Electron spin resonance (ESR) studies of the planar bis(maleonitriledithiolato)nickel anion radical, Ni(mnt) 2 -, have been carried out from the motional narrowing region to the glassy limit in ethanol (EtOH), tributyl phosphate (TBP), 4-allyl-2-methoxyphenol (eugenol), and a mixed solvent containing equal volumes of dimethylformamide and chloroform (DMF-CHCl 3 ). In all solvents, axially symmetric Brownian rotational diffusion produces agreement between the experimental and calculated ESR line widths of Ni(mnt) 2 -when the anisotropic Zeeman interaction makes the dominant contribution to the widths. The long in-plane axis of the radical is the diffusional symmetry (or parallel) axis; the rotational motion is fastest about this axis with D | /D ⊥ ) 3.0-4.0 (D | and D ⊥ are the diffusion constants for reorientation about the parallel and perpendicular axes, respectively). In EtOH and DMF-CHCl 3 , however, the calculated widths are not in agreement with experiment in the fast motional region near the minimum experimental width; possible reasons for this lack of agreement are discussed (spectra were taken in the fast motional region for the other solvents but not at temperatures high enough to observe the minimum experimental width). The calculated widths were determined as a function of the correlation time τ 2 (0) ) (6D ⊥ ) -1 . The temperature dependence of τ 2 (0) is discussed in terms of the modified Stokes-Einstein-Debye (SED) model and the Vogel-Tammann-Fulcher (VTF) equation. The results from the SED model indicate that Ni(mnt) 2 -has the strongest interaction with EtOH and the weakest interaction with TBP. The VTF analyses give the solvent-dependent parameters D (a measure of the strong or fragile nature of the liquid) and T 0 (a temperature below the glass transition temperature, T g ). The values of D and T 0 obtained from the Ni(mnt) 2 -line widths are (a) compared with the values from viscosities, dielectric relaxation, and light scattering and (b) used to calculate values of T g , which are compared with experiment. For both comparisons, the agreement is generally good, although some differences are found.

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