Thomas W. Swaddle

Silicate complexes of aluminum(III) in aqueous systems

Aluminum(III) forms complexes with aqueous silicic acid in neutral or mildly acidic media, and these species are important in the protection of plant and animal life against aluminum toxicity. In highly alkaline media, surprisingly high concentrations of various aluminosilicate solute species can be achieved at least temporarily, the lifetime of the homogeneous solution depending on the pH, the nature of the cation(s) present, and the Al and Si concentrations. The longevity of aluminosilicate complexes in solution may have technological consequences — for example, in causing silica carryover in the Bayer process for aluminum production. The kinetics of silicate exchange on small, acyclic aluminosilicate solute species in alkaline solutions are much faster than on either the corresponding all-silicate ions or cyclic aluminosilicate species. The kinetic lability of small aluminosilicate species is attributable in part to the ability of AlIII to expand its coordination number easily from four to six in acyclic structures, but also to the availability of OH groups for condensation reactions on aluminate centers, even at high pH where SiOH functions become deprotonated. One implication is that, contrary to the traditional picture of zeolite formation from structured ‘secondary building units’ pre-existing in solution, cyclic and cage-like aluminosilicate solute species are not directly involved in the crystallization of solid aluminosilicates such as zeolites from aqueous solutions but simply serve as reservoirs for small, active, acyclic species responsible for crystal growth.

Electrode reactions of metal complexes in solution at high pressures

Abstract Techniques for measuring the pressure dependencies of the electrode potentials (characterized by the cell reaction volume ΔVcell) and electrode kinetics (volume of activation ΔV‡) of metal complexes are described. Volumes of reaction ΔV for net redox reactions can be obtained by the combination of ΔVcell values. We show how ΔVcell can be broken down into contributions of the couple of interest and the reference electrode, and how the solvational and intrinsic components of the former can be evaluated. Values of ΔV‡ measured for electrode reactions of aqueous metal complex couples are precisely one-half of the ΔV‡ found for the corresponding outer-sphere self-exchange reactions, and can generally be accounted for by an extension of Marcus theory developed for the latter. When ΔV is known, the theory can be extended to ΔV‡ for redox ‘cross’ reactions; thus, ΔV‡ can serve as a criterion of a simple, adiabatic, outer-sphere reaction mechanism.

Estimation of the outer-sphere contribution to the activation volume for electron exchange reactions using the mean spherical approximation

Abstract The outer-sphere contribution to the volume of activation of homogeneous electron exchange reactions is estimated for selected solvents on the basis of the mean spherical approximation (MSA), and the calculated values are compared with those estimated by the Strank-Hush-Marcus (SHM) theory and with activation volumes obtained experimentally for the electron exchange reaction between tris(hexafluoroacetylacetonato)ruthenium(III) and -(II) in acetone, acetonitrile, methanol and chloroform. The MSA treatment, which recognizes the molecular nature of the solvent, does not improve significantly upon the continuous-dielectric SHM theory, which represents the experimental data adequately for the more polar solvents.

The role of solvent in inorganic reaction mechanisms, as elucidated by high pressure studies

Abstract Pressure effects on the rates of simple inorganic reactions in solution act primarily on intermolecular interactions, particularly solvation. They afford the possibility of “tuning” solvent properties without altering the solvent chemically. Outer-sphere self-exchange electron transfer reactions of transition metal complexes provide an opportunity to test this idea quantitatively, through an adaptation of Marcus-Hush theory. The theory is only moderately successful for organic solvents, as the solvent “seen” by the reactant behaves as if it is less compressible than bulk solvent. Furthermore, for multiply-charged ions or for solvents of low relative permittivity, the calculations become unstable even without the likely complication of ion pairing. For aqueous systems, the theory is successful except where one or more of the following complications applies: (a) changes in total spin multiplicity accompany the electron transfer process in complexes with flexible ligand systems; (b) cation-mediated pathways emerge in anion-anion self-exchange reactions; (c) the reactants are so small that the assumption that the solvent is a continuous dielectric fails; or (d) an inner-sphere sphere mechanism is operative.

Aqueous silicate exchange dynamics and 29Si nuclear magnetic relaxation: the importance of protonation equilibria

At high alkalinities, deprotonation of H3SiO4–(the main vehicle of Si exchange between aqueous silicate species) and its polymers results in reduced polymerization rates and a greater proportion of small silicate units.