Description of the Ground State Wave Functions of Ni Dithiolenes Using Sulfur K-edge X-ray Absorption Spectroscopy

The pterin-dithiolene cofactor is an essential component of the catalytic sites of all molybdoenzymes except nitrogenase. Understanding its bonding to transition metals allows for development of
electronic structure/function correlations in catalysis. The electronic structure description for a series of
bis(dithiolene) complexes ([NiL2]Z, L = 1,2-Me2C2S2; Z = 2−, 1−, 0) using sulfur XAS provides the basis
for extension to the biologically relevant metal-containing dithiolenes. The transition dipole integral has
been developed for the dithiolene sulfur through correlation of XAS pre-edge energy positions of sulfide-,
thiolate-, and enedithiolate-S. The ground state wave functions of all three NiL2 complexes have more
than 50% S character experimentally demonstrating the noninnocent behavior of the dithiolene ligand.
The S K-edge experimental results are correlated with spin-unrestricted, broken-symmetry density functional
calculations. These show only limited spin polarization in the neutral complex and delocalized, ligand based
ground states for the mono- and dianionic complexes. These XAS and DFT results are correlated with
other spectroscopic features and provide insight into reactivity.