Kinetic Evaluation of Ligand Hemilability in Transition Metal Complexes

Hemilability is the property of hybrid ligands to undergo a reversible metal chelate opening process by rupture of the weakest coordinative bond, yielding a coordinatively unsaturated complex. While experimental evidences of the process rely on the observation of fluxional behavior in the metal complex or on the spectroscopic detection of the species with different chelating bites, various effects are speculatively attributed to hemilability. In this review, this property is discussed in the context of its effects on the rate and mechanisms of bimolecular reactions. Examples of kinetics performed on ligand substitution, oxidative addition, alkyne tautomerization, and dehydrobromination reactions illustrate that the ligand hemilability process can be detected through simple kinetic analyses and can be quantitatively evaluated with respect to alternative reaction pathways. Since this concept affects various fields of chemistry, e.g. catalysis, supramolecular chemistry, molecular sensing, and materials, the kinetic approach may assist the rational design of ligands with the expected properties.