Empirical force-field calculations of strain-energy contributions to the thermodynamics of complex formation. Part 1. The difference in stability between complexes containing five- and six-membered chelate rings

Abstract

The technique of conformational potential-energy minimisation has been applied to the ligands ethylenediamine (en) and propane-1,3-diamine (pd), and to four complexes of Ni^II with en, pd, 3-azapentane-1,5-diamine, and 4-azaheptane-1,7-diamine. The method of Boyd, as modified by Snow, using a general force field including contributions from non-bonded interactions, bond-stretching, and valence-angle deformations and torsional strain, has been used. The difference in the enthalpy of complex formation between five- and six-membered chelate rings is due mainly to steric-strain considerations. Valence-angle deformations, together with non-bonded interactions, represent the major contributions to this difference. Predicted differences in the total strain energy between complexes containing five- and six-membered chelate rings have been found to be in good agreement with experimentally determined values.