Binuclear complexes of the ligand 3,6-bis(2-pyridylthio)pyridazine involving homo- and heterodiatomic binuclear centers (Cu-Cu, Cu-Co, Cu-Zn). Crystal structure of bis[.mu.-3,6-bis(2-pyridylthio)pyridazine-N1,.mu.-N2,.mu.-N3,N4](.mu.-chloro)dicopper(II) triperchlorate-acetonitrile