Variable antiferromagnetic exchange in a series of binuclear copper(II) complexes of tetradentate (N4) and hexadentate (N6) diazine ligands. Magnetism versus structure, bridging and terminal ligand groups, and chelate ring size. Crystal and molecular structure of [.mu.-1,4-bis((6-methylpyrid-2-yl)amino)phthalazine](.mu.-hydroxo)(.mu.-nitrato-O)-bis(nitrato)dicopper(II)-0.5-water, Cu2C20H19N9O10.cntdot.0.5H2O