Reports of the Academy of Sciences of the USSR

1. Volume 134, No. 2

CHEMISTRY

S. N. ANDREEV and V. G. KHALDIN

COMPLEX FORMATION IN THE SYSTEM $\mathrm{CoCl_2}$—$\mathrm{HCl}$—$\mathrm{H_2O}$

(Presented by Academician I. I. Chernyaev, 5 V 1960)

Despite the large number of works devoted to the study of complex formation in aqueous solutions of $\mathrm{CoCl_2}$ (${}^{1-12}$), information on the composition and structure of the complex compounds formed in these solutions is contradictory. Data on the quantitative content of complex compounds in the system $\mathrm{CoCl_2}$—$\mathrm{HCl}$—$\mathrm{H_2O}$ are absent from the literature.

In the present work an attempt is made to establish the composition and quantitative content of complex ions of Co(II) in the system $\mathrm{CoCl_2}$—$\mathrm{HCl}$—$\mathrm{H_2O}$ at varying concentrations of the components. The identification of the complexes formed in these solutions was carried out by us by comparing the electronic absorption spectra of the solutions with the spectra of crystalline complex compounds of Co(II).

Figure 1 shows the nearest environment of the cobalt ion in crystalline Co(II) complexes, as established by X-ray structural analysis. From regular octahedra $[\mathrm{Co(H_2O)_6}]^{+2}$ is built the crystal lattice of the crystalline hydrate $[\mathrm{Co(H_2O)_6}](\mathrm{ClO_4})_2$ (${}^{13}$); the complexes $[\mathrm{Co(H_2O)_4(ClH_2O)_2}]$ form the crystal lattice of $\mathrm{CoCl_2 \cdot 6H_2O}$ (${}^{14}$); from the complexes $[\mathrm{CoCl_4(H_2O)_2}]$ and $[\mathrm{CoCl_6}]$ are composed the chain and layered lattices of the crystals $\mathrm{CoCl_2 \cdot 2H_2O}$ (${}^{15}$) and $\mathrm{CoCl_2}$ (${}^{16}$). The tetrahedral complexes $[\mathrm{CoCl_4}]^{-2}$ and $\mathrm{Cs}^+$ ions form the crystal lattice of $\mathrm{Cs_2CoCl_4}$ (${}^{17}$).

Fig. 1. Structure of the inner sphere of Co(II) complex compounds in crystals:
a — $\mathrm{Co(ClO_4)_2 \cdot 6H_2O}$, b — $\mathrm{CoCl_2 \cdot 6H_2O}$, c — $\mathrm{CoCl_2 \cdot 2H_2O}$, d — $\mathrm{CoCl_2}$, e — $\mathrm{Cs_2CoCl_4}$

The data on the electronic absorption spectra of $\mathrm{CoCl_2 \cdot 6H_2O}$, $\mathrm{CoCl_2 \cdot 2H_2O}$, $\mathrm{CoCl_2}$, and $\mathrm{Cs_2CoCl_4}$, presented in the article by S. V. Grum-Grzhimailo and I. I. Plyusnina (${}^{18}$), were obtained mainly for powder preparations and do not allow calculation of the values of the absorption coefficient of Co(II) complexes. In addition, when working with finely crystalline samples of crystalline hydrates, one cannot be certain of the homogeneity of the chemical composition of such preparations.

In this connection, we measured the absorption spectra, in natural and polarized light, of thin plates (1 mm—0.01 mm) cut from large crystals of $\mathrm{Co(ClO_4)_2 \cdot 6H_2O}$, $\mathrm{CoCl_2 \cdot 6H_2O}$, $\mathrm{CoCl_2 \cdot 2H_2O}$, and $\mathrm{Cs_2CoCl_4}$. The measurements were carried out on an SF-11 spectrophotometer. A Frank–Ritter prism was used as the polarizer. The absorption spectrum

Table 1

Wavelengths corresponding to the maxima of the absorption curves of complex compounds of Co(II), and values of the absorption coefficient — $\chi_\lambda$, at the maxima of the absorption curves. Experimental temperature 20°

* The numbers above the line denote $\lambda_{\max}$; those below the line denote the corresponding $\chi_\lambda$.

of small crystals of $\mathrm{CoCl_2}$ was measured on a microspectrophotometer kindly made available to us by M. V. Savost’yanova.

Table 1 gives the wavelengths corresponding to the maxima in the absorption curves of the crystalline Co(II) complexes studied, as well as the values of the absorption coefficient $\chi_\lambda$, calculated per 1 gram-ion of $\mathrm{Co^{+2}}$ for natural radiation. Measurements carried out by us in polarized radiation showed that the pleochroism of the crystals considered is reduced to the case of “absorption” (19)—a change in the orientation of the crystal is not accompanied by the appearance of new frequencies in its absorption spectrum.

The absorption spectra of aqueous $\mathrm{CoCl_2}$ solutions in the presence of HCl were studied at $\mathrm{CoCl_2}$ concentrations of 1 mole/l, 0.1 mole/l, 0.01 mole/l, and 0.001 mole/l; the HCl concentration in each series of experiments was varied from 0 to 11 mole/l, $t = 20^\circ\mathrm{C}$.

