CHEMISTRY

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

APPLICATION OF SPECTROPHOTOMETRY TO THE STUDY OF THE COMPOSITION AND STRUCTURE OF COBALT CHLORIDE COMPLEXES

The crystal-field bands in the absorption spectrum of complex ions are directly connected with the structure of the ligand field. This circumstance has made it possible, on the basis of spectrophotometric measurements, to obtain considerable information concerning the composition, stability, and structure of complex particles in liquid media. In contrast to liquid media,

Fig. 1. Absorption spectra of melts in the systems CsCl—CoCl₂ (a), RbCl—CoCl₂ (b). The numbers correspond to the following molar ratios MCl/CoCl₂: a: 1 — 0, 2 — 0.9, 3 — 1, 4 — 2, 5 — 326; b: 1 — 1, 2 — 2, 3 — 47.

the connection between the composition and structure of crystalline substances is not direct. However, the application of spectrophotometry in physicochemical analysis can in this case also provide an indication of the composition and structure of complex compounds. The absorption spectra of anhydrous CoCl₂ and of complexes containing the ion \([ \mathrm{CoCl}_4 ]^{2-}\) have been studied repeatedly \((^{1-6})\). Taking into account X-ray structural data makes it possible to conclude \((^{4-6})\) that the absorption band with maxima at 530 and 580 mμ belongs to the octahedral ion \([ \mathrm{CoCl}_6 ]^{4-}\), while the band with maxima at 412, 450, 535, 625, 660, and 685 mμ is associated with the existence of tetrahedral ions \([ \mathrm{CoCl}_4 ]^{2-}\).

We obtained absorption spectra of crystalline melts in systems composed of anhydrous CoCl₂ and alkali-metal chlorides.

The substance or mixture of substances in the molten state was applied in a thin layer to the walls of a closed quartz test tube, and the absorption of the thin polycrystalline film obtained in this way was measured on

spectrophotometer SF-4. The experimental data are presented in Figs. 1–4.

Figure 1 presents the absorption spectra of melts in the systems CoCl₂—CsCl (a) and CoCl₂—RbCl (b). Curve 1a represents the visible region of the absorption band of CoCl₂, which has an octahedral structure. The band has maxima at 535 and 590 mμ. The absorption of a CoCl₂ melt with CsCl at a molar ratio of 1 : 1 is represented by curve 3a. As can be seen, the curve retains the character of the octahedral band, but acquires a right-hand shoulder. It was assumed that this is connected with insufficient accuracy of the stoichiometry of the composition, which gives rise to traces of strongly absorbing tetrahedral complexes. Indeed, a melt with the molar ratio

Fig. 2. Absorption spectrum of melts in the systems KCl—CoCl₂ (a), NaCl—CoCl₂ (b). The numbers correspond to the following molar ratios MCl/CoCl₂.
a: 1 — 1, 2 — 1.8, 3 — 2, 4 — 900;
b: 1 — 1, 2 — 2, 3 — 4, 4 — 5, 5 — 105

of CsCl to CoCl₂ equal to 0.9 has no right-hand shoulder in the absorption pattern (curve 2a). Thus, on fusion of CsCl with CoCl₂ in the ratio 1 : 1, an octahedral complex of composition CsCoCl₃ is probably formed. This compound is blue in color, solidifies as needles, and does not hydrate in air.

Curve 4a represents the absorption spectrum of a CsCl melt with CoCl₂ in the ratio 2 : 1. The absorption spectrum fully corresponds to the absorption pattern characteristic of a tetrahedral complex of composition Cs₂CoCl₄ (⁴–⁶). The compound obtained is stable in air for an arbitrarily long time. Melts of CsCl with CoCl₂ containing larger molar fractions of CsCl retain the spectral structure characteristic of tetrahedral complexes (as illustrated by curve 5a for a melt with ratio 326). The RbCl—CoCl₂ system is analogous to the one just considered. It should merely be noted that the absorption band of the RbCl melt with CoCl₂ in the ratio 1 : 1 has no tetrahedral right-hand shoulder. The needle-shaped blue crystals of RbCoCl₃ are stable in air for a long time; however, with time they nevertheless undergo hydration, passing into a pink hydrate form.

