CHEMISTRY

Academician I. V. TANANAEV and B. F. DZHURINSKII

STUDY OF THE INTERACTION IN THE SYSTEMS Co(NO₃)₂—KBr, Co(NO₃)₂—KI IN A NITRATE MELT

Previously we studied the interaction in the system Co(NO₃)₂—KCl in a nitrate melt (¹). The present work is devoted to the investigation of the interaction in the systems Co(NO₃)₂—KBr, Co(NO₃)₂—KI in a nitrate melt and under direct contact of the components.

The method of high-temperature spectrophotometry has already been described by us (¹). As solvent we used a molten mixture,

Fig. 1. Changes in the absorption spectrum of a 0.0025 M solution of Co(NO₃)₂ associated with the formation of bromide complexes at 160° (a) and 200° (b). The numbers correspond to the following ratios

\[ \frac{C_{\mathrm{KBr}}}{C_{\mathrm{Co(NO_3)_2}}}: \quad 1—0,\quad 2—8,\quad 3—16,\quad 4—32,\quad 5—64,\quad 6—128,\quad 7—256,\quad 8—400, \]

\[ 8^a \text{—saturated solution} \]

containing 45 mol.% LiNO₃ and 55 mol.% KNO₃. Since KBr and, especially, KI are unstable in an acidic nitrate melt, while Co(NO₃)₂ is unstable in an alkaline nitrate melt, measures were taken to maintain the appropriate acidity of the melt. This was achieved by preliminary single melting and freezing for the system with KBr, and fourfold melting and freezing for the system with KI, of an acidic solution of Co(NO₃)₂ in the nitrate melt; for each experiment a fresh portion of the Co(NO₃)₂ solution treated in this way was taken.

Figure 1 presents changes in the absorption spectrum of a 0.0025 M solution of Co(NO₃)₂ associated with the formation of bromide complexes. With increasing KBr concentration, the following appear successively: a hidden maximum at 580 mµ, maxima at 610, 640, 660, and 695 mµ, and a hidden maximum at about 730 mµ. The independence of their appearance, especially evid—

… when comparing data relating to different temperatures, makes it possible to assign the observed maxima to the following individual complex ions: \([\mathrm{CoBr}]^+\), \([\mathrm{CoBr}_2]\), \([\mathrm{CoBr}_3]^-\), \([\mathrm{CoBr}_4]^{2-}\), \([\mathrm{CoBr}_5]^{3-}\), \([\mathrm{CoBr}_6]^{4-}\). S. A. Shukarev and O. A. Lobanova \((^2)\), in a spectrophotometric study of acetone solutions of cobalt bromide complexes, found absorption maxima at 720, 700, 670, 640, and 580 mµ, which they attributed to the formation of the following complex compounds, respectively: \(\mathrm{Li}_4[\mathrm{CoBr}_6]\), \(\mathrm{Li}_2[\mathrm{CoBr}_4 \cdot 2\,\mathrm{acet.}]\), \(\mathrm{Li}[\mathrm{CoBr}_3 \cdot 3\,\mathrm{acet.}]\), \([\mathrm{CoBr}_2 \cdot 4\,\mathrm{acet.}]\),

Fig. 2. Isomolar series in the system \(\mathrm{Co(NO_3)_2}—\mathrm{K_2Br_2}\) at a total concentration of \(0.025\ \mathrm{M/l}\).
\[ n=\frac{C_{\mathrm{K_2Br_2}}}{C_{\mathrm{Co(NO_3)_2}}}. \]
\(1 — 160^\circ,\ 2 — 200^\circ;\ a — 610,\ b — 680,\ c — 660,\ d — 700\ \text{m}\mu\)

\([\mathrm{CoBr}\cdot 5\,\mathrm{acet.}]\mathrm{Br}\). They declare the complex compound of composition \(\mathrm{Li}_3[\mathrm{CoBr}_5\cdot\mathrm{acet.}]\) to be disproportionating. Meanwhile, the values found by Lobanova \((^3)\) for the successive stability constants, \(K_4=21,\ K_5=8,\ K_6=2\), contradict the idea of disproportionation of this complex. It should also be said that the curves on the basis of which the individual absorption maxima were isolated \((^{2,3})\) clearly indicate the existence of a principal maximum at 700 mµ for \(\bar n=5\) and at 670 mµ for \(\bar n=4\). From the curve for \(\bar n=2\), nothing definite can be said at all.

