Electron spin resonance was used to investigate structural features of Cu-II complexes with dNbpy (4,4'-di(5-nonyl)-2,2'-bipyridine) and dnNbpy (4,4'-di-n-nonyl-2,2'-bipyridine) ligands in methyl isobutyrate (MIB) and toluene. With 1 or 2 equiv of either dNbpy or dnNbpy ligands, (CuBr2)-Br-II forms predominantly the neutral complexes Cu-II(dnNbpy)Br-2 and Cu-II(dnNbpy)Br-2, respectively. Bromine atom transfer between [Cu-I(dnNbPY)(2)](+)[(CuBr2)-Br-I](-) and ethyl 2-bromoisobutyrate (EBriB) in MIB does not yield Cu-II(dnNbpy)Br-2. Instead, it leads to the complex [Cu-II(dnNbpy)(2)Br](+)[(CuBr2)-Br-I](-). The latter species is also formed in an equilibration reaction of Cu-II(dnNbpy)Br-2 with [Cu-I(dnNbpy)(2)](+)[(CuBr2)-Br-I](-) (K greater than or equal to 100 M-1/2, 23 degreesC, MIB).