Spontaneous reduction of copper(II) complexes of the ligand 3,6-bis(2-pyridylthio)pyridazine. Crystal structures of bis[3,6-bis(2-pyridylthio)pyridazine-N1,N2]aquocopper(II) diperchlorate trihydrate and bis[.mu.-3,6-bis(2-pyridylthio)pyridazine-N4,.mu.-N1,.mu.-N2,N3]dicopper(I) diperchlorate