An Objective Alternative to IUPAC's Approach To Assign Oxidation States

Abstract

The IUPAC has recently clarified the term oxidation state (OS), and provided algorithms for its determination based on the ionic approximation (IA) of the bonds supported by atomic electronegativities (EN). Unfortunately, there are a number of exceptions and ambiguities in IUPAC's algorithms when it comes to practical applications. Our comprehensive study reveals the critical role of the chemical environment on establishing the OS, which cannot always be properly predicted using fix atomic EN values. By identifying what we define here as subsystems of enhanced stability within the molecular system, the OS can be safely assigned in many cases without invoking exceptions. New insights about the effect of local aromaticity upon OS are revealed. Moreover, we prove that there are intrinsic limitations of the IA that cannot be overcome. In this context, the effective oxidation state (EOS) analysis arises as a robust and general scheme to derive an OS without any external guidance.

Can We Safely Obtain Formal Oxidation States from Centroids of Localized Orbitals?

“…No localized orbital corresponding to a N-O π bond is found and, instead, the O moiety bears two p-type lone-pairs. However, as sketched in Figure 4, the centroid of the σ orbital is closer to N than to O atom, and it lies within the atomic basin of N. Thus, for both CA and BA criteria the electron pair corresponding to the N-O σ bond should be assigned to N, leading to a final formal OS of (0) for O and (−3) for N. It is remarkable that the same alternative assignment was obtained using EOS analysis for the same level of theory, and even for a multireference wave function [5]. It appears that the actual electronic structure of this molecule is at odds with the straightforward application of the ionic approximation.…”

“…Moreover, Postils et al recognized that when the π-ligand exhibited some local spin (e.g., triplet character), the OS assignment driven by aromaticity should be (−1), as in the excited-state 8π electron rings become Baird aromatic [44]. This was indeed nicely predicted by EOS analysis [5].…”

“…The frontier EFOs are almost degenerate in these two examples, indicating a very unpolarized bond, and hence a very poor description of the electron distribution by any ionic model. Recently, Postils et al [5] applied EOS analysis to a set of over a hundred molecular systems. Most of them were included in the IUPAC reports for being either particularly challenging or ambiguous, but the set also included a number of additional examples including π-adducts, high-valent compounds and TM carbenes.…”

“…Moreover, a number of spectroscopic techniques and detailed information about the geometrical arrangement of the atoms are indirect experimental probes of the oxidation states (in the former case, referred as spectroscopic oxidation state). Yet, several recent works have exposed some intrinsic limitations of the ionic approximation [5,6]. Postils et al [5] concluded that, in π-adducts, the local spin state of the π-system determines its formal charge, and concomitantly that of the TM.…”

“…Yet, several recent works have exposed some intrinsic limitations of the ionic approximation [5,6]. Postils et al [5] concluded that, in π-adducts, the local spin state of the π-system determines its formal charge, and concomitantly that of the TM. The cycloheptatryenil (C 7 H 7 ) moiety can be found on his π-system as formally (+1) or (−3), fulfilling Hückel's rule, but also (−1) if it appears in triplet state, following instead Baird's rule.…”