Molecular Motion of the Bis(maleonitriledithiolato)nickel Trianion in Solution

Electron spin resonance (ESR) has been used to study the reorientational motion of the bis­(maleonitriledithiolato)nickel trianion, [Ni­(mnt)₂]^3–, in diethylene glycol dimethyl ether (diglyme). [Ni­(mnt)₂]^3– has one unpaired electron and was prepared by reducing the dianion, [Ni­(mnt)₂]^2–, with potassium metal. The trianion and dianion are members of the redox series [Ni­(mnt)₂]ⁿ⁻ with n = 0, 1, 2, and 3. The monoanion, [Ni­(mnt)₂]⁻, also has S = 1/2 and its rotational diffusion in diglyme was the subject of previous ESR studies. This made possible the comparison of the reorientational data for two different oxidation states of the same planar complex in the same solvent. Differences were found; isotropic rotational diffusion produced agreement between the trianion’s experimental and calculated spectra, whereas the monoanion’s simulations required axially symmetric reorientation with diffusion about the long in-plane axis three times faster than that about the two perpendicular axes. At a given temperature, the monoanion’s reorientation rates about the long in-plane axis and two perpendicular axes were faster than the trianion’s isotropic rate by factors of ∼27 and ∼9, respectively. These differences suggest that [Ni­(mnt)₂]⁻ and [Ni­(mnt)₂]^3– have different shapes and sizes in solution; the monoanion is approximately a prolate ellipsoid, whereas the trianion is larger and more spherical. [Ni­(mnt)₂]^3– appears to be ion-paired, whereas in accord with results from other techniques, [Ni­(mnt)₂]⁻ is not.