Oxamidato complexes. 1. A study of the formation of complexes between copper(II) and H2pmox (H2pmox = N,N'-bis(2-pyridylmethyl)oxamide). Synthesis, crystal structure, and magnetic properties of the alternating-chain compound [Cu2(pmox)(H2pmox)(H2O)2](NO3)2.cntdot.8H2O