Labile and Inert Complexes

As chemists, when we talk about a stable complex, we are usually referring to the thermodynamic stability of the complex. The dissociation constant, Kd, of a complex is a measure of its thermodynamic stability; the size reflects how likely the complex is to dissociate under normal conditions. The smaller the value of Kd, the smaller the tendency for a complex to dissociate and the greater its stability.

The relative kinetic stabilities of complexes are described by the terms labile and inert.

Labile complexes undergo rapid exchange of its ligands, with half lives for substitution reactions, t1/2 < 1 minute

e.g. Labile complex

Inert complexes undergo slow exchange of its ligands, with half lives for substitution reactions, t1/2 > 1 minute

e.g. Inert complex

The cations that most commonly form inert complexes are Cr3+, Co3+, Pt2+ and Pt4+.

There is no connection between the thermodynamic stability of a complex and its lability towards substitution. For example, values of ΔhydGo for Cr3+ and Fe3+ are about the same, but [Cr(OH2)6]3+ undergoes substitution substantially more slowly than
[Fe(OH2)6]3+.


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Author: Kathryn Will
Document modification date: 07 May 2003