Pulsed ENDOR and ESEEM Study of [Bis(maleonitriledithiolato)nickel]^-:  An Investigation into the Ligand Electronic Structure

Abstract

By a combination of Q-band pulsed ENDOR (electron nuclear double resonance) and X-band ESEEM (electron stimulated echo envelope modulation) techniques, we have determined the hyperfine tensors for ethylene (C1) and cyano (C2) carbons and N, of [Ni(mnt)(2)](-), along with the quadrupole tensor for nitrogen. These measurements give pi electron spin densities of rho(C1) approximately 0.03 in the C1 2p(z)() orbital, rho(C2) < 0.003, rho(N) approximately 0.01, such that in total, approximately 0.15 of the spin resides on the ligand atoms C and N, while the rest resides in the NiS(4) core, giving rho(NiS(4)(-)) = 0.85. These results are compared with extended Hückel and density functional (BLYP) MO calculations, as well as with Xalpha calculations reported earlier.

The Electronic Structure and Spectroscopy of Metallo‐Dithiolene Complexes

“…It is comforting to note that the results of extended Hückel, Xa, and DFT calculations support the experimental results, in that the bulk of the spin density is localized on the NiS 4 core, with the majority located on the four S atoms of the two mnt 2À ligands (355,381). The large sulfur contribution to the b 2g (381).…”

“…The assumption of opposite signs for the parallel and perpendicular dipolar Ni hyperfine coupling constants by Maki and co-workers (387, 388) resulted in 25-50% of the spin density being localized on the Ni center. The results of Hoffman et al (381) provide an estimate of r(Ni) $ 0.32, which is on the low side of that suggested by Maki and co-workers (387,388). Since the total spin density localized on the NiS 4 core is 0.85, this results in $50-60% of the spin density localized on the four dithiolene sulfur atoms.…”

“…c Xa Spin densities are from Sano et al (355). The EHMO and BLYP calculations are described in (381).…”

“…It is also interesting to compare the spin density ratio of specific atoms on the mnt 2À ligand in order to assess the degree of p delocalization over the entire ligand in [Ni(mnt) 2 ] 1À and [Ni(tds) 2 ] 1À . Specifically, the r(C ethylene )/r(S) ratio is 0.16, which indicates the remarkable ability of the S 3p z orbitals to efficiently couple with the 2p orbitals of the ethylenic carbon p z orbitals to promote extensive p-type delocalization (381). To summarize, the key result of these ENDOR and ESEEM studies, with respect to the redox chemistry of [Ni(mnt) 2 ] 1À and [Ni(tds) 2 ] 1À , is that the electroactive b 2g redox orbital is primarily ligand based.…”

Electron and Spin Density Topology of the H‐Cluster and Its Biomimetic Complexes

“…As discussed above, advanced paramagnetic spectroscopic techniques, such as electron-nuclear double resonance (ENDOR) and electron spin echo envelope modulation (ESEEM), provide another way to get quantitative information about the atomic spin densities indirectly from the metal hyperfine and, preferably, the ligand super-hyperfine interactions. [18] This technique is, however, limited to paramagnetic systems. Another direct way of determining the atomic spin densities is by analyzing the results of the electronic structure calculations from either the wave functionbased methods (ab initio MO) or the density functional theory (DFT) calculations.…”