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

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 calcula ted by making use of isodesmic equations. The relative stabilities of sever al ethylenediamine-N,N,N',N'-tetraacetic acid-transition metal complexes, a nd homologues thereof, are discussed first and compared with published stab ilities of similar complexes based upon strain analyses carried out on isol ated crystal structures. Second, after having established that the computat ional results are in good agreement with the published stabilities, a more comprehensive analysis of complex stabilities was carried out on (S,S)-ethy lenediamine-N,N'-disuccinic acid-transition metal complexes, and homologues thereof. Theoretical indications are that complex stabilities increase wit h 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-cha in ligators. (C) 2000 Elsevier Science B.V. All rights reserved.