Solvents and their role in determining inorganic mechanisms

Abstract

-Two aspects of the mechanisms of the reactions of metal complexes in nonaqueous solutions are discussed in this paper. Firstly, the contribution made by studies in non-aqueous solvents, and in particular dipolar aprotic solvents, to the understanding of the mechanisms of substitution reactions at transition metal centres is considered. Secondly, the mechanisms of electron transfer reactions between transition metal centres are discussed with particular reference to recent work on the role of the solvent. THE SUBSTITUTION REACTIONS OF TRANSITION METAL COMPLEXESIt is helpful to an understanding of these substitution processes to discuss why it is that dipolar aprotic solvents have had a smaller impact on the study of transition metal substitution kinetics than on similar studies in carbon chemistry. The value of the dipolar aprotic solvent to the understanding of the chemistry of carbon has derived from the importance of the bimolecular processes of substitution. In the bimolecular mechanisms the rate of nucleophilic attack by an anionic nucleophile shows a first order dependence on the concentration of the nucleophile, and the rate in any solvent is directly proportional to the activity of the nucleophilein the solvent. Thus anions, which have activities most characteristically dependent on transfer from protic to dipolar aprotic solvents, have rates of nucleophilic attack which can vary dramatically with the nature of the solvent.The dependence of such rates on solvent characteristics is now well understood and was most elegantly expressed by my Australian colleague, Prof.A.J.Parker in his important review in 1969 (Ref.1). In simple terms, Parker showed that small anions, hard in the Pearson sense (Ref.2), had greatly incrsd rates of bimolecular nucleophilic attack in dipolar aprotic solvents compared with protic solvents mainly because of the hydrogen bonding contribution to solvation of these anions in the latter solvents. Provided the changes in the solvation of the attacked substrate and the transition state are not important in comparison with these changes in anion solvation, differences in rate of as much as i6 fold can be found for bimolecular nucleophilic attack by say chloride ion in dimethylacetamide (DMA) compared with the rate in methanol, being faster in dimethylacetamide. Large polarizable anionic nucleophiles, soft in the Pearson sense (Ref.2), do not show this tendency, indeed their reactions are often slower in dipolar aprotic solvents because these ions are better solvated in these media.Unimolecular processes in carbon chemistry can also show marked solvent dependence particularly in those cases where considerable charge separation occurs in the transition state and thus the departing anion can manifest a good deal of its separate ion character. In such cases, if the departing ion is for instance the chloride ion, the reaction is significantly faster in a protic solvent compared with an aprotic solvent because of the magnitude of the hydrogen-bonding contribution to the solvation of...

Non-aqueous electrolyte solutions in chemistry and modern technology

“…Therefore the alkaline hydrolysis of esters, which is one of the most investigated secondorder model reactions in water and aqueous mixtures, is unsuited for the investigation of specific solvent effects despite many efforts 385 ~3 88 >. Most inorganic reactions of solvolysis and nucleophilic substitution of transition metal complexes are rather insensitive towards a change from protic to aprotic dipolar solvents 368) . Since the maximum coordination number of the complexes cannot be exceeded, in most cases the rate-determining step is a dissociation leaving a transition state with a positive charge almost one unit higher than the initial complex and an anion or, in few cases, a molecule.…”

Non-Aqueous Electrolyte Solutions in Chemistry and Modern Technology

“…Most inorganic reactions of solvolysis and nucleophilic substitution of transition metal complexes are rather insensitive towards a change from protic to aprotic dipolar solvents 368) . Since the maximum coordination number of the complexes cannot be exceeded, in most cases the rate-determining step is a dissociation leaving a transition state with a positive charge almost one unit higher than the initial complex and an anion or, in few cases, a molecule.…”

Titrations in nonaqueous solvents

“…The general area of reaction kinetics and solvation in nonaqueous solvents has been reviewed by Caldin (53). Solvent effects on rates and mechanisms of transition metal complexes have been considered by Burgess ( 50 this topic, and points out that the role of solvents in determining inorganic mechanisms is much smaller than in carbon chemistry owing to solvolytic effects and to the counteracting of changes in anion solvation by changes in solvation of the cationic residue (314).…”