Metrical Oxidation States of 2-Amidophenoxide and Catecholate Ligands: Structural Signatures of Metal–Ligand π Bonding in Potentially Noninnocent Ligands

Catecholates and 2-amidophenoxides are prototypical “noninnocent”
ligands which can form metal complexes where the ligands are best
described as being in the monoanionic (imino)­semiquinone or neutral
(imino)­quinone oxidation state instead of their closed-shell dianionic
form. Through a comprehensive analysis of structural data available
for compounds with these ligands in unambiguous oxidation states (109
amidophenolates, 259 catecholates), the well-known structural changes
in the ligands with oxidation state can be quantified. Using these
correlations, an empirical “metrical oxidation state”
(MOS) which gives a continuous measure of the apparent oxidation state
of the ligand can be determined based on least-squares fitting of
its C–C, C–O, and C–N bond lengths to this single
parameter (a simple procedure for doing so is provided via a spreadsheet
in the Supporting Information). High-valent d0 metal complexes,
particularly those of vanadium­(V) and molybdenum­(VI), have ligands
with unexpectedly positive, and generally nonintegral, MOS values.
The structural effects in these complexes are attributed not to electron
transfer, but rather to amidophenoxide- or catecholate-to-metal π
bonding, an interpretation supported by the systematic variation of
the MOS values as a function of the degree of competition with the
other π-donating groups in the structures.