CHEMISTRY

A. M. GOLUB and V. V. SKOPENKO

CATIONIC-TYPE SILVER SELENOCYANATE COMPLEXES

(Presented by Academician I. V. Tananaev, February 3, 1961)

Silver halides and pseudohalides have increased solubility in the presence of an excess not only of the corresponding anions, but also of silver cations. The possibility of formation of cationic complexes of the type $\mathrm{Me}_2\mathrm{X}^+$, where $\mathrm{X}$ is an anion, was indicated by V. A. Kistyakovskii ($^1$), who found the complexes $\mathrm{Ag}_3\mathrm{J}^{2+}$ in solution ($^2$). K. Hellwig came to the same conclusion, observing the migration of iodine to the cathode during electrolysis of the liquid phase of the system $\mathrm{AgNO}_3$—$\mathrm{AgJ}$—$\mathrm{H_2O}$ ($^3$).

Fig. 1. Dependence of $\psi_i$ on the concentration of $\mathrm{AgNO}_3$.
$1$—$\psi_2$; $2$—$\psi_3$; $3$—$\psi_4$

Fig. 2. Dependence of $\psi_i$ on the concentration of $\mathrm{AgClO}_4$.
$1$—$\psi_2$; $2$—$\psi_3$; $3$—$\psi_4$

K. Hellwig studied the solubility of certain silver salts in a solution of $\mathrm{AgNO}_3$ and isolated, in crystalline form, compounds of the composition: $2\mathrm{AgNO}_3 \cdot \mathrm{AgX}$ and $\mathrm{AgNO}_3 \cdot \mathrm{AgJ}$, where $\mathrm{X} = \mathrm{J}', \mathrm{CNS}', \mathrm{Y} = \mathrm{Cl}', \mathrm{Br}', \mathrm{J}'$.

Regarding such compounds as double salts, S. V. Gorbachev ($^4$) calculated their dissociation constants in accordance with the scheme:

\[ \mathrm{AgNO}_3 \cdot \mathrm{AgX} \rightleftharpoons \mathrm{AgNO}_3 + \mathrm{AgX}. \]

I. A. Kablukov ($^5$), by thermal analysis of the $\mathrm{AgBr}$—$\mathrm{AgNO}_3$ system, established the existence of the compound $\mathrm{AgNO}_3 \cdot \mathrm{AgBr}$. In the melt ($^6$), the compounds $2\mathrm{AgNO}_3 \cdot \mathrm{AgJ}$, $\mathrm{AgNO}_3 \cdot \mathrm{AgJ}$, and $\mathrm{AgNO}_3 \cdot \mathrm{AgCl}$ were also demonstrated.

K. Liser ($^7$), measuring the depression of the freezing point and the electrical conductivity of aqueous solutions of $\mathrm{AgNO}_3$ in which $\mathrm{AgJ}$ had been dissolved, confirmed the existence of cationic complexes. B. V. Nekrasov also cal-

considers compounds of the type $\mathrm{Ag}_n\mathrm{Hal}(\mathrm{NO}_3)_{n-1}$ as complexes with a halide ion as the central ion $(^8)$. Using Hellwig’s data on the solubility of silver halides, K. B. Yatsimirsky $(^9)$ calculated the overall dissociation constants of the cationic complexes $\mathrm{Ag}_3\mathrm{J}^{2+}$ $(8.0\cdot 10^{-15})$, $\mathrm{Ag}_2\mathrm{Br}^{+}$ $(2\cdot 10^{-10})$ and $\mathrm{Ag}_2\mathrm{Cl}^{+}$ $(2\cdot 10^{-7})$.

Since the $\mathrm{CNSe}^{-}$ ion has a marked similarity to the $\mathrm{J}^{-}$ ion, it could be assumed that cationic complexes also exist on the basis of silver selenocyanate. It was of interest to compare the stabilities of the selenocyanate complexes with those of the halide complexes, and also to clarify whether the outer-sphere ion affects the course of the complex-formation reaction. For this purpose we studied the solubility of $\mathrm{AgCNSe}$ in aqueous solutions of $\mathrm{AgNO}_3$ and $\mathrm{AgClO}_4$.

