Oxamidato complexes. 3. A study of the formation of homo- and heteropolymetallic Cu(II)H2apoxM(II) complexes (H2apox = N,N'-bis(3-aminopropyl)oxamide; M(II) = Mn, Co, Ni, Cu, Zn, Cd). Synthesis and crystal structure of [Cu2(apox)(NCO)2]