The new nonsymmetric dinuclear copper(II) complex [Cu(2)L(1)(OAcW(ClO4) (7) was synthesized by complexation of Cu(OAc). H2O with a new nonsymmetric dinucleating ligand (5) which' is formed in situ by condensation of 2-formyl-6-((4-methylpiperazin-1-yl)methyl)phenol (3a) with 2-(aminoethyl)pyridine. Complex 7 crystallizes in the monocli,nic space group P2(1)/n with cell constants a = 7.486(1) Angstrom, b 22.472(2) Angstrom, c = 16.892(2) Angstrom, beta = 102.7(1)degrees, V = 2771.6(6) Angstrom(3), and Z = 4. The structure was determined at 130 K from 4890 out of a total of 7034 reflections with R(F) = 0.060 and R(wF) = 0.060. The crystal structure establishes the presence of a dinuclear copper core with distinct copper sites. One of the copper ions is coordinated by two sp(3) nitrogens whereas the other copper is coordinated by two sp(2) nitrogens. The two copper ions are bridged by a phenoxo group, by a syn-syn bidentate acetato bridge, and by a monodentate acetato bridge resulting in a Cu-Cu distance of 3.0293(10) Angstrom. The copper centers are weakly antiferromagnetically coupled with 2J = -15 cm(-1). This weak antiferromagnetic coupling is reflected in the large isotropic shifts for the ligand protons in the H-1 NMR spectrum of 7. 2D-COSY experiments and selective substitutions, leading to p-methyl-substituted complex 8, were performed to allow assignments of the proton resonances. The complexation behavior of nonsymmetric ligand 5 is studied by microcalorimetric and UV-vis spectroscopic measurements. These techniques unambiguously establish the stepwise complexation behavior of the ligand; nonsymmetric dinuclear complex 7 is formed via the initial formation of a mononuclear copper complex. The enthalpy of formation of the first complexation step is -8.4 (+/-0.2) kcal/mol (K = 1.8 (+/-0.1) x 10(8)); for the second step a value of Delta H = -1.9 (10.1) kcal/mol (K = 1.1 (+/-0.1) 10(5)) was obtained using microcalorimetry.