CHEMISTRY

M. V. ANDREEVA, E. V. STROGANOV

SPECTROPHOTOMETRIC STUDY OF ETHANOL SOLUTIONS OF CoCl₂ AND CoBr₂ AT HIGH CONCENTRATIONS OF Cl⁻ AND Br⁻ ANIONS

(Presented by Academician I. I. Chernyaev, April 4, 1963)

Information on the composition of complex compounds formed in ethanol solutions of Co(II) halides is contradictory \(^{(1–7)}\). There are no data on the structure of these complexes.

The spectral absorption curves of the system CoBr₂—C₂H₅OH—LiCl, measured at varying LiBr concentration by S. A. Shukarev and O. A. Lobanova, give grounds to suppose the existence in this system of several Co(II) compounds.

In the present work an attempt is made to determine the composition and structure of those Co(II) halide complexes which are formed in anhydrous ethyl alcohol at extremely high concentrations of Cl⁻ and Br⁻ anions. On an SF-10 recording spectrophotometer, with a spectral slit width of 1–1.5 mμ, we measured the absorption curves of solutions of the following composition: CoCl₂—C₂H₅OH—LiCl and CoBr₂—C₂H₅OH—LiBr. The concentration of Co²⁺ ions was varied from \(5 \cdot 10^{-4}\) g/l to \(10^{-1}\) g-mol/l; the concentration of LiCl and LiBr was varied from 0 to 4.5 g-mol/l.

Fig. 1. Spectral absorption curves of crystals Cs₂CoCl₄ and Cs₃CoCl₅ (1), and of a CoCl₂—C₂H₅OH—LiCl solution (2);
\(C_{\mathrm{CoCl_2}} = 0.01\,M;\quad C_{\mathrm{LiBr}} = 4.5\,M\)

An increase in the concentration of LiCl and LiBr in solutions of CoCl₂ and CoBr was accompanied by an increase in optical density in the region 500–750 mμ, a shift of the absorption curves toward lower frequencies, and the appearance of new absorption maxima. At concentrations \(C_{\mathrm{LiCl}} > 1.5\) g-mol/l and \(C_{\mathrm{LiBr}} > 2.5\) g-mol/l, the spectrum of the solutions becomes stabilized; a further increase in the concentration of Li halides is not reflected in the positions of the absorption maxima, nor in their relative intensity. This fact can be explained only by the formation of Co(II) acid complexes in which all the coordination valences of the Co²⁺ ion are saturated by Cl⁻ and Br⁻ anions.

The question of the composition and structure of these complexes was resolved by the method of classical structural spectral analysis—comparison of the spectra of the systems studied with the spectra of crystalline complexes \(^{(8–21)}\) of known composition and structure. The absorption curves measured by us in the region 600–750 mμ for solutions of CoCl₂ and CoBr₂ (at a Li halide concentration of 4.5 g-mol/l) are very similar to the absorption curves of crystals Cs₂CoCl₄, Cs₃CoCl₅ and Cs₂CoBr₄, Cs₃CoBr₅ reported in the literature \(^{(8,14–16,21)}\). Absor—

the absorbing centers in these double salts are complex ions \([\mathrm{Co}\Gamma_4]^{-2}\), having the form of a tetrahedron \((^{22, 23})\).

However, the literature data on the spectra of the crystals differ somewhat from one another, which can be explained by different values of the spectral slit width in the experiments of different authors and by differences in the calibration of the spectral instruments. Therefore, the absorption spectra of the complexes \(\mathrm{Cs_2CoCl_4}\), \(\mathrm{Cs_3CoCl_5}\), \(\mathrm{Cs_2CoBr_4}\), and \(\mathrm{Cs_3CoBr_5}\) were measured by us anew on an SF-10 spectrophotometer. The data of measurements carried out in the spectral region \(600\text{--}750\ \mathrm{m}\mu\) are given in Figs. 1 and 2.

The absorption curves of the solutions and crystals of the double salts are analogous, but the crystal curves are shifted by \(\sim 5\ \mathrm{m}\mu\) toward the long-wavelength region

Fig. 2. Spectral absorption curves of crystals \(\mathrm{Cs_2CoBr_4}\) and \(\mathrm{Cs_3CoBr_5}\) (1), and of a \(\mathrm{CoBr_2—C_2H_5OH—LiBr}\) solution (2); \(C_{\mathrm{CoBr_2}} = 0.01\ M;\ C_{\mathrm{LiBr}} = 4.5\ M\)

in comparison with the absorption curves of solutions of Co(II) halides. The reason for this should be sought not in phenomena of a chemical nature, but in the design features of the instrument, because of which an increase in the optical density of the samples studied is accompanied by a small displacement of their absorption maxima toward longer wavelengths. The crystals studied by us, even in very thin layers, have a large optical density (1–1.5).

For a final decision on the question of the existence in the solutions studied of the tetrahedral ions \([\mathrm{CoCl_4}]^{-2}\) and \([\mathrm{CoBr_4}]^{-2}\), a careful comparison was made of the absorption spectra of solutions and crystals of double salts in the region \(400\text{--}600\ \mathrm{m}\mu\). This portion of the spectrum is usually ignored in the spectrophotometric investigation of complex formation in solutions of Co(II) halides, since the values of the absorption coefficient of the \(\mathrm{Co}^{+2}\) ion for light of these wavelengths are 2–3 orders of magnitude smaller than for radiation with \(\lambda \sim 700\ \mathrm{m}\mu\) \((^{21})\). At the same time, the crystals \(\mathrm{Cs_2CoCl_4}\), \(\mathrm{Cs_3CoCl_5}\), \(\mathrm{Cs_2CoBr_4}\), and \(\mathrm{Cs_3CoBr_5}\) have, in this spectral region, a large number of absorption bands with a sharply expressed structure \((^{14-16, 21})\).

In Figs. 1 and 2 are shown the spectral absorption curves of crystals and solutions, measured by us on an SF-10 spectrophotometer in the region \(400\text{--}600\ \mathrm{m}\mu\) with a spectral slit width of \(0.5\text{--}1\ \mathrm{m}\mu\). The frequencies of maximum absorption on the light-absorption curves of the solutions completely reproduce the frequencies in the absorption spectra of the crystals.

Thus, at high concentrations of lithium halides, the system \(\mathrm{CoCl_2—C_2H_5OH—LiCl}\) is a spectral analog of the crystals \(\mathrm{Cs_2CoCl_4}\) and \(\mathrm{Cs_3CoCl_5}\); the system \(\mathrm{CoBr_2—C_2H_5OH—LiBr}\) is a spectral analog of the crystals \(\mathrm{Cs_2CoBr_4}\) and \(\mathrm{Cs_3CoBr_5}\). This fact gives grounds to assert that in the first of these systems the \(\mathrm{Co}^{+2}\) ions form ions of composition

\([\mathrm{CoCl}_4]^{-2}\), having tetrahedral structures; in the second system, \([\mathrm{CoBr}_4]^{-2}\) ions are formed, having the same structure.

Leningrad Textile Institute
named after S. M. Kirov

Received
29 III 1963

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