Facing the Challenges of Borderline Oxidation State Assignments Using State-of-the-Art Computational Methods

The oxidation state
(OS) of metals and ligands in inorganic complexes
may be defined by carefully curated rules, such as from IUPAC, or
by computational procedures such as the effective oxidation state
(EOS) or localized orbital bonding analysis (LOBA). Such definitions
typically agree for systems with simple ionic bonding and innocent
ligands but may disagree as the boundary between ionic and covalent
bonds is approached, or as the role of ligand noninnocence becomes
nontrivial, or high oxidation states of metals are supported by heavy
dative bonding, and so on. This work systematically compares IUPAC,
EOS, and LOBA across a series of complexes where OS assignment is
challenging. These systems include high-valent transition metal oxides,
transition metal complexes with noninnocent ligands such as dithiolate
and nitrosyl, metal sulfur dioxide adducts, and two transition metal
carbene complexes. The differences in OS assignment by the three methods
are carefully discussed, demonstrating the synergy between EOS and
LOBA. In addition, a clarity index for LOBA OS assignments is introduced
that provides an indication of whether or not its predictions are
close to the ionic–covalent boundary.