5 nickel compounds for example nickel complexes of 2 2 39 thio bis 4 1 1 1 3 tetramethylbutyl phenol such as the 1 1 or 1 2 complex optionally with additional ligands such as n butylamine triethanolamine or n cyclohexyl diethanolamine nickel dibutyldithiocarbamate nickel salts of 4 hydroxy 3 5 di tert butylbenzylphosphonic acid monoalkyl esters such as of the methyl ethyl or butyl ester nickel complexes of ketoximes such as of 2 hydroxy 4 methyl penyl undecyl ketoxime nickel complexes of 1 phenyl 4 lauroyl 5 hydroxy pyrazole optionally with additional ligands 2 6 sterically hindered amines for example bis 2 2 6 6 tetramethylpiperidyl sebacate bis 1 2 2 6 6 pentamethylpiperidyl sebacate n butyl 3 5 di tert butyl 4 hydroxybenzyl malonic acid bis 1 2 2 6 6 pentamethylpiperidyl ester condensation product of 1 hydroxyethyl 2 2 6 6 tetramethyl 4 hydroxy piperidine and succinic acid condensation product of n n 39 2 2 6 6 tetramethylpiperidyl hexamethylendiamine and 4 tert octylamino 2 6 dichloro 1 3 5 s triazine tris 2 2 6 6 tetramethylpiperidyl nitrilotriacetate tetrakis 2 2 6 6 tetramethyl 4 piperidyl 1 2 3 4 butane tetra carbonic acid 1 1 39 1 2 ethanediyl bis 3 3 5 5 tetramethylpiperazinone such amines include hydroxylamines derived from hindered amines such as di 1 hydroxy 2 2 6 6 tetramethylpiperidin 4yl sebacate 1 hydroxy 2 2 6 6 tetramethyl 4 benzoxypiperidine 1 hydroxy 2 2 6 6 tetramethyl 4 3 5 di tert butyl 4 hydroxy hydrocinnamoyloxy piperdine and n 1 hydroxy 2 2 6 6 tetramethyl piperidin 4.