Examples of metal complexes are complexes of a transition metal such as manganese, iron, cobalt, nickel, copper, vanadium or ruthenium, with a complexing agent, especially nitrogen-containing compound such as a bipyridyl, a trimethyl-triazonane or other poly(tertiary amine) or a (poly)histidine; such metal complexes can be used in an amount of 1-25 mol % with respect to the number a aldehyde or ca