The charge-transfer bands of I<sup>−</sup> in thin film spectra of MCl<sub>2</sub> (M = Mn, Co, and Ni)-rich mixtures of tetra-<i>n</i>-alkyl* (A* = butyl, pentyl, hexyl, and heptyl, respectively), and tributylpropyl-ammonium iodides have been found to resemble those of crystalline potassium −, rubidium −, and cesium-iodides. (The band separation corresponds to 0.87 to 0.97 eV as against 0.94 to 1.15 eV found in the spectra of crystalline KI, RbI, and CsI, and 0.94 eV for 5<sup>2</sup>P<sub>3/2</sub> ← 5<sup>2</sup>P<sub>½</sub> transitions in the case of iodine atom.) In addition to the usual <i>d-d</i> bands, the charge-transfer bands of CoX<sub>4</sub>− and NiX<sub>4</sub>− which are ordinarily obscured by the absorption of X<sup>−</sup> are also observed. The band assignments of NiX<sub>4</sub>− have been made in the light of the calculated electronic energy levels of tetrahalonickel (II) complexes using the Liehr-Ballhausen secular determinants for the d system in a cubic tetrahedral field based on the four ligand-field parameters (NiCl<sub>4</sub>−: λ = -275, B = 740.00, C/B = 3.97, Dq = 358; NiCl<sub>2</sub>Br<sub>2</sub><sup>−</sup>: λ = -275, B = 713.75, C/B = 3.93, Dq = 346; and NiCl<sub>2</sub>I<sub>2</sub><sup>−</sup>: λ = -275, B = 637.75, C/B = 4.7, Dq = 346 cm<sup>−1</sup>). Similarly, the charge-transfer bands of Br<sup>−</sup> and Cl<sup>−</sup> are also obtained in thin film spectra of MCl<sub>2</sub> (M = Mn, Co, and Ni) in tetrabutyl<sub>-</sub>, and trioctylpropyl-ammonium bromides, and tetrabutyl-ammonium chloride, respectively. The degree of dissociation of MX<sub>4</sub>− is negligible in pure (A<sub>4</sub>N<sup>+</sup>)<sub>2</sub>MX<sub>4</sub>− (where A stands for either the same or different alkyl groups) as apparent from the decrease in the electrical conductances with [MX<sub>2</sub>]. The species, (A<sub>4</sub>N<sup>+</sup>)<sub>2</sub>MX<sub>4</sub>−, appear to be held together with increased intermolecular forces at low temperatures as evident from the (increase in the Eη, energy of activation of viscosity) kinematic viscosity measurements. The data on the energies of activation of conductances and viscosities as functions of composition and temperature suggest an increased molecular association (as their ratio Eη/EΛ ≈ 1) in such systems. The conductances and viscosities have been analyzed in terms of different functional forms based on the free volume model. The Walden's product rule has been found applicable in these systems.

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