The electronic and EPR spectra of Cu(RC02)2 [R = (CH3)2CH-, C2Hg-,(CH3)$-] have been measured in pyridine-chlorobenzene solvent mixtures. The results have been interpreted in terms of equilibrium between mononuclear and binuclear forms of copper(l1) carboxylate complexes with pyridine. The dimerization constant, &, and €b, Em values for bi-and mononuclear complexes have been determined in various mixed solvent compositions. The Kc values decrease with the basicity of the carboxylate ligand. The observed tendency is opposite to that obtained for Cu(ll) carboxylate complexes with dnethylsubstituted pyridines. The axial position of pyridine ligands in the elongated octahedron structure of mononuclear forms seems to be responsible for this correlation. It appears also that detailed comparison of physico-chemical properties of different copper complexes is possible only when the geometric structures of the species in equilibrium are identical. Furthermore, steric effect dominates the behaviour of complexes studied in binary solvent mixtures,
EXPERIMENTAL PART
Cu(ll) carboxylates were obtained as was described in [4]. Mononuclear Cu(ll) carboxylate complexes, C~(RC02)2Pyn.H20, were obtained by dissolving the solid Cu(ll) carboxylate in the pure pyridine at -315 K and cooling to -255 K. The blue precipitates, recrystallized from the pure amine, were stored over pyridine moisture (not stable at the air). Binuclear Cu(ll) carboxylate complexes, C U ~( R C O ~) ~P Y ~, were obtained * extrapolated results.
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