The stability of metal complexes

Complex formation between a metal ion (Mm+) and a ligand (Lp-) produces a series of complexes ML(m-p)+, ML2(m-2p)+, etc.. The magnitude of the stability constants of these complexes (defined by βn = [MLn(m-np)+]/[Mm+][Lp-]n) will depend upon a variety of factors, e.g. on the nature of the metal acceptor ion, on the nature of the donating atoms, on the structure of the ligand and on various steric and conformational aspects.
The present thesis treats a few of these factors in detail. The first part is concerned with the stability of chelate rings of the type (I) (formed by an α-amino-acid), and discusses the effect of replacing the α-hydrogen atoms of glycine (II) by methyl groups, and of linking these to form aliphatic rings of varying size.
To this end the compounds III - VII were prepared in a state of high purity and their proton constants were determined at 20.0°C. in a medium of constant ionic strength (μ = 0.10M, KCl) by a precise potentiometric titration procedure. Stability constants of complexes formed by each of these ligands and the divalent ions Co++, Ni++, Cu++ and Zn++ were determined under the same conditions. In each case the presence of 1:1 and 2:1 complexes was established and there was an indication of a 3:1 complex with nickel, small changes in the stability constants β1 and β2 could be explained in terms of the inductive effects of the substituents, and by changes in the bond angles of the chelate ring produced by substitution of one or both hydrogen atoms on the α-carbon atom, or by including this atom in an aliphatic ring.
Closely connected with this work was the problem of the nature of complexes formed by piperazine-NN′-diacetic acid (VIII).
The zwitterion shown in VIII must be in the chair configuration but, if both nitrogen atoms are included in 'the formation of a chelate ring, the molecule must assume the less favourable boat conformation - as is known to occur in the complex of palladous chloride and NN′-dimethyl-piperazine (Hassel and Pedersen, 1959). Complexes of VIII were found to have a comparatively low stability and it appears that the ligand cannot bond effectively to small ions through both nitrogen atoms. This hypothesis was supported by measurements on piperidine-N-acetic acid (IX).
The second part of this work is concerned with the problem of relating the stabilities of metal complexes formed by a series of ligands in which a nitrogen atom is replaced in turn with phosphorus and arsenic. Various factors (e.g. ease of oxidation) prevented the use of purely aliphatic ligands and the only readily accessible ones were not ideally suited to the main purpose. However a comparison of the complexing ability of nitrogen and arsenic in the potentially tridentate ligands X and XI could be carried out, again using the technique of potentiometric titrations. Complex formation was studied with various metal ions, particular attention being paid to copperII, nickel, zinc, silver, and mercuryII. The experimental data were amplified by investigations of the complexes of XII and XIII.
It was apparent that the behaviour was that expected for metals belonging to the acceptor types classified by Ahrland, Chatt, and Davis (1958) as class (a) and class (b).
Measurements with mercuryII and the ligands X - XVI were carried out by measuring the E.M.F. and the pH of a cell containing a reversible mercury electrode. In measurements with silver ions and the same ligands a simultaneous measurement of pH and p[Ag] permitted calculation of the stability constants of the metal complex, MJ, and of the protonated species MHL; this latter species was found to have a stability comparable to that of ML when the ligand was XI, XII, XIV, and XV.
The complexes of diphenylphosphine-P-propionic acid (XIV) and its arsenic analogue (XV) have been examined in 20% dioxan solution. The nitrogen analogue (XVI) has been prepared and its complexes studied to enable a comparison of the donor properties of Group V elements to be made.
From measurements with the ligands X-XVI it is clear that with class (a) metals bonding is predominantly through the carboxyl groups (and nitrogen) rather than through arsenic or phosphorus. With class (b) metals, however, the strength of bonding increased in the order N < As and N < P, and the contributions to the stabilities of the metal complexes by the formation of chelate rings involving the carboxyl groups are small.
[For the diagrams to accompany this abstract, please consult the PDF.]