PNNL

Abstract

[Ni(dddt)2]– (dddt = 5,6-dihydro-1,4-dithiine-2,3-dithiolate) and [Ni(edo)2]– (edo = 5,6-dihydro-1,4-dioxine-2,3-dithiolate) are two donor-type nickel bis(dithiolene) complexes, with the tendency of donating low binding energy electrons. These two structurally similar complexes differ only with respect to the outer atoms in the ligand framework where the former has four S atoms while the latter has four O atoms. Herein, we report a negative ion photoelectron spectroscopy (NIPES) study on these two complexes to probe electronic structures of the anions and their corresponding neutrals. The NIPE spectra exhibit the adiabatic electron detachment energy (ADE) or, equivalently, the electron affinity (EA) of the neutral [Ni(L)2]0 to be relatively low for this type complexes, 2.780 and 2.375 eV for L = dddt and edo, respectively. The 0.4 eV difference in ADEs shows significant substitution effect for sulfur in dddt by oxygen in edo, i.e., noninnocence of the ligands, which has decreased the electronic stability of [Ni(edo)2]– by lowering its electron binding energy by ~0.4 eV. The observed substitution effect on gas-phase EA values correlates well with the measured redox potentials for [Ni(dddt)2]–/0 and [Ni(edo)2]–/0 in solutions. The singlet-triplet splitting (EST) of [Ni(dddt)2]0 and [Ni(edo)2]0 is also determined from the spectra to be 0.57 and 0.53 eV, respectively. Accompanying DFT calculations and molecular orbital (MO) composition analyses show significant ligand contributions to the redox MOs and allow the components of the orbitals involved in each electronic transition and spectral assignments to be identified.