen atom,M is two hydrogens, two metals with one valence or a metal with two or more valences, y is a number from 0 to 16, and each Z is bound to a peripheral position of the phenyl rings and is, independently of the other halogen, C1 -C18 alkyl, C1 -C18 alkoxy, C1 -C18 alkylthio or C2 -C18