Oxidation State Localized Orbitals: A Method for Assigning Oxidation States Using Optimally Fragment-Localized Orbitals and a Fragment Orbital Localization Index

Oxidation states represent the ionic
distribution of charge in
a molecule and are significant in tracking redox reactions and understanding
chemical bonding. While effective algorithms already exist based on
formal Lewis structures as well as using localized orbitals, they
exhibit differences in challenging cases where effects such as redox
noninnocence are at play. Given a density functional theory (DFT)
calculation with chosen total charge and spin multiplicity, this work
reports a new approach to obtaining fragment-localized orbitals that
is termed oxidation state localized orbitals (OSLO), together with
an algorithm for assigning the oxidation state using the OSLOs and
an associated fragment orbital localization index (FOLI). Evaluating
the FOLI requires fragment populations, and for this purpose a new
version of the intrinsic atomic orbital (IAO) scheme is introduced
in which the IAOs are evaluated using a reference minimal basis formed
from on-the-fly superposition of atomic density (IAO-AutoSAD) calculations
in the target basis set and at the target level of theory. The OSLO
algorithm is applied to a range of challenging cases including high
valent metal oxide complexes, redox noninnocent NO and dithiolate
transition metal complexes, a range of carbene-containing TM complexes,
and other examples including the potentially inverted ligand field
in [Cu­(CF3)4]−. Across this
range of cases, OSLO produces generally satisfactory results. Furthermore,
in borderline cases, the OSLOs and associated FOLI values provide
direct evidence of the emergence of covalent interactions between
fragments that nicely complements existing approaches.