No abstract

Ion-pair association constants have been determined from conductance measurements for the association of chloride and bromide ions with cations of the type cis-and fra/M-[CoXY(en)2]+, where "en" represents ethylenediamine, and X and Y represent either of the ligands chlorine or bromine. The data have been treated by the equation of Fuoss, Onsager, and Skinner.2 The solvents investigated are , -dimethylformamide, N,N-dimethylacetamide, and dimethyl sulfoxide. The results are presented together with limiting ionic conductances at 25°. The results emphasize that both the dipolar nature of the cis complexes and the hydrogen-bond donor properties of the weakly acid nitrogen protons of the ethylenediamine ligands are important in the association process.

Rate data are reported for two anion-cation reactions, the SN2 decompositions of trimethylsulphonium thiocyanate and trimethylsulphonium bromide, and for a dipole-dipole reaction between methyl bromide and dimethyl sulphide in a protic and a dipolar aprotic solvent of the same dielectric constant. The entropy and enthalpy of ion-pair formation are compared with the entropy and enthalpy of activation of the anion-cation reaction and with the overall entropy and enthalpy change for the decomposition of trimethylsulphonium bromide in dimethylacetamide. The anion-cation reaction has a high activation energy and a large positive entropy of activation, and the reverse dipole-dipole reaction has a low activation energy and a large negative entropy of activation. This situation is less pronounced in 88% methanol-water than in dimethylacetamide. Solvent and ionic strength influence the rate of the anion-cation reaction much more than that of the dipole-dipole reaction, although both reactions pass through the same transition state. Changes in the equilibrium and rate constants for anion-cation reactions, on transfer from dipolar aprotic to protic solvent, can be predicted from medium effects on anion-dipole reactions, on trihalide equilibria, and on the e.m.f. of halogen-halide cells. All are influenced by the much greater hydrogenbonding solvation of anions and the weaker solvation of dipolar molecules, transition states, or cations, by protic solvents relative to dipolar aprotic solvents.THE SN2 decomposition of trimethylsulphonium bromide [forward reaction (l)] is an excellent model for demonstrating some features of bimolecular anion-cation reactions in solution. Such reactions are more common in inorganic systems than with carbon compounds, but the latter provide a simpler, better understood model. Following the work of Hughes and Ingold2 and of Moel~yn-Hughes,~ perhaps more is known about the kinetics and mechanism of bimolecular substitution at a saturated carbon atom and, in particular, at a methyl carbon atom than about most reactions.We considered earlier4 the effect of ion association and of ionic strength on the rate of anion-cation reactions, using forward reaction (1) in ethanol a t 100" as a model. We now report rates for both the forward and the backward reactions (l), and thermodynamic data for the equilibrium (l), in dimethylacetamide, a dipolar aprotic solvent, and in 88% (w/w) methanolwater, a protic solvent of the same dielectric constant. ki CH,-S+(CH,), + Br-+ CH,Br + (CH,),S (I) kb Solvent, ionic strength, and temperature strongly influence the equilibrium constant, K = kf/kb, for reaction (1) because, contrary to classical ideas,2 the anion-cation reaction is much more susceptible to changes in these variables than is the dipole-dipole reaction. Much of the discussion is along conventional linesJe7 for solvent and salt effects on reaction rate. We feel, however, that the large effects which have been observed for two simple reactions of different charge type, but having the same transition state, ...

A spectrophotometric method for the evaluation of ion association constants has been developed. This method involves the preparation of solutions containing only the compound under consideration. A function relating the association constants to the concentration of compound and extinction coefficients has been derived. The association constants are best determined by an iterative method using a computer. The association constant for the cis-diohlorobisethylenediaminecobalt(111) cation, cis-[Co en2 Cl2]+, and chloride ion in NN-dimethylformamide (DMF) at 30� has been calculated by this new method and is compared with that of another established procedure. The association constant has also been determined for the association of cis-[Co en2 Cl2]+ and iodide in DMF at 30�.

Ion association constants at 30� have been determined for the cis-[Co en, Cl2]+Cl- ion pair in NN-dimethylformamide (DMF), NN-dimethylacetamide (DMA), and at 20.0�, 25.0�, and 30.0� in dimethyl sulphoxide (DMSO), by a spectrophotometric method. Association constants for the cis-[Co en2 Cl2]+Br- and the trans- [Co en2 Cl2]+Cl- ion pairs have also been determined in DMF at 30�.