Dithio-oxamides as Ligands: Crystal Structures and Vibrational Analyses of Bis(N,N′-dicyclohexyldithio-oxamidato)palladium(II), Bis(N,N′-dibenzyldithio-oxamide)copper(II) Diperchlorate, and Dichloro(N,N′-dimethyldithio-oxamide)zinc(II