Exchange coupling in the alternating-chain compounds catena-di-.mu.-chloro-bis(4-methylpyridine)copper(II), catena-di-.mu.-bromobis(N-methylimidazole)copper(II), catena-hexanedione)bis(thiosemicarbazonato)copper(II), and catena-octanedione bis (thiosemicarbazonato)copper(II)