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