A series
of mononuclear six-coordinate tungsten compounds spanning
formal oxidation states from 0 to +VI, largely in a ligand environment
of inert chloride and/or phosphine, was interrogated by tungsten L-edge
X-ray absorption spectroscopy. The L-edge spectra of this compound
set, comprised of [W0(PMe3)6], [WIICl2(PMePh2)4], [WIIICl2(dppe)2][PF6] (dppe = 1,2-bis(diphenylphosphino)ethane),
[WIVCl4(PMePh2)2], [WV(NPh)Cl3(PMe3)2], and [WVICl6], correlate with formal oxidation state and
have usefulness as references for the interpretation of the L-edge
spectra of tungsten compounds with redox-active ligands and ambiguous
electronic structure descriptions. The utility of these spectra arises
from the combined correlation of the estimated branching ratio of
the L3,2-edges and the L1 rising-edge energy
with metal Zeff, thereby permitting an
assessment of effective metal oxidation state. An application of these
reference spectra is illustrated by their use as backdrop for the
L-edge X-ray absorption spectra of [WIV(mdt)2(CO)2] and [WIV(mdt)2(CN)2]2– (mdt2– = 1,2-dimethylethene-1,2-dithiolate),
which shows that both compounds are effectively WIV species
even though the mdt ligands exist at different redox levels in the
two compounds. Use of metal L-edge XAS to assess a compound of uncertain
formulation requires: (1) Placement of that data within the context
of spectra offered by unambiguous calibrant compounds, preferably
with the same coordination number and similar metal ligand distances.
Such spectra assist in defining upper and/or lower limits for metal Zeff in the species of interest. (2) Evaluation
of that data in conjunction with information from other physical methods,
especially ligand K-edge XAS. (3) Increased care in interpretation
if strong π-acceptor ligands, particularly CO, or π-donor
ligands are present. The electron-withdrawing/donating nature of these
ligand types, combined with relatively short metal–ligand distances,
exaggerate the difference between formal oxidation state and metal Zeff or, as in the case of [WIV(mdt)2(CO)2], exert the subtle effect of modulating the
redox level of other ligands in the coordination sphere.
