MONOMER AND DIMER COMPLEXES OF COPPER(II) ACETATE WITH PYRIDINE AND PICOLINES .2. SOLUTION EQUILIBRIA BY SPECTROPHOTOMETRIC AND NMR MAGNETIC MEASUREMENTS

Equilibrium constants (K) for the reaction: [Cu(OAc)2.L]2+2L reversible 2[Cu(OAc)2.L2], where OAc is the acetate anion and L = pyridine (py), beta-picoline (beta-pic) or gamma-picoline (gamma-pic), were determined from spectrophotometric and variable-temperature magnetic measurements in chloroform solutions. Good agreement was found between the two methods. The small K values (0.01-0.45) found for the equilibria are consistent with the observed instability of the monomers in chloroform, and increase in the order: py < beta-pic < gamma-pic. The position of the beta-picoline system, for which no crystalline monomer could be isolated, and the anomalous behaviour of alpha-picoline within this series are discussed in terms of crystal-packing requirements arising from the steric properties of these ligands. Thermodynamic parameters for the equilibria were obtained from the temperature dependence of K. The values of DELTA-H(Kelvin) and DELTA-S(Kelvin) are in close agreement with those found for adduct formation of related copper(II) chelates with these ligands, and are discussed in terms of contributions from electronics and structural reorganization, bond formation and solvation effects.