Electron spin resonance studies of the reorientational motion of Ni(mnt)2(-).

Electron spin resonance studies of the planar bis(maleonitriledithiolato)nickel complex ion, Ni(mnt)(2)(-), have been carried out from the motional narrowing region to the glassy limit in a series of ethers: 2-methyltetrahydrofuran (MTHF), diethylene glycol dimethyl ether (diglyme), triethylene glycol dimethyl ether (triglyme), and tetraethylene glycol dimethyl ether (tetraglyme). Analyses of the spectra show that Ni(mnt)(2)(-) is reorienting a factor of 3 faster about its long in-plane axis in all of these solvents; i.e., axially symmetric rotational diffusion produces agreement between the experimental and calculated line widths with D(parallel)/D(perpendicular) = 3.0 +/- 0.2; D(parallel) and D(perpendicular) are the diffusion constants for reorientation about the long in-plane (parallel) and perpendicular axes, respectively. The temperature dependence of the reorientational correlation time tau(2)(0) = (6D(perpendicular))(-1) determined from the widths is in agreement with the modified Stokes-Einstein-Debye model; the results indicate that Ni(mnt)(2)(-) has relatively strong (but not associative) interactions with the ethers. The experimental values of tau(2)(0) and the solvents' viscosities, self-diffusion constants, and dielectric relaxation times are compared and found to have a common temperature dependence. The ESR data also are compared with values of tau(solv), the correlation time obtained when a fluorescent probe is excited and its emission is monitored as the nonequilibrium solvent distribution relaxes. tau(solv) and tau(2)(0) are found to have a common temperature dependence in MTHF, tetraglyme, and two other solvents (ethyl alcohol and 1-butanol) in which Ni(mnt)(2)(-) has been studied. The factors determining these transport properties are discussed.

Molecular motion of a nickel-bis(dithiolato) complex in solution.

The molecular motion of the planar bis(maleonitriledithiolato)nickel anion, Ni(mnt)(2)(-), has been studied as a function of temperature using electron spin resonance (ESR) in several polar solvents; they are ethyl alcohol, eugenol, dimethyl phthalate, tri-n-butyl phosphate, tris(2-ethyl-hexyl)phosphate, diglyme, and a dimethylformamide-chloroform mixed solvent. Calculated spectra in agreement with the experimental X-band spectra are obtained using axially symmetric reorientation when the long in-plane axis is the unique (parallel) axis of the rotational diffusion tensor with D parallel/D perpendicular = 3.0-4.0; D parallel and D perpendicular are the diffusion constants for reorientation about the parallel and perpendicular axes, respectively. The reorientational model required for the simulations is either in or close to the Brownian rotational diffusion limit. In the slow motional (low temperature) region, the spectra can be simulated using the glassy g values. As the temperature increases, however, agreement is obtained only if the intermediate g factor, g(y), for the non-axially symmetric Zeeman interaction increases while g(x), g(z), and the motional model remain unchanged; this scheme and others for which gx and g(z) are possibly temperature-dependent are discussed. The values of D perpendicular from the simulations are in general agreement with those from earlier analyses of the width of the central spectral feature. The simulations and width analyses indicate (as do electrochemical, conductivity, and vapor-phase osmometry data) that the paramagnetic species reorienting in solution has a shape similar to that of the Ni(mnt)(2)(-) ion.