Synthesis, structure, and magnetism of a new type of .pi.-molecular complex containing binuclear copper(II) complexes and benzene: bis[2,2-dimethyl-7-(phenylimino)-3,5,7-octanetrionato]dicopper(II)-benzene and bis[2,2-dimethyl-7-[(4-nitrophenyl)imino]-3,5,7-octanetrionato]dicopper(II)-bis(benzene)