Fusibility in the KCl—CoCl₂ system was studied by M. S. Golubeva and A. G. Bergman (⁷). Two congruently melting compounds, CoCl₂·2KCl and 5CoCl₂·9KCl, and one incongruently melting compound, CoCl₂·KCl, were found. The spectrophotometric measurements are in agreement with these data, Fig. 2a. Curve 1a relates to the melt CoCl₂·KCl. As can be seen, it combines features of the spectra of octahedral and tetrahedral complexes, with a quantitative predominance of octahedral ones.

Curves 2 and 3 (Fig. 2a) represent the absorption spectra of the compounds \(5\mathrm{CoCl}_2 \cdot 9\mathrm{KCl}\) and \(\mathrm{CoCl}_2 \cdot 2\mathrm{KCl}\), respectively. Increasing the molar fraction of KCl in its melts with \(\mathrm{CoCl}_2\) does not lead to the appearance of new structures (illustrated by curve 4a for a melt with a molar ratio of 940). Tetrahedral complexes are stable in air indefinitely.

According to thermal-analysis and crystal-optical measurements \((^8)\), in the \(\mathrm{NaCl}—\mathrm{CoCl}_2\) system below the eutectic point one compound of composition \(2\mathrm{NaCl} \cdot \mathrm{CoCl}_2\) is formed. The absorption spectrum of a melt of composition \(\mathrm{NaCl} \cdot \mathrm{CoCl}_2\) indicates the formation of a tetrahedral complex (Fig. 2b, curve 1). In a melt of composition \(2\mathrm{NaCl} \cdot \mathrm{CoCl}_2\), two phases are visually observed: a tetrahedral phase

Fig. 3. Absorption spectra of melts in the \(\mathrm{LiCl}—\mathrm{CoCl}_2\) system. The numbers correspond to the following molar ratios \(\mathrm{LiCl}/\mathrm{CoCl}_2\):
\(1\) — 0.085, \(2\) — 2, \(3\) — 200

Fig. 4. Differential absorption curves of \(0.00248\,M\) solutions of \(\mathrm{Co(NO}_3)_2\) in nitrate melt containing various amounts of KCl. Curves \(1, 2, 3\) refer to the following pairs of molar concentrations of KCl, respectively: 0.139 and 0.000, 0.282 and 0.139, 1.635 and 0.282

and a weakly absorbing octahedral phase. The absorption spectrum of the tetrahedral phase is shown by curve 2 (Fig. 2b). Melts with a higher NaCl content continue to retain both of the indicated phases. At the same time, the fraction of the octahedral phase increases. For melts of composition \(4\mathrm{NaCl} \cdot \mathrm{CoCl}_2\) and \(5\mathrm{NaCl} \cdot \mathrm{CoCl}_2\), we succeeded in separating these phases. The corresponding absorption spectra are represented by curves 3 and 4 (Fig. 2b). A melt with a ratio of NaCl to \(\mathrm{CoCl}_2\) of 105 contains only the octahedral dendritic phase, curve 5 (Fig. 2b). Evidently, it is a limited solid substitution solution of \(\mathrm{CoCl}_2\) in NaCl. In this case the character of coordination of the \(\mathrm{Co}^{2+}\) ion is retained.

Melts of \(\mathrm{CoCl}_2\) with LiCl are homogeneous over the entire range of component ratios, both in appearance and in optical properties. In view of the closeness of the ionic radii of \(\mathrm{Co}^{2+}\) (0.82) and \(\mathrm{Li}^{+}\) (0.78), their mutual and continuous substitutions in the crystal lattices of \(\mathrm{CoCl}_2\) and LiCl are possible, with preservation of the octahedral cubic crystal field for the \(\mathrm{Co}^{2+}\) ion. It is seen from Fig. 3 that the absorption spectra of melts in the \(\mathrm{LiCl}—\mathrm{CoCl}_2\) system retain the features characteristic of octahedral complexes over a wide range of component ratios. The optical data are in agreement with the results of studying fusibility in the same system \((^9)\) and contradict the results of Ferrari and Baroni \((^{10})\) (the fusibility curve has two minima and a maximum corresponding to the compound \(\mathrm{Li}_2\mathrm{CoCl}_4\)).

