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Diagrams that plot the free energies of formation of aqueous species, or enthalpies of formation of ionic solids, against the oxidation number n for metallic elements have been exploited by a number of authors in order to display the relative stabilities of oxidation states. The lack of experimental data for unstable oxidation states, and difficulties in estimating required enthalpies/free energies of formation, have limited the utility of such diagrams.
The stability index An of a metallic element M in the +n oxidation state is defined by the equation:
An = DeltaHof(Mn+, g) - an(n + 1)/(r + b)
where a is an empirical constant (taking the recommended value 1150 kJ mol-1
A), and r is the metallic radius of M in A. b is a constant which reflects the covalent radius of the element bonded to M in a binary compound, and is allocated a representative value of 1.0 A, although this can be varied in order to compare, e.g., fluorides with iodides. Stability index diagrams plot An against n. Examples given for elements of the 3d and 4f series, and for the Group 13 elements, demonstrate the value of such diagrams, and their construction helps students and teachers to rationalize the relative stabilities of oxidation states.
A letter from Derek W. Smith in our April 2000 issue addresses the above.
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