In all cases, at an HCl concentration in solution from 0 to 4 mole/l, only an absorption band at 515 m$\mu$ is observed in the spectra of the solutions, identical with the absorption band of the ion $[\mathrm{Co(H_2O)_6}]^{+2}$ in the spectrum of crystalline hydrate $\mathrm{Co(ClO_4)_2\cdot 6H_2O}$. At an HCl concentration of 4 mole/l, numerous absorption bands appear in the spectrum of the $\mathrm{CoCl_2}$ solution, exactly coinciding with the absorption bands of the complex $[\mathrm{CoCl_4}]^{-2}$ in the $\mathrm{Cs_2CoCl_4}$ crystal. With increasing HCl concentration, the 515 m$\mu$ absorption band shifts toward 535 m$\mu$, which is entirely explainable by the overlap of the absorption bands of the ions $[\mathrm{CoCl_4}]^{-2}$ and $[\mathrm{Co(H_2O)_6}]^{+2}$. Frequencies corresponding to the complexes $[\mathrm{Co(H_2O)_4(ClH_2O)_2}]$ and $[\mathrm{CoCl_6}]$ are not observed in the spectra of $\mathrm{CoCl_2}$ solutions in HCl.

The absorption bands of the ion $[\mathrm{CoCl_4(H_2O)_2}]^{-2}$, located at 516 m$\mu$ and 620 m$\mu$, are overlapped by the corresponding absorption bands of the complexes $[\mathrm{Co(H_2O)_6}]^{+2}$ and $[\mathrm{CoCl_4}]^{-2}$. At the same time, crystalline hydrate $\mathrm{CoCl_2\cdot 2H_2O}$, in which the immediate environment of the $\mathrm{Co^{+2}}$ ion consists of four $\mathrm{Cl^-}$ ions and two $\mathrm{H_2O}$ molecules, has a strongly pronounced absorption at $\lambda = 280$—290 m$\mu$. The absorption coefficient of this crystalline hydrate at

\(\lambda = 280\) m\(\mu\) is 10–15 times greater than the values of the absorption coefficient of crystals of \(\mathrm{Co(ClO_4)_2\cdot 6H_2O}\) and \(\mathrm{Cs_2CoCl_4}\). Our measurement of the optical density of \(\mathrm{CoCl_2}\) solutions in concentrated \(\mathrm{HCl}\), with varying concentration of the latter, showed the absence of appreciable amounts of the complexes \([\mathrm{CoCl_4(H_2O)_2}]^{-2}\).

As a first approximation it may be assumed that, upon transfer of the ions \([\mathrm{Co(H_2O)_6}]^{+2}\) and \([\mathrm{CoCl_4}]^{-2}\) from the crystalline phase into solution, the values of the absorption coefficient of these ions do not undergo significant changes.

A comparison of the absorption curves of the crystalline hydrate \(\mathrm{Co(ClO_4)_2\cdot 6H_2O}\) and dilute aqueous solutions of Co(II) salts supports this assumption.

Since the values of the absorption coefficient of the ions \([\mathrm{Co(H_2O)_6}]^{+2}\) and \([\mathrm{CoCl_4}]^{-2}\) in the crystalline phase had been measured by us, it seemed reasonable to calculate, from the optical-density values of \(\mathrm{CoCl_2}\) solutions, approximate values of the concentrations of the listed ions. The data obtained by us in these calculations are approximate in character and are given in Table 2.

Table 2

Distribution of \(\mathrm{Co}^{+2}\) ions among complex compounds in the \(\mathrm{CoCl_2}\)—\(\mathrm{HCl}\)—\(\mathrm{H_2O}\) system at varying concentrations of \(\mathrm{CoCl_2}\) and \(\mathrm{HCl}\), \(t = 20^\circ\) (in percent)

| HCl concentration, g-mol/L | \multicolumn{2}{c}{\(\mathrm{CoCl_2}\) concentration in solution, g-mol/L: 1} | \multicolumn{2}{c}{\(\mathrm{CoCl_2}\) concentration in solution, g-mol/L: 0.1} | \multicolumn{2}{c}{\(\mathrm{CoCl_2}\) concentration in solution, g-mol/L: 0.01} | \multicolumn{2}{c}{\(\mathrm{CoCl_2}\) concentration in solution, g-mol/L: 0.001} |
|---:|---:|---:|---:|---:|---:|---:|---:|---:|
| | \([\mathrm{Co(H_2O)_6}]^{+2}\) | \([\mathrm{CoCl_4}]^{-2}\) | \([\mathrm{Co(H_2O)_6}]^{+2}\) | \([\mathrm{CoCl_4}]^{-2}\) | \([\mathrm{Co(H_2O)_6}]^{+2}\) | \([\mathrm{CoCl_4}]^{-2}\) | \([\mathrm{Co(H_2O)_6}]^{+2}\) | \([\mathrm{CoCl_4}]^{-2}\) |
| 0 | 100 | 0 | 100 | 0 | 100 | 0 | 100 | 0 |
| 4 | 95 | 5 | 100 | 0 | 100 | 0 | 100 | 0 |
| 5 | 80 | 20 | 98 | 2 | 99 | 1 | 100 | 0 |
| 6 | 60 | 40 | 90 | 10 | 90 | 10 | 92 | 8 |
| 7 | 45 | 55 | 65 | 35 | 70 | 30 | 80 | 20 |
| 8 | 30 | 70 | 35 | 65 | 40 | 60 | 55 | 45 |
| 9 | 10 | 90 | 20 | 80 | 25 | 75 | 35 | 65 |
| 10 | | | 15 | 85 | 20 | 80 | 25 | 75 |
| 11 | | | 10 | 90 | 15 | 85 | 20 | 80 |