Thus, the cobalt ion in chloride complex compounds may be found both in octahedra and in tetrahedra, showing a preference for tetrahedra. The tendency to enter octahedra decreases in the series Cs—Li for complex compounds with an alkali-metal atom in the outer sphere. At the same time, the closeness of the ionic radii of Co\(^{2+}\), Na\(^{+}\), and Li\(^{+}\) creates conditions for the entry of Co\(^{2+}\) into the NaCl and especially LiCl lattices with preservation of octahedral coordination. The tendency toward the formation of higher complexes decreases in the series Cs—Li.

In an octahedral crystal field the ground \({}^{4}F\) level of the Co\(^{2+}\) ion splits into three sublevels \(\Gamma_{4}\), \(\Gamma_{5}\), and \(\Gamma_{2}\). The next level, which is not split in an octahedral field, is \({}^{4}P\). Accordingly, the bands corresponding to the transitions \({}^{4}F(\Gamma_{4}) \to {}^{4}F(\Gamma_{5})\) and \({}^{4}F(\Gamma_{4}) \to {}^{4}F(\Gamma_{2})\), at small splittings, should lie in the infrared region. The band of the \({}^{4}F(\Gamma_{4}) \to {}^{4}P(\Gamma_{4})\) transition in a weak crystal field should lie in the visible region. Indeed, the center of gravity of the principal band in the visible region for CoCl\(_2\) and MeCoCl\(_3\) compounds lies at 590 and 600 mµ. Taking for the \(F—P\) transition

\[ \gamma = 3.5(E_{1} - E_{2}) + 15400 \]

\((^{11})\), where \(E_{1} - E_{2}\) is the splitting of the \(F\) level and 15400 cm\(^{-1}\) is the distance between the \(F\) and \(P\) levels in the free Co\(^{2+}\) ion, we find \(E_{1} - E_{2} = \sim 3300\) cm\(^{-1}\) for CoCl\(_2\) and 2200 cm\(^{-1}\) for MeCoCl\(_3\). As can be seen, if the displacement of the \(P\) level in the crystal field is disregarded, introduction of an alkali metal into the outer sphere of the complex sharply reduces the strength of the crystal field and, consequently, leads to a decrease in the already weak excess of the stabilizing force of the octahedral field over the tetrahedral one \((^{12})\). It is characteristic that the replacement of Na\(^{+}\) and Li\(^{+}\) ions by Co\(^{2+}\) ions in alkali chlorides is not associated with a displacement of the octahedral band characteristic of CoCl\(_2\), even in the case of solid solutions with LiCl.

The fact that the cobalt ion prefers a tetrahedral sphere is reflected in the chemistry of solutions. According to data from the preceding work \((^{13})\), differential curves of the absorption spectra of two Co(NO\(_3\))\(_2\) solutions in a nitrate melt containing different amounts of KCl were constructed. In our opinion, the resulting picture characterizes the absorption bands of those complexes which are present in the concentration interval of addends under study in larger amounts. Curve 3 in Fig. 4 is analogous to the curves that we obtained for tetrahedral crystalline complexes. As can be seen, the absorption maxima of the lower complexes belong, approximately, to 620 and 660 mµ. Earlier \((^{13})\) we already suggested that these bands correspond to the complex particles [CoCl\(_2\)] and [CoCl\(_3\)]\(^{-}\). Taking into account that the absorption maximum of the higher nitrate complex belongs to 550 mµ \((^{13})\), while the maximum of the octahedral [CoCl\(_6\)]\(^{4-}\) ion is found at about 590 mµ, the complexes [CoCl\(_2\)] and [CoCl\(_3\)]\(^{-}\) should probably be assigned a tetrahedral structure and the composition [CoCl\(_2\)(NO\(_3\))\(_2\)]\(^{2-}\) and [CoCl\(_3\)NO\(_3\)]\(^{2-}\).

Institute of General and Inorganic Chemistry
named after N. S. Kurnakov
Academy of Sciences of the USSR

Received
4 III 1961

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