Hemilability

Hemilability is a property of certain multidentate ligands in coordination chemistry and homogeneous catalysis. In a hemilabile system, one donor site forms a strong bond to a metal center while a second donor site is bound more weakly and can reversibly dissociate. This creates a temporarily open coordination position that can accommodate substrates or reactive intermediates during a catalytic cycle, without requiring dissociation of the entire ligand framework.

Typically, hemilabile ligands are bidentate or tridentate, featuring two or more donor sites with differing binding strengths. A common motif is a phosphine donor paired with a pendant ether or thioether donor (P,O- or P,S-type ligands). The phosphine tends to bind robustly to late transition metals, while the pendant donor is more labile and can detach to free a coordination site when a substrate approaches or a step in the catalytic cycle requires reorganization. Other examples include N,O-, S,O-, or P,N- donor combinations that exhibit similar reversible binding at the second site.

The utility of hemilability lies in its ability to modulate the metal’s coordination environment in situ. By providing a controllable, temporary vacancy, hemilabile ligands can facilitate substrate binding, stabilize reactive intermediates, or enable key steps such as migratory insertion or reductive elimination in catalytic cycles. This design principle is exploited in various homogeneous catalytic systems, including hydroformylation, hydrogenation, and cross-coupling, where selective lability enhances activity, selectivity, or turnover without compromising ligand integrity.

See also: chelation, multidentate ligands, ligand design in catalysis.