Hydricity

Hydricity is a thermodynamic measure of a chemical species' ability to donate a hydride ion (H−) to another species. It is defined as the standard free energy change associated with hydride transfer from a donor to a hydride acceptor, or equivalently the standard free energy of heterolytic cleavage of a donor–hydrogen bond to give a cationic donor and a free hydride. In practical terms, hydricity is reported as ΔG°(H− transfer) for a defined solvent and temperature, with smaller values indicating stronger hydride donors. Because hydride transfer depends on the solvent, temperature, and the identity of the donor and acceptor, hydricity values are system-specific and are typically presented in units of kcal/mol or kJ/mol for a chosen solvent (e.g., acetonitrile or dimethyl sulfoxide).

Hydricity is related to, but distinct from, redox potential and pKa. It can be framed within proton–coupled electron transfer thermodynamics, linking hydricity to both the electron-donating ability of the donor and its H− affinity. This makes hydricity a useful descriptor for catalytic cycles that involve hydride transfer, such as CO2 reduction to formate, hydrogenation, and various organometallic transformations.

Measurement and computation: experimental hydricity values are obtained from hydride-transfer equilibria with well-characterized acceptors, or inferred from calorimetry and electrochemical data; computational hydricities are estimated from thermodynamic cycles or DFT calculations, often validated against known references.

See also: hydride donor, hydride affinity, proton-coupled electron transfer.