Thus, the positions of the main absorption bands found by us and by the authors cited basically coincide and can be assigned to the corresponding complexes. The successive shift of the absorption bands into the long-wavelength region with increasing content of \(\mathrm{Br}^-\) ions in the complex up to the complex of composition \([\mathrm{CoBr}_6]^{4-}\) indicates a successive weakening of the crystal field surrounding the \(\mathrm{Co}^{2+}\) ion, while the octahedral configuration is essentially preserved. The presumed positions of the absorption bands of the individual complexes were calculated on the basis of the concept of averaging the influence of the crystal field of the ligands when, in the octahedral complex \([\mathrm{Co(NO_3)_6}]^{-4}\), \(\mathrm{NO_3^-}\) ions are replaced by \(\mathrm{Br^-}\) ions. It turned out that complexes of the compositions \([\mathrm{Co(NO_3)Br_5}]^{4-}\), \([\mathrm{Co(NO_3)_2Br_4}]^{4-}\), \([\mathrm{Co(NO_3)_3Br_3}]^{4-}\), \([\mathrm{Co(NO_3)_4Br_2}]^{4-}\), and \([\mathrm{Co(NO_3)_5Br}]^{4-}\) should absorb in the wavelength region 695, 658, 604, and 580 mµ. The good agreement between the calculated and experimental positions of the absorption maxima indicates the existence of the observed individual complex ions.

Figure 2 presents the data of the isomolar series in the system \(\mathrm{Co(NO_3)_2}—\mathrm{K_2Br_2}\) at a total concentration of \(0.025\ \mathrm{M}\). As can be seen, complexes are formed in the system that tend toward the composition \([\mathrm{CoBr}_2]\) at \(\lambda=610\ \text{m}\mu\) and a temperature of \(200^\circ\), and toward higher coordination numbers (with respect to \(\mathrm{Br^-}\)) at greater wavelengths and \(160^\circ\).

Figure 3 presents the changes in the absorption spectrum of a 0.0025 $M$ solution of $\mathrm{Co(NO_3)_2}$ in a nitrate melt, associated with the formation of iodide complexes. As the concentration of $\mathrm{KJ}$ is increased, the following appear successively: a hidden maximum near $590\,m\mu$, maxima at 610, 650, 695, 750, and $780\,m\mu$, which may be assigned, respectively, to the complex ions $[\mathrm{Co(NO_3)_5J}]^{4-}$, $[\mathrm{Co(NO_3)_4J_2}]^{4-}$, $[\mathrm{Co(NO_3)_3J_3}]^{4-}$, $[\mathrm{Co(NO_3)_2J_4}]^{4-}$, $[\mathrm{Co(NO_3)J_5}]^{4-}$, and $[\mathrm{CoJ_6}]^{4-}$. On the basis of the positions of the absorption bands of the octahedral ions $[\mathrm{Co(NO_3)_6}]^{4-}$ ($555\,m\mu$) and $[\mathrm{CoJ_6}]^{4-}$ ($780\,m\mu$), the positions of the bands of the intermediate complexes were calculated under the assumption that the octahedral configuration is retained and that the influence of the ligand crystal fields is averaged. The calculated absorption bands for the ions $[\mathrm{Co(NO_3)_5\cdot J}]^{4-}$, $[\mathrm{Co(NO_3)_4J_2}]^{4-}$, $[\mathrm{Co(NO_3)_3J_3}]^{4-}$, $[\mathrm{Co(NO_3)_2J_4}]^{4-}$, $[\mathrm{Co(NO_3)J_5}]^{4-}$ lie in the wavelength region, respectively, 585, 615, 650, 695, and $730\,m\mu$. As can be seen, with the exception, apparently, of the tetrahedrally distorted $[\mathrm{Co(NO_3)J_5}]^{4-}$, the calculated bands coincide with the observed ones, which confirms the existence of octahedral mixed iodide complexes of $\mathrm{Co^{2+}}$.

Fig. 3. Changes in the absorption spectrum of a 0.0025 $M$ solution of $\mathrm{Co(NO_3)_2}$ associated with the formation of chloride complexes at 160°. The numbers correspond to the following ratios:

$$ \frac{C_{\mathrm{KJ}}}{C_{\mathrm{Co(NO_3)_2}}}: \quad 1—0,\quad 2—32,\quad 3—48,\quad 4—64,\quad 5—80,\quad 6—96,\quad 7—128, $$

$1^a$ — absorption curve of $\mathrm{J_2}$.

In addition to the absorption bands already noted, in the region of $400\,m\mu$ there is another very intense band, increasing with increasing $\mathrm{KJ}$ concentration and probably associated with the allowed transition

$$ \mathrm{Co^{2+}J^-}+h\nu=\mathrm{Co^+J}. $$

In view of the presence of strong oxidizing agents in the nitrate melt, such a transition may promote oxidation of the $\mathrm{J^-}$ ion. Indeed, at 160° a solution of $\mathrm{KJ}$ in the nitrate melt, subjected to the treatment described above,* is practically stable for a long time. In the presence of $\mathrm{Co^{2+}}$ ions in the melt, however, slow decomposition of $\mathrm{KJ}$ occurs with liberation of $\mathrm{J_2}$. The rate of decomposition increases with increasing $\mathrm{KJ}$ concentration. Unfortunately, it was not possible to isolate this band, since the absorption band of $\mathrm{J_2}$ is located in the same wavelength region.

