Determination and comparison of stability constants of vanadium(V), molybdenum(VI), and tungsten(VI) aminocarboxylate complexes

Abstract

The solution structure and equilibria of vanadium(V), molybdenum(VI), and tungsten(VI) complexes formed by ethylenediaminetetra-acetic (H₄edta), ethylenediamine-NN′-diacetic (H₂edda), nitrilotriacetic (H₃nta), and iminodiacetic (H₂ida) acids have been investigated potentiometrically and spectrophotometrically. If Y^n– represents the fully dissociated ligand anion, all the 1 : 1 complexes have the formula [VO₂Y]^(n–1)–, [MoO₃Y]^n–, and [WO₃Y]^n–. A 1 : 2 vanadium(V)-ida, a 2 : 1 molybdenum(VI)- or tungsten(VI)–edta and two protonated 1 : 1 vanadium(V)–edta chelate complexes are also formed. The formation constants of the 1 : 1 complexes increase with the number of chelate rings for the same metal and decrease from V^V to Mo^VI and W^VI for a given ligand. The oxometal ions have a greater affinity for the nitrogen of the amine function than for the acetate oxygen.