Studies of some ternary complexes of copper(II) involving π-bonding ligands. Part 2

Abstract

The proton–ligand formation constants of HL, formation constants of the binary complexes [CuL]⁺ and [CuL₂], and formation constants of the ternary complexes [CuAL]⁺, where A = 2,2′-bipyridyl, 1,10-phenanthroline, 2-(2′-pyridyl) benzimidazole, or 2-(2′-pyridyl)imidazoline, and L = salicylamide ion, salicylanilide ion, 5-bromosalicyl-amide ion, or 5-bromosalicylanilide ion have been determined in dioxan–water (1 : 1, v/v) solutions, and 0.2 mol dm^–3 Na[ClO₄], at 30 °C. The mixed-ligand stability constants K_CuAL′, for the reaction [CuA]²⁺+ L^–⇌[CuAL]⁺, have been calculated by three methods, namely, graphical linear plot, computer calculations considering complete formation of [CuA]²⁺ prior to the formation of [CuAL]⁺, and also considering all possible species to be present in solution. The values so obtained are very similar and the concentration plot confirms that the major species present in the solution are only [CuA]²⁺ and [CuAL]⁺. The value of Δ log K= log K_cuAL′–log K_cuL is positive, except when A is 2-(2′-pyridyl)imidazoline. The majority of the mixed-ligand complexes have been synthesized and characterized. Electronic spectra confirm that there is no signifcant interaction between the two ligands through the metal dπ orbitals in the mixed-ligand complexes. The greater stability of the ternary complexes may be due to a lowering of repulsion between metal dπ electrons and the ligand electrons in these compounds.