and R are defined as above, and R and R can be combined to form a single divalent radical, and wherein X is a halogen; or plex consists essentially of bis(N-decyl-fl-mercaptoethylamine)nickel (II). (d) A S,S-disubstituted-a-diketobis(B-mercaptoethylimine)nickel (II) complex having the formula aluminum compound is ethylaluminum dichloride. 13.