The substitution of \(\mathrm{H_2O}\) molecules in the hydrate shell of the \(\mathrm{Co}^{+2}\) ion by \(\mathrm{Cl^-}\) ions proceeds to an appreciable extent only at high concentrations of \(\mathrm{HCl}\) in solution. The degree of dissociation of \(\mathrm{HCl}\) at such concentrations is small. This gives grounds to consider that the process

\[ [\mathrm{Co(H_2O)_6}]^{+2} + 4\mathrm{Cl^-} = [\mathrm{CoCl_4}]^{-2} + 6\mathrm{H_2O} \]

proceeds only in the case when there is a considerable amount of undissociated \(\mathrm{HCl}\) molecules in the solution, acting as a dehydrating agent.

A similar assumption is confirmed by the fact that the concentration of \([\mathrm{CoCl_4}]^{-2}\) ions in the \(\mathrm{CoCl_2}\)—\(\mathrm{HCl}\)—\(\mathrm{H_2O}\) system increases when acetone or sulfuric acid is introduced into the solution.

In connection with the above, one may suppose a high strength of the bond of the \(\mathrm{Co}^{+2}\) ion with the \(\mathrm{H_2O}\) molecules that form its immediate environment in aqueous solutions, which agrees with literature data on the energy of attachment of water to the \(\mathrm{Co}^{+2}\) cation \((^{20,\,21})\).

The circumstance that, in the electronic absorption spectrum of the \(\mathrm{CoCl_2}\)—\(\mathrm{HCl}\)—\(\mathrm{H_2O}\) system, only the frequencies of the complex ions \([\mathrm{Co(H_2O)_6}]^{+2}\) and \([\mathrm{CoCl_4}]^{-2}\) are observed may be explained as follows: either the process of substitution of \(\mathrm{H_2O}\) molecules in the ion \([\mathrm{Co(H_2O)_6}]^{+2}\) by \(\mathrm{Cl^-}\) anions, accompanied by a change in the coordination number of Co(II), proceeds abruptly, without the formation of intermediate complex compounds, or this same pro-

the process proceeds in several stages, but the concentration of the intermediate complexes is very small.

Leningrad State University
named after A. A. Zhdanov

Received
28 III 1960

REFERENCES

1. A. Potylitsyn, ZhRFKhO, 16, 206 (1884).
2. F. G. Donnan, H. Basset, J. Chem. Soc., 81, 939 (1902).
3. A. Kouchubei, ZhRFKhO, 46, 1055 (1914).
4. J. Groh, Zs. anorg. Chem., 146, 305 (1925).
5. O. R. Howell, A. Jackson, J. Chem. Soc., 1936, 1268; 1937, 973.
6. A. Kiss, M. Gerendas, Zs. phys. Chem., A 180, 117 (1937).
7. L. R. Dawson, J. H. Chaudet, J. Chem. Phys., 19, 771 (1951).
8. L. J. Katzin, J. Am. Chem. Soc., 76, 3089 (1954).
9. C. J. Ballhausen, C. K. Jorgensen, Acta Chem. Scand., 9, 397 (1955).
10. K. P. Mishchenko, I. S. Pominov, ZhFKh, 31, 2026 (1957).
11. K. B. Yatsimirskii, V. D. Korablyova, Izv. Vyssh. Ucheb. Zaved., Khim. i khim. tekhnol., 4, 19 (1958).
12. A. K. Babko, M. M. Tananaiko, Ukr. khim. zhurn., 24, 298 (1958).
13. A. F. West, Zs. Kristallogr., 91, 480 (1935).
14. E. V. Stroganov, I. I. Kozhina, S. N. Andreev, Vestn. LGU, 10, 109 (1958).
15. B. K. Vainshtein, DAN, 68, 301 (1949).
16. B. F. Ormont, Structures of Inorganic Compounds, Moscow, 1950.
17. G. N. Tishchenko, Z. G. Pinsker, DAN, 100, 913 (1955).
18. S. V. Grum-Grzhimailo, I. I. Plyusnina, Crystallography, no. 3, 175 (1958).
19. B. B. Tatarskii, Crystal Optics and Immersion Method for Determining Substances, Leningrad, 1949.
20. K. B. Yatsimirskii, Thermochemistry of Complex Compounds, Moscow, 1951.
21. S. N. Andreev, V. G. Khaldin, E. V. Stroganov, ZhOKh, 29, 1798 (1959).