Electron counting in solids: oxidation states, partial charges and ionicity

TL;DR

This paper discusses the assignment of ionic charges in solids, emphasizing the usefulness of formal oxidation states while cautioning against overinterpreting partial charges, which depend on analysis methods and properties studied.

Contribution

It clarifies the roles of oxidation states and partial charges, highlighting their appropriate interpretation and limitations in solid-state chemistry.

Findings

Formal oxidation states are useful for understanding solids.

Partial charges should be interpreted with caution and context.

Analysis methods influence charge assignment and conclusions.

Abstract

In this short viewpoint, we discuss the assignment of ionic charges in solids. We argue that formal oxidation states serve a useful function, and that absolute values of partial charges should be interpreted and applied with caution; the charge assigned can never be definitive and depends on the type of property studied and the type of analysis performed. Careful analysis can be used to avoid unphysical conclusions such as a recent report on the Ti(III) nature of Ti in stoichiometric TiO$_2$.