Figure 3 shows part of the absorption curve of $\mathrm{J_2}$ in the nitrate melt acidified for the decomposition of $\mathrm{KJ}$.

Fig. 4. Solid-phase reactions in the systems $\mathrm{Co(NO_3)_2\cdot 6H_2O}$—$\mathrm{KCl}$ (1), $\mathrm{Co(NO_3)_2\cdot 6H_2O}$—$\mathrm{KBr}$ (2), $\mathrm{Co(NO_3)_2\cdot 6H_2O}$—$\mathrm{KJ}$ (3).

Figure 4 presents the absorption curves of samples obtained by pressing $\mathrm{Co(NO_3)_2\cdot 6H_2O}$ with potassium halides. Absorption was measured relative to a disk pressed from the corresponding potassium halide with an admixture of $\mathrm{KNO_3}$. Even upon mixing crystals of $\mathrm{Co(NO_3)_2\cdot 6H_2O}$ with potassium halides, the pink color of the $\mathrm{Co(NO_3)_2\cdot 6H_2O}$ crystals changes...

to blue (the color of chloride and bromide complexes) or dark green (the color of iodide complexes). This reaction is at equilibrium, and the state of its equilibrium depends strongly on the temperature and humidity of the air and on the hygroscopicity of the potassium halide used. Therefore, in order to obtain the higher cobalt complexes, the mixture of crystals of $\mathrm{Co(NO_3)_2\cdot 6H_2O}$ with $\mathrm{KCl}$ and $\mathrm{KJ}$ was preliminarily dried at $30$–$50^\circ$. As expected, in this case higher halide complexes were formed, with an even content of halogen ions: $\mathrm{K_2[CoCl_4]}$ and, possibly, $\mathrm{K_2[CoCl_2(NO_3)_2]}$ at $\lambda = 690$ and $\sim 630$ m$\mu$ (curve 1), $\mathrm{K_4[CoBr_6]}$ and $\mathrm{K_2[CoBr_4\cdot 2H_2O]}$ at $\lambda = 725$ and $\sim 670$ m$\mu$ (curve 2), $\mathrm{K_4[CoJ_6]}$ and, probably, $\mathrm{K_2[CoJ_4\cdot 2H_2O]}$ at $\lambda = 782$ and $\sim 700$ m$\mu$ (curve 2). On curve 2 the right-hand shoulder of the intense band of the allowed transition is also visible; in this case it is not accompanied by oxidation of $\mathrm{KJ}$, owing to the absence of electron acceptors. In Fig. 1 a weak band at 535 m$\mu$ is also visible, possibly attributable to $\mathrm{K_2[CoCl_6]}$. The absence in Figs. 2 and 3 of the maxima found in solutions and melts at 695 m$\mu$ for the system with $\mathrm{KBr}$ and at 750 m$\mu$ for the system with $\mathrm{KJ}$ is additional confirmation of the existence in the liquid state of complex ions of composition $[\mathrm{CoX}\Gamma_5]^{4-}$, where $\mathrm{X}$ is the solvent or the anion of the solvent melt, and $\Gamma$ is $\mathrm{J^-}$ or $\mathrm{Br^-}$.

Summarizing the study of the interaction of the $\mathrm{Co^{2+}}$ ion with halogen ions in a nitrate melt, it should be said that all the complexes investigated are formed exothermically, with increasing stability in the series $\mathrm{Cl}$, $\mathrm{Br}$, $\mathrm{J}$. According to Lobaneva’s data ($^3$), in acetone solutions the stability of complex halides of $\mathrm{Co^{++}}$ in terms of $\Delta F$ decreases in the series $\mathrm{Cl}$, $\mathrm{Br}$, $\mathrm{J}$; the endothermicity decreases in the series $\mathrm{Br}$, $\mathrm{Cl}$, $\mathrm{J}$, and $\Delta S$ compensates the endothermicity of complex formation. As can be seen, in the nitrate melt, owing to the exothermicity of the substitution processes of $\mathrm{NO_3^-}$ ions by halogen ions, the stability of the complexes follows the series of greatest energetic advantage.

Institute of General and Inorganic Chemistry
named after N. S. Kurnakov

Received
4 VI 1960

CITED LITERATURE

1. I. V. Tananaev, B. F. Dzhurinskii, DAN, 134, No. 6 (1960).
2. S. A. Shukarev, O. A. Lobaneva, Vestn. LGU, No. 16, issue 3, 72 (1956).
3. O. A. Lobaneva, Author’s abstract of dissertation, L., 1959.