Toward a comprehensive definition of oxidation state (IUPAC Technical Report)

Abstract

A generic definition of oxidation state (OS) is formulated: "The OS of a bonded atom equals its charge after ionic approximation". In the ionic approximation, the atom that contributes more to the bonding molecular orbital (MO) becomes negative. This sign can also be estimated by comparing Allen electronegativities of the two bonded atoms, but this simplification carries an exception when the more electronegative atom is bonded as a Lewis acid. Two principal algorithms are outlined for OS determination of an atom in a compound; one based on composition, the other on topology. Both provide the same generic OS because both the ionic approximation and structural formula obey rules of stable electron configurations. A sufficiently simple empirical formula yields OS via the algorithm of direct ionic approximation (DIA) by these rules. The topological algorithm works on a Lewis formula (for a molecule) or a bond graph (for an extended solid) and has two variants. One assigns bonding electrons to more electronegative bond partners, the other sums an atom's formal charge with bond orders (or bond valences) of sign defined by the ionic approximation of each particular bond at the atom. A glossary of terms and auxiliary rules needed for determination of OS are provided, illustrated with examples, and the origins of ambiguous OS values are pointed out. An electrochemical OS is suggested with a nominal value equal to the average OS for atoms of the same element in a moiety that is charged or otherwise electrochemically relevant.

Comprehensive definition of oxidation state (IUPAC Recommendations 2016)

“…1 It scales trends in redox and acid-base properties, as well as in physical properties such as magnetism, and is a key component when tracking the course of chemical reactions. Thus, the concept of OS is important, and so is an agreedupon definition of what OS is and the algorithmic manner in which it is to be calculated.…”

“…The present authors have published a comprehensive Technical Report on OS [1]. It provides a summary of previous work and discussions concerning OS, and gives numerous examples of the application and use of algorithms to determine OS.…”

“…Several criteria were considered 3 for the sign of the ionic approximation [1], but only one of them is reflective of current use and common values of OS; the atom's contribution to the bonding MO, which is associated with the atomic-orbital energy as illustrated in Fig. 1.…”

“…4 Therefore, this contribution does not concern the electrons' origin upon bond formation (p. 1048, Ref. [1]), only their final allegiance. As Fig.…”

“…6 Of several scales discussed in Appendix B of Ref. [1], only the Allen electronegativity (Section 6.1) is truly independent of OS, since it relates to the average valence-electron energy of the free atom [3][4][5], as if the bonds implied in Fig. 1 were abstracted away (Fig.…”