Chelating tendencies of N,N′-bis(2-hydroxyphenyl)ethylenediamine-N,N′-diacetic acid

Abstract

The proton association and metal complex formation constants of N,N′-bis(2-hydroxyphenyl)ethylenediamine-N,N′-diacetic acid (H₄ hedda) have been determined utilising potentiometric and spectrophotometric techniques. The ligand is highly selective for the trivalent metal ion Fe^III, the formation constant for the Fe^III–H₄hedda complex [log K_FeL= 40.1 at 25 °C and I= 0.10 mol dm^–3(KNO₃)] being one of the highest reported so far for any 1 : 1 complex involving a multidentate ligand. Binding to the divalent metal ions Mg^II, Ca^II, Mn^II, Co^II, Ni^II, Cu^II, Zn^II, Cd^II and Pb^II, and to the trivalent metal ion Lu^III, takes place in a stepwise manner, and the order in which the ligand donor groups become co-ordinated to the metal ions has been deduced.