The crystal structure of a novel compound Cu-II(dTbpy)Br-2 (dNbpy = 4,4 ' -di(5-nonyl)-2,2 ' -bipyridine), which is used in the reverse atom transfer radical polymerization, is reported. Cu-II(dNbpy)Br2 crystallizes in the triclinic Pi space group with a = 12.5283(11) Angstrom, b = 15.0256(14) Angstrom, alpha = 17.7900(16) Angstrom, alpha = 90.350(2)degrees, beta = 99.360(2)degrees, gamma 107.937(2)degrees, and Z = 2. The Cu-II center in the complex has a distorted square planar geometry and is coordinated by two nitrogen atoms of a single dNbpy ligand (Cu-N = 2.011(7) and 2.022(7) Angstrom) and two bromine atoms (Cu-Br = 2.3621(14) and 2.3567(13) Angstrom). The similarity of the absorption spectra in the solid state and in solution suggested that the geometry of the complex remained unchanged upon dissolution. In the presence of dNopy, Cu-II(dNbpy)Br2 undergoes Br substitution to form ionic [Cu-II(dNbDy)(2)Br](+)[Br](-). DeltaH degrees and DeltaS degrees values for this equilibrium were negative and dependent on the polarity of the medium. It was found that, under the typical polymerization conditions (T greater than or equal to 90 degreesC and the total copper concentration in the range 1.0 x 10(-2)-1.0 x 10(-1) M), (CuBr2)-Br-II and 2 equiv of dNbpy will predominantly form the neutral Cu-II(dNbDy)Br-2 complex. In a polar medium under the same conditions, [Cu-II(dNbpy)2Br](+)[Br](-) is preferred.