Hemilability

In coordination chemistry and catalysis hemilability (hemi - half, lability - a susceptibility to change) refers to a property of many polydentate ligands which contain at least two electronically different coordinating groups, such as hard and soft donors. These hybrid or heteroditopic ligands form complexes where one coordinating group is easily displaced from the metal centre while the other group remains firmly bound; a behaviour which has been found to increase the reactivity of catalysts when compared to the use of more traditional ligands.

In general, catalytic cycles can be divided into 3 stages:

Traditionally the focus of catalytic research has been on the reaction taking place in the second stage, however there will be energy changes associated with the beginning and end steps due to their effect on the coordination sphere and geometry of the complex, as well as its oxidation number in cases of oxidative addition and reductive elimination. When these energy changes are large they can dictate the turn-over rate of the catalyst and hence its effectiveness.

Hemilabile ligands reduce the activation energy of these changes by readily undergoing partial and reversible displacement from the metal centre. Hence a co-ordinately saturated hemilabile complex will readily reorganise to allow the coordination of reagents but will also promote the ejection of products due to re-coordination of the labile section of the ligand. The low energy barrier between the fully and hemi coordinated states results in frequent inverconvertion between the two, which promotes a fast catalytic turn-over rate.

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