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Potentiometric pH-titration and electron spin resonance studies have shown the existence of mono-meric, dimeric and dinuclear vanadyl chelates of certain polyaminocarboxylic acids. The ligands studied include ethylenediaminetetraacetic (edta), ethyleneglycolbis(β- aminoethyl ether)-N,N?-tetra-acetic (egta), diethylenetriaminepentaacetic (dtpa) and triethylenetetraaminehexaace- tic (ttha) acids. All the ligands form 1 : 1 chelates. The addition of ethylene glycol to the aqueous solution in each case provides circumstances for the study of the monomeric chelates, though it is not equally effective in all cases. The 1 : 1 vanadyl egta chelate above pH 5.5 gave rise to ΔMS = 2 and ΔMS = 1 transitions from which structural and magnetic parameters have been determined and interpreted in terms of the formation of a dimeric form of a hydrolytic product of the 1 : 1 chelate. Firm evidence for the formation of dinuclear chelates occurs only in the vanadyl ttha system. Here the room tem- perature e.s.r. spectrum shows the presence of weak exchange coupling between the vanadyl ions in the dinuclear chelate.

The e.s.r. spectra of frozen aqueous solutions containing vanadyl (DL)- and (DD)-tartrates are consistent with the formation of dimeric species. Computer simulation of the spectra, assuming a monoclinic dimer model, has enabled the determination of the magnetic parameters associated with vanadyl ions, the internuclear separations between the metal ions in the dimers and their relative orientation. For vanadyl (DL)-tartrate dimer, the excellent agreement between structural parameters for the solid and frozen glass was taken to indicate that the same structure exists in both states. In the case of vanadyl(DD)-tartrate, however, although the overall symmetry is maintained, a slight reduction in the vanadyl vanadyl separation is believed to occur as a result of incorporation into an ethylene glycol-water glass.

The triplet state e.s.r. spectra of bis(dimethylglyoximato)copper(II) in ethanol solutions have been recorded and the results interpreted in terms of a magnetic dipole-dipole interaction between pairs of copper(II) ions in a dimeric complex. Consideration of a spin Hamiltonian which takes into account the symmetry relationship of the transition metal ions in the dimeric complex has provided a set of structural and magnetic parameters by computer lineshape simulation procedures. The results indicate that the symmetry of the pair system is similar to that observed for the dimeric units in the solid complex but that the calculated distance between the metal ions is somewhat larger than that determined by crystallographic methods for the solid complex.

Measurements of 27Al nuclear magnetic resonance of aqueous solutions containing aluminium chelates of hydroxy carboxylic acids show that the signal intensity depends to a marked extent on the pH of the solution. This effect has been interpreted in terms of polymerization of the aluminium chelates which results in a broadening of the signal through magnetic dipole-dipole coupling between the various 27Al nuclei.

The electron spin resonance spectra of copper(II)- tris(hydroxymethyl)amino-methane complexes in aqueous solution provides evidence of the formation of dimeric species. These species are found to be different from those postulated by previous investigations. In dimethylformamide dimeric species are formed at room temperature and frozen solution in 1 : 1 and 1 : 2 metal-to-ligand mole ratios, though the formation of the dimer is critically dependent on the addition of base. Low field, ΔM = 2, signals observed for these systems have been interpreted in terms of dipole-dipole coupling between pairs of copper(II) ions. Structures for the copper(II) dimers have been proposed which are compatible with the copper-copper separation estimated by computer simulation of the spectra.

Electron spin resonance measurements have been made on the copper(II) chelates of o-methylmercaptobenzoic acid, S-carboxydiethyldithiocarbamic acid, thiazolidine-4-carboxylic acid, 5-hydroxy-1,3-benzoxathiol-2-one and diacetyl bisthiosemicarbazone. The e.s.r. spectra of dimethylformamide solutions of the chelates have been interpreted in terms of dimeric species which are believed to exist at room temperature, as well as in frozen solutions in the case of o-methyl-mercaptobenzoic acid, S-carboxydiethyldithiocarbamic acid, and 5-hydroxy-1,3-benzoxathiol-2-one, whereas the dimeric forms of the remaining chelates are formed solely in frozen solution. The results have been interpreted as arising from dipole-dipole coupling between the copper(II) ions in their dimeric forms, and a line simulation procedure employed to extract the distance between the copper(II) dipoles. This information has been used to suggest the possible structures of the dimeric species. The effect of the introduction of zinc(II) ions during the formation of the copper(II) chelates has been assessed in each case.

Autooxidation of tin(II) in citric acid-sodium citrate solutions has been studied and the rate of oxidation was described by a rate law first order in tin(II) concentration. The oxidation is inhibited by relatively small concentrations of nitrate ion though nitrate is not consumed in the reaction. It has been found that during the oxidation of tin(II) to tin(IV) keto-acid breakdown products of citric acid are formed and that their presence is critical in overcoming the inhibiting effect of the nitrate ion.