Synthesis and characterization of dinickel(II) and dipalladium(II) complexes of the macrocyclic binucleating ligand 3,13-dimethyl-3,13-dinitro-1,5,11,15-tetra-azacycloeicosane-8,18-dithiol (L5). Crystal structure of the complex [Ni2(L5– 2H)][NO2]2·3.5H2O