Fac–mer equilibria of coordinated iminodiacetate (ida2–) in ternary CuII(ida)(H–1B)– complex formation (B = imidazole, benzimidazole) in aqueous solution

pH potentiometric and spectrophotometric investigations on the complex formation equilibria of CuII with iminodiacetate (ida2–) and heterocyclic N-bases, viz. imidazole and benzimidazole (B), in aqueous solution in binary and ternary systems using different molar ratios of the reactants indicated the formation of complexes of the types, Cu(ida), Cu(ida)(OH)^–, (ida)Cu(OH)Cu(ida)–, Cu(B)^2+, Cu(H–1B)^+, Cu(ida)(H–1B)^–, (ida)Cu(B)Cu(ida) and (ida)Cu(H–1B)Cu(ida)–. Formation constants of the complexes at 25plus-minus1^at a fixed ionic strength, I = 0,1 mol dm–3 (NaNO3) in aqueous solution were evaluated and the complex formation equilibria were elucidated with the aid of speciation curves. Departure of the experimental values of the reproportionation constants (delta logKcu) of ternary Cu(ida)(H–1B)– complexes from the statistically expected values, despite their formation in appreciable amounts at equilibrium, were assigned to fac( f )-mer(m) equilibria of the ida2– ligand coordinated to CuII, as the N-heterocyclic donors, (H–1B)–, coordinate trans- to the N-(ida2–) atom in the binary Cu(ida)f complex to form the ternary Cu(ida)m(H–1B)– complexes.