Kinetics (and thermodynamics) of substitution reactions

We're first going to look at some simple descriptions of kinetics of substitution reactions, and then move onto a more detailed examination of their thermodynamics. In order to do this, we first need to understand the difference between the two.

Thermodynamics is a measure of the extent to which a particular species will form from other species when the system has reached equilibrium.

Kinetics refers to the speed at which the transformation leading to attainment of equilibrium will occur.

Equilibrium constants, often represented by capital K, tell you about the position of equilibrium, they don't tell you anything about how fast the reaction will go

Rate constants, usually represented by lower case k, tell you about the rate of reaction

In terms of the kinetics of the substitution reactions of hexa-aquo complexes [M(OH2)6]n+, we can divide the complexes into labile and inert..

Labile complexes undergo rapid substitution reactions e.g. within 60 seconds

Inert complexes undergo slow substitution reactions e.g. minutes, days, weeks, years...

For example:

[Cu(OH2)6]2+ (aq) + NH3 (aq) instantaneous

[Cr(OH2)6]3+ (aq) + NH3 (aq) several hours

As for metal hexa-aquo ions in the first transition series, most are labile. The only two that are not are Cr(III) and Co(III). And that is why we often start off with a solution of Co(II) when we want to make complexes of Co(III): Co(II) is labile so we can do rapid substitution reactions on it and then oxidise it pretty easily to Co(III), whereas if we started off with Co(III), which is inert, our substitution reaction would be much slower.

We can measure the rate of these reactions by dissolving a hexa-aquo complex in water labelled with 18O and then monitoring how long it takes for the labelled water to exchange with the unlabelled ones in the inner coordination sphere. You ought to be aware that for some hexa-aquo ions in the 2nd, and perticularly the 3rd transition series, substituion reactions can take a long time... thousands of years in some cases!

While we're on the subject of hexa-aquo ions, you ought to be aware that they are to some extent acidic. Attaching a water molecule to a metal centre takes some of the electron density from the oxygen and consequently makes the hydrogens easier to pull off as H+. Hence if you take something like hydrated iron(III) chloride, dissolve it in water, and measure the pH, you will find that it is somewhat acidic. You can find a Table of the acidities of some metal aquo complexes here.

On the next page we start to discuss equilibrium constants of substitution reactions.


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This page was written by Dr Mike Morris, March 2001

Last updated March 2012