Hydricities Hydricity scale

DuBois has studied many examples of these reactions and from these data and related experiments has generated die "hydricity scale" shown in Table 12.2. Particularly electron-rich complexes and third-row metal complexes are stronger hydride donors than less-electron-rich complexes and first-row analogs. 

DuBois and co-workers introduced the thermochemical cycle in Scheme 4 as a means to determine the hydride donor power (AG°h-) or hydricity, of a cationic metal hydride. The method requires the knowledge of metal-hydride acidity (p- a) data and the electrode potentials for the oxidation of the metal-hydride conjugate base to two-electron oxidized counterpart, either by two successive one-electron processes (Equation (10), Scheme 4), or by one two-electron process (Equation (11), Scheme 4). The thermochemical cycle is derived from one that was introduced by Parker and co-workers for use in organic systems/ The accuracy of values on an absolute scale rests upon the... 

Scheme 4 Thermochemical cycle for determination of metal-hydride hydricities from pKa and electrode potential data. The quantity C in Equations (10) and (11), when electrode potentials and pKa data are obtained in the same solvent and E° data are referred to the Cp2Fe/Cp2Fe scale, is 333 kJ mor (solv = MeCN) and 387 kJ mor (solv = DMSO).

The general sequence of reactivity is, after Waters quinols > mono-hydric phenols > aldehydes, ketones, hydroxy acids, 1,2 glycols > alcohols, olefins. Here again we see that free radicals fit into a broad scale of reactivity. 

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