The vanadium complex used according to the invention is preferably a complex of vanadium (III), (IV) or (V), especially (IV) or (V), with a ligand as described above, especially preferably a complex VOL2, VO2 L2 or VOL2 (OR26) where LH is a chelant as described above and R26 is an optionally substituted alkyl, aryl, aralkyl or alkenyl group (where such groups are as defined above), preferably such