Kinetics of copper(II)–ethylenediamine complex formation

Abstract

The temperature-jump method has been used to study the complex formation of copper(II) with ethylenediamine (en) at 25° and ionic strength 0·1M. Both the neutral form of the ligand and the protonated H en⁺ species react according to the overall processes (i) and (ii) where n= 1 or 2. The values of k₁=(3·8 ± 1·6)× 10⁹ and k₂=(Cu en_n–1)²⁺+ en [graphic omitted] Cu en_n(i), (Cu en_n–1)²⁺+(H en)⁺ [graphic omitted] Cu en_n+ H (ii)(1·9 ± 0·3)× 10⁹ l. mole^–1 sec.^–1 are relatively large for reaction with a neutral ligand and seem to indicate either the presence of an internal conjugate base mechanism, or a classical associative S_N2 mechanism. The substantially lower values of k′₁=(1·4 ± 0·3)× 10⁵ and k′₂=(3·1 ± 2·1)× 10⁴ l. mole^–1 sec.^–1 reflect the fact that deprotonation of the intermediate must occur before chelation. Results of temperature-jump and stopped-flow experiments at pH ⩽ 4·3 indicate the presence of a dissociative path involving direct attack of a proton on [Cu en]²⁺.