Nickel Dithiolenes Revisited:  Structures and Electron Distribution from Density Functional Theory for the Three-Member Electron-Transfer Series [Ni(S₂C₂Me₂)₂]^0,1-,2-

Abstract

The complexes [Ni(S2C2Me2)2](z) (z = 0, 1-, 2-) have been isolated for the purpose of investigating their electronic structures in a reversible three-member electron-transfer series. Members are interrelated by reversible redox reactions with E(1/2)(0/1-) = -0.15 V and E(1/2)(1-/2-) = -1.05 V versus SCE in acetonitrile. The three complexes have nearly planar structures of idealized D(2)(h) symmetry. As the series is traversed in the reducing direction, Ni-S and C-S bond lengths increase; the chelate ring C-C bond length decreases from the neutral complex to the monoanion and does not change significantly in the dianion. Structural trends are compared with previous results for [Ni(S2C2R2)2)](1-,2-). Following the geometrical changes, values of nu(Ni)(-)(S) and nu(C)(-)(S) decrease, while the value of nu(C)(-)(C) increases with increased reduction. Geometry optimizations at the density functional theory (DFT) level were performed for all members of the series. Geometrical parameters obtained from the calculations are in good agreement with the experimental findings. The 5b(2g) orbital was identified as the LUMO in [Ni(S2C2Me2)2], the SOMO in [Ni(S2C2Me2)2](1-), and the HOMO in [Ni(S2C2Me2)2]2-. Unlike in the situation in the [M(CO)2-(S2C2Me2)2]z series (M = Mo, W; z = 0, 1-, 2-), the apparent contribution from the metal d orbital in the electroactive orbital is not constant. In the present series, the d(xz) contribution increases from 13 to 20 to 39% upon passing from the neutral to the monoanionic to the dianionic complex. Accurate calculation of EPR g-values of [Ni(S2C2Me2)2]1- by DFT serves as a test for the reliability of the electronic structure calculations.

The Electronic Structure and Spectroscopy of Metallo‐Dithiolene Complexes

“…28, where formally high-valent Ni Figure 28. 2À complexes, considerable metal-ligand redox interplay is anticipated in the more oxidized members of this series (328). Such interplay allows various oxidation state assignments to be postulated for both the metal and the associated dithiolene ligands.…”

“…Extensive electronic structure and spectroscopic studies of metallo-bis(dithiolenes) have been undertaken as a result of their potential involvement in electrically conducting (115,116,142,145,146,201,202,270,359,(361)(362)(363) and optoelectronic device technologies (113,119,160,248,(364)(365)(366)(367)(368)(369)(370)(371)(372)(373)(374), as well as their presence in the active sites of certain pyranopterin Mo and pyranopterin W enzymes (11, 13, 17, 19, 27, 28, 36, 45- There have been a considerable number of MO calculations performed on four-coordinate square-planar metallo-bis(dithiolene) complexes at various levels of theory (283,327,328,(379)(380)(381)(382)(383)(384). The most important differences in the results of these various calculations rests in the energy level ordering of the valence MOs as well as the degree of metal-sulfur covalency.…”

“…Very recently, a combined DFT and X-ray crystallographic study of the [Ni(S 2 C 2 Me 2 ) 2 ] 0,1À,2À electron-transfer series was undertaken by Holm and co-workers (328). This study has proved to be extremely important, in that it details the intimate relationship between the geometric and electronic structures of all members of this electron-transfer series at parity of the dithiolene ligand.…”

“…In particular, Kirk and co-workers (23) showed that RR spectroscopy is an extremely useful tool in making band assignments in oxo-molybdenum-mono(-dithiolenes). Specifically, the important c xy in-plane covalency in oxo-metal-mono(dithiolenes) suggests that this interaction, in concert with ligand p-delocalization, plays a fundamental role in the unusual ability of these ligands to support multiple redox states (86,87,110,281,(327)(328)(329)(330)(331) and to electronically buffer (16) metal centers to the large changes in charge that accompany these redox changes. Another role hypothesized for this in-plane covalency is to facilitate electron transfer in pyranopterin molybdenum and tungsten enzymes (20-24, 106, 110).…”

Toward a comprehensive definition of oxidation state (IUPAC Technical Report)

“…The SCCS conjugation is supported by the shortening of the CS bonds, expansion of CC bond and a corresponding change in vibration frequencies upon oxidation to the neutral complex [183]. The 8−N rule is obeyed on S as the most electronegative atom, and both C and S have octets.…”