Potocnak, I: Low-dimensional compounds containing cyano groups. IV. Bis(2,2'-bipyridine-kappa(2)N,N1) (dicyanamido-kappaN')copper(II) perchlorate and mu-dicyanamido-kappa(2)N1:N5-bis[bis(2,2'-bipyridine-kappa(2)N,N')copper(II)]