Exchange interaction through extended molecular bridges: magnetic properties of .mu.-4,4'-bipyridine and .mu.-pyrazine copper(II) binuclear complexes and crystal structures of (.mu.-4,4'-bipyridine)bis[(diethylenetriamine)(perchlorato)copper(II)] perchlorate and aquo(4,4'-bipyridine)(diethylenetriamine)copper(II) perchlorate