Gravar-mail: Bis{μ(3)-cis-N-(2-carboxyl­ato-5-chloro­phen­yl)-N′-[3-(dimethyl­amino)­prop­yl]oxamidato(3−)}bis­(perchlorato-κO)bis­(N,N,N′,N′-tetra­methyl­ethylene­diamine)­tetra­copper(II)