Search

The chemical shift of the Cl-35 NMR signal was used as a probe for the investigation of the solvation of the chloride ion in solvent mixtures of water-acetonitrile and water-dimethyl sulfoxide. First it was shown that the chemical shift is independent of the nature of the cation and the salt concentration; that is, the counter ion has no effect on the experimental parameter. The chemical shift then should reflect the composition of the solvation sphere. In acetonitrile-water mixtures, chloride ion shows a strong preference for water; presumably the result of hydrogen bonding between H2O and Cl(-). In DMSO-water mixtures however, there appears to be nearly equal competition for Cl(-) solvation sites. A possible explanation of this unexpected behavior is advanced. (Author), Prepared in cooperation with Carleton Univ., Ottawa (Ontario), Dept. of Chemistry.

A summary is given of a program aimed at the elucidation of the factors which determine the rate of solvent exchange (and ligand substitution) reactions with transition metal ions in solution. In many octahedral complexes it has become apparent that the reaction is mainly dissociative in nature, and that it is relatively independent of the entering group. Hence the composition of the second coordination sphere (solvation shell) plays an important role in the reaction rate. It was necessary to work out techniques for the determination of the composition of the solvation shell of a complex ion. Several methods of accomplishing this were developed; all of them depending on nuclear magnetic resonance measurements. Then exchange rates with metal ions and dimethylformamide were studied in depth. Some other systems were examined in less detail. There has resulted from the work a useful technique for the study of the kinetics and mechanisms of solvolysis reactions. (Author)

From the preface: The subject of the mechanistic study of ligand substitution reactions is currently undergoing an exciting growth. New fast-reaction techniques have removed the upper limit on rates that can be measured, and extension to less familiar central metal atoms has begun in earnest. This might seem the wrong moment for review of the field. As yet, definitive treatment is possible only for those complexes involving monodentate ligands with cobalt(III) and platinurn(II). But, because information is so extensive for these systems, it is clear that they are functioning as models from which concepts and experiments are generated for application over the fast-growing range of the subject. We believe that this is an important moment to reopen debate on fundamentals so that concepts will be most felicitously formulated to aid growth of understanding. This monograph is centrally concerned with three aspects of those fundamentals. We have attempted to develop an approach to classification of ligand substitution reactions that is adapted to what seem to have emerged as the characteristic features of these reactions and is susceptible to operational tests. (We do recognize that any such scheme of ideas is necessarily obsolescent once it is formulated since new experiments will certainly follow immediately.) We have tried to evaluate the basis for making generalizations about ligand substitution processes and to formulate tests to show whether new reactions fall within familiar patterns. Finally, we have sought to base the models of ligand substitution processes in the language of molecular-orbital theory. We believe that MO theory is most useful, because it may be used to correlate rate data on complexes with the extensive information available from spectral and magnetic studies, yet differs from crystal-field theory in providing a natural place for consideration of the bonding electrons, which must be a principal determinant of reaction processes. To keep this essay