The mechanism of substitution reactions in octahedral coba1t(111) complexes of the type [CoXY en,]"+, where en represents 1,2-diaminoethane, and X and Y monodentate ligands, has been extensively re~iewed.l-~ In aquation studies, where studies of oxygen exchange in aquo complexes would have been useful in the interpretation of mechanism, the techniques are difficult, and not a great amount of work has been a t t e m~t e d .~,~ In most of the non-aqueous solvent studies7-10 the differentiation between unimolecular (SN1) and bimolecular solvolytic SOL) mechanisms of substitution has proved difficult because intermediates containing the coordinated solvent have not been isolated.Recent work in dipolar aprotic ~o l v e n t s , l~-~* where stable complexes containing the coordinated solvent have been prepared, has emphasized the need for solvent exchange results, and in these systems n.m.r. spectroscopy will prove a useful technique in that proton resonances in such ligands as dimethyl sulphoxide (DMSO) and NN-dimethylformamide (DMF) are shifted significantly by coordination. 15 We have studied exchange of the solvent DMSO-d, for DMSO in complex ions of the type cis-[CoX(DMSO) en2I2+ where X = C1-, Br-, and NO;, and in the complex ion cis-[Co(DMSO), en,I3+ a t the probe temperature (35") in a Varian A60 analytical n.m.r. spectrometer. (DNSO-d, was used as supplied by Merck, Sharpe & Dohme at 99.9% purity.) Rate constants were calculated from the areas of the methyl proton resonances of the coordinated and free dimethyl sulphoxide molecules.
The aquation of each of
three octahedral chloro(solvent)bisethylene- diaminecobalt(III) ions, involving the solvent ligands
dimethyl sulphoxide (DMSO), dimethylformamide (DMF), and dimethylacetamide
(DMA), has been examined in solutions of different pH. In solutions of pH less
than 6, the predominant reaction is replacement of the solvent molecule by
water in an SN1 process, involving a trigonal bipyramidal transition
state and resulting in a mixture of the trans- and cis- chloroaquo complexes.
The slow loss of chloride ion is an accompanying side reaction. In solutions of
pH greater than 6 base hydrolysis becomes important, and in alkaline solutions
the rate of solvent loss is too fast to measure by conventional techniques. The
rate of liberation of chloride ion is also greatly increased with increasing
pH. Values of the activation energy, and entropy of activation, are reported
for the solvent replacement reaction at a pH of 1.90.
The kinetics and mechanism of the
substitution of chloride ion and thiocyanate ion into the cis-chlorodimethylformamidebisethylenediaminecobalt(111) ion, cis-[CoCl(DMF) en2]2+,
have been studied in ,NN-dimethylformamide (DMF).
The chloride entry shows mixed kinetics
which are accounted for by two paths, a slow ion pair dissociation, and a fast
bimolecular attack of chloride on an ion pair. The steric course of the
reaction, which is chloride dependent, shows that the ion pair dissociative
mechanism is more important at low chloride concentrations.
The thiocyanate entry shows similar
characteristics. Thiocyanate enters faster than chloride in the bimolecular
path, but in the ion pair dissociative path, which is significant at lower
anion concentration, both chloride and thiocyanate enter at the same rate.
Thermodynamic acidity
constants have been measured over the temperature range 5-50� for aqueous
solutions of sodium 4?-dimethylaminoazobenzene- 4-sulphonate (methyl orange)
and sodium 4?-dimethylaminoazobenzene-2- sulphonate (ortho-methyl orange). From
these data values of the standard enthalpy, entropy, and heat capacity changes
have been calculated for these compounds. These results are discussed in
conjunction with previous spectrophotometric and other data with reference to
the nature of the equilibrium systems involved in these protonation reactions.
It is concluded that existing evidence does not allow an unequivocal assignment
of the sites of protonation of these and related molecules.
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