Crystal and molecular structures of bis[.mu.-9,9,9-trifluoro-8-(trifluoromethyl)-6-methyl-5-azanon-5-ene-1,8-diolato(2-)-.mu.-0,N,0']-dicopper(II), Cu2C20H26F12N2O4, and bis[.mu.-8,8,8-trifluoro-7-(trifluoromethyl)-5-methyl-4-azaoct-4-ene-1,7-diolato(2-)-.mu.-0,N,0']-dicopper(II), Cu2C18H22F12N2O4. Effects of varying chelate ring size on geometry and antiferromagnetic exchange in di-.mu.-oxo-bridged dinuclear copper(II) complexes