Comparison of the dimerisation kinetics of the iron(III)–nitrilotriacetate and –bis-picolinate complexes

Abstract

The kinetics of the dimerisation of iron(III)–nitrilotriacetate and –bis-picolinate complexes have been investigated by use of the temperature-jump technique. In the case of the nitrilotriacetate complex which is considered as oxo-bridged between the iron atoms the reaction rates are dependent on acidity whereas for the bis-hydroxy-bridged bis-picolinate dimer no acid-dependence is found. For reaction (i)k₅₆= 1·05 ± 0·2 × 10³ mol^–1 s^–1 and 2Fe(OH)(pic)₂ [graphic omitted] Fe₂(OH)₂(pic)₄(i)k₆₅= 1·6 ± 0·1 s^–1(where pic = picolinic acid), and for reaction (ii)k₃₄= 1·2 ± 0·5 × 10⁵ mol^–1 s^–1 and k₄₃= 2·5 ± 1 × 10⁶ mol^–1 s^–1(where nta = nitrilotriacetic acid) at 25 °C and l= 0·2M. The mechanisms of the Fe(OH)(nta^–)+ Fe(nta) [graphic omitted] Fe₂(nta)₂O^2–+ H⁺(ii) reactions are discussed.