A semi-empirical molecular orbital study of complex stabilities of hexadentate polyaminocarboxylate ligands and transition metal ion complexes

Abstract

Semi-empirical molecular orbital calculations were performed in vacuo on a series of metal-hexadentate polyaminocarboxylate ligand complexes using the ZINDO/1 Hamiltonian. Stabilisation energies for the complexes were calculated by making use of isodesmic equations. The relative stabilities of several ethylenediamine- N, N, N′, N′-tetraacetic acid-transition metal complexes, and homologues thereof, are discussed first and compared with published stabilities of similar complexes based upon strain analyses carried out on isolated crystal structures. Second, after having established that the computational results are in good agreement with the published stabilities, a more comprehensive analysis of complex stabilities was carried out on ( S, S)-ethylenediamine- N, N′-disuccinic acid-transition metal complexes, and homologues thereof. Theoretical indications are that complex stabilities increase with an increase in the number of 6-membered rings formed upon complexation, i.e. complexation is favoured with an increase in the length of the side-chain ligators.