Solvent effects on the kinetics of the reactions of nickel(II) and cobalt(II) ions with 2,2′-bipyridyl and 2,2′,2″-terpyridyl

Abstract

The rates and activation parameters for substitution of bipyridyl and terpyridyl at NiII and Corl ions have been measured in water and some non-aqueous solvents. A comparison is made with the corresponding rates of solvent exchange for these ions, and the applicability of the model usually suggested for aqueous solutions is examined. The behaviour of water is apparently not general. Approximate relationships are described linking the divergence of substitution rates from exchange rates with properties of the solvent which reflect its structure, such as its enthalpy of evaporation and its fluidity.MUCH evidence has accumulatedl in support of the hypothesis that, in the substitution of a ligand (L) into the first hydration sphere of a metal cation (M2+), the rate-determining step involves the loss of a solvent molecule (S) from that sphere. This is thought to be preceded by the rapid formation of an ' outer-sphere complex,' as in equation (1). This reaction schemepredicts the correct rate law, first-order in M2+ and in L.If k , is the first-order rate constant for exchange of a solvent molecule between the first hydration sphere and the bulk solvent, is the second-order rate constant for the overall forward reaction, and KO is the equilibrium constant for the formation of the outer-sphere complex, then the kinetic behaviour in the simplest case, with M2+ ion in large excess, should be given by equation (2). Even for aqueous solutions, this equationis obeyed much less accurately than is often supposed. For instance, the observed values for the reactions of nickel(I1) ion with uncharged ligands vary by a factor of ca. 20 (leaving aside the reactions of aliphatic diamines and related ligands for which a special mechanism is proposed).4 These variations in ki remain largely unexplained. They are sometimes rationalised by adjusting the value of K O ; for uncharged ligands this is not independently measurable ,5 but large variations would not be expected (cj. below) and the interpretation is unconvincing. Moreover, the lifetime of such a complex, if its formation is diff usion-controlled, would not be much longer than the time of an encounter.The outer-sphere-complex treatment leads to the same transition state as the interchange mechanism MS,

Rates of Reaction of Nickel (II) with Terpyridine in Dimethyl Sulfoxide

“…The present kinetic results fall into the pattern observed previously (6,7,19) for substitution reactions of Ni(I1) ion in DMSO solution ; these results are, for convenience, collected together in Table 4. Activation parameters are available only for the present systems (Table 3), plus a value of 12.6 kcal mol-' for the activation enthalpy, and -7.7 cal deg-' mol-' for the activation entropy, for the bipyridine reaction.…”

“…We are indebted to a referee for drawing to our attention the work of Bennetto and Caldin (19), on the (terpy + Ni2+) reaction, which appeared while the present work was in the process of being refereed ; the activation parameters from this work (19), quoted in Table 3, were for comparison purposes computed from the original data (19) using the present procedure (18). The rate constants obtained in the present work are some 30% higher than the comparable data of Bennetto and Caldin (19), the activation enthalpies being in good agreement.…”

Thermodynamic analysis of 1,3-dicarbonylic monochelates of iron(III) from equilibrium and kinetic measurements

“…A very useful consideration is that the existence of such isokinetic correlations for a series of reactions is a good indication that the essential reaction mechanism is the same throughout a given series. Consequently, these results may predict mechanisms in structurally similar reactions when working in analogous experimental conditions [18].…”

Reaction kinetics and solvation in non-aqueous solvents

“…The third possibility has been considered by Coetzee but is regarded by him as unlikely (64,77,78). The second possibility was investigated by Bennetto, Caldin et al, as an explanation of the correlations of rate and activation parameters with physical properties of solvents related to the strengths of their structures, such as their enthalpies of evaporation; a general picture based on the Frank-Wen model was developed, with stress on the influence of L on the local solvent structure (79,80,81). Of these possible explanations, the first is favoured by Coetzee, partly because (as we have noted) monodentate ligands show much smaller deviations (77,78).…”

“…' Since solvent effects depend, as we have seen, on polarisability as well as dipole nment or dielectric constant, and also on specific solute-solvent interactions such as hydrogen-bond-U ing in which the solvent may behave either as donor or acceptor, it appears unprofitable to seek for a single property that will characterise the behaviour of a solvent towards all solutes. We do, however, wish to understand solute-solvent interactions eventually in terms of the properties of individual solvent molecules such as size, shape, dipole •ircment, charge distribution and polarisability; as aids towards such understanding we can also use macroscopic properties such as dielectric constant, free volume, viscosity (79) , heat capacity (88), enthalpy and entropy of evaporation (89) , and cohesive energy density (90).…”