Structural and mechanistic studies of co-ordination compounds. Part IX. Acidity and proton-exchange kinetics of some octahedral cobalt(III) amine complexes

Abstract

The preparation and characterization of the cations trans-[Co(cyclam)(A)OH₂]²⁺(cyclam = 1,4,8,11-tetra-aza-cyclotetradecane; A = CN, NCS, and NO₂) and trans-[Co(cyclam)(NH₃)OH]²⁺ are described. pK_a Values of these and other related aquo-complexes in aqueous solution have been determined by pH titration at 25·0 °C. The amine proton-exchange kinetics of the cations trans-[Co(am)₄(NH₃)₂]³⁺ and -[Co(am)₄(NH₃)Cl]²⁺[(am)₄= cyclam or two ethylenediamine (en) molecules] have been followed by n.m.r. spectroscopy at 34·5 °C. Within a given molecule, the axial ammine ligands exchange faster than the in-plane cyclam and en ligands. For a given series of analogous complexes, the proton-exchange rate constant of the axial ammine ligands decreases in the following order of (am)₄: cyclam > (en)₂ > (NH₃)₄. The acidity of the corresponding aquo-complexes trans-[Co(am)₄(A)OH₂]ⁿ⁺(A = CN, N₃, NCS, NO₂, NH₃ and OH₂) also decreases along this series. These results are in accordance with the nephelauxetic effect of the amine ligands on the central cobalt(III) ion. Solvation effects on the order of acidity of the complexes are discussed and it is concluded that these are of little importance for axial aquo- and amine ligands of analogous complexes.