Abstract
The use of centroids of localized orbitals as a method to derive oxidation states (OS) from first-principles is critically analyzed. We explore the performance of the closest-atom distance criterion to assign electrons for a number of challenging systems, including high-valent transition metal compounds, π-adducts, and transition metal (TM) carbenes. Here, we also introduce a mixed approach that combines the position of the centroids with Bader’s atomic basins as an alternative criterion for electron assignment. The closest-atom criterion performs reasonably well for the challenging systems, but wrongly considers O-H and N-H bonds as hydrides. The new criterion fixes this problem, but underperforms in the case of TM carbenes. Moreover, the OS assignment in dubious cases exhibit undesirable dependence on the particular choice for orbital localization.
Highlights
Heuristic concepts play a key role in chemical knowledge
We explore the performance of the closest-atom distance criterion to assign electrons for a number of challenging systems, including high-valent transition metal compounds, π-adducts, and transition metal (TM) carbenes
The possibility of using centroids of localized orbitals is very attractive, as one could apply the same strategy for molecules and solids on equal footing
Summary
Heuristic concepts play a key role in chemical knowledge. It should not be a surprise that most of these chemical concepts are not observable in a strict quantum mechanical sense (they have no few detractors claiming its inherent arbitrariness). Still, they have undoubtedly proven very useful for shedding light into chemical phenomena and more importantly, for achieving true predictions (i.e., without performing an experiment or even a computational exercise). The problem of a chemical concept arises when it comes to its quantification as recently stressed by Grunenberg [1]. There is, a concept of utmost relevance in chemistry, whose flaw was the lack of a well-established definition, namely the oxidation state (OS)
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