Results of the investigation and their discussion

Silver selenocyanate was obtained by an exchange reaction between reagent-grade $\mathrm{AgNO}_3$ and $\mathrm{KCNSe}$ (99%). Silver perchlorate was obtained from silver nitrate. The latter was converted into the oxide, which was then dissolved in reagent-grade $\mathrm{HClO}_4$. The resulting $\mathrm{AgClO}_4$ was recrystallized before use.

A series of solutions of $\mathrm{AgNO}_3$ and $\mathrm{AgClO}_4$ was prepared; these were saturated with freshly prepared $\mathrm{AgCNSe}$. The mixtures were thermostated at $20^\circ$ until equilibrium was attained, after which the solid phase, consisting of $\mathrm{AgCNSe}$, was filtered off. Silver and selenium were determined in the filtrate. For this purpose an aliquot of the solution was treated with nitric acid, total silver was determined as $\mathrm{AgCl}$ (gravimetrically), and selenium was determined argentometrically $(^{10})$. Experiments on the solubility of $\mathrm{AgCNSe}$ in solutions of $\mathrm{AgNO}_3$ and $\mathrm{AgClO}_4$ were repeated 2–3 times. Construction of the logarithmic dependence of the solubility of $\mathrm{AgCNSe}$ on the concentration of the silver salt showed that, in the concentration interval 0.7–3.1 mole/liter, the complexes $\mathrm{Ag}_2\mathrm{CNSe}^{+}$, $\mathrm{Ag}_3\mathrm{CNSe}^{2+}$ and even $\mathrm{Ag}_4\mathrm{CNSe}^{3+}$ are formed. The existence of the last com-

Table 1

Solubility of $\mathrm{AgCNSe}$ in $\mathrm{AgNO}_3$ solutions

Table 2

Solubility of $\mathrm{AgCNSe}$ in $\mathrm{AgClO}_4$ solutions

Previously, only tentative suggestions had been made regarding the existence of a complex of this composition (3). Data on the composition of these complexes were confirmed in calculating their dissociation constants by Leden’s method (11). The results of the investigation are presented in Tables 1–2 and in Figs. 1–2.

The function $\Phi$ was calculated approximately as $\dfrac{S[\mathrm{Ag}^{+}]}{\mathrm{Pr}}$ (12), where $[\mathrm{Ag}^{+}]$ is the equilibrium concentration of silver ions, $S$ is the solubility of AgCNSe. The value of the solubility product (Pr) of AgCNSe was taken as $4 \cdot 10^{-16}$ (13).

As is seen from Figs. 1, 2, by extrapolating the values of $\psi_i$ to zero concentration of silver ions, $\beta_i$ was found (intercepts on the ordinate), and the dissociation constants of the corresponding complexes were then calculated as $\dfrac{1}{\beta_i}$.

The values thus obtained are given in Table 3.

From the data given above it is evident that, in aqueous solutions in the presence of silver nitrate and perchlorate, complexes of the same composition exist. It should be noted that formation of $\mathrm{Ag}_{4}\mathrm{CNSe}^{3+}$ in the presence of silver nitrate, as compared with $\mathrm{AgClO}_{4}$ solutions, is more difficult (see Fig. 1). In both solutions there is good agreement between the dissociation constants of the simplest complexes. For the $\mathrm{Ag}_{4}\mathrm{CNSe}^{3+}$ complex, the dissociation constants differ somewhat (see Table 3). The solubility of silver selenocyanate in the presence of $\mathrm{AgClO}_{4}$ and $\mathrm{AgNO}_{3}$ also differs.

Table 3

Values of the dissociation constants of the complexes

In strength, the cationic complexes of silver based on the selenocyanate ion are very close to the corresponding iodide complexes. Thus, the dissociation constant of $\mathrm{Ag}_{3}\mathrm{J}^{2+}$ is $8.0 \cdot 10^{-15}$ (9), while the dissociation constant of $\mathrm{Ag}_{3}\mathrm{CNSe}^{2+}$ is $5.6 \cdot 10^{-13}$ (in the presence of $\mathrm{AgNO}_{3}$).

To trace the difference in the formation of cationic complexes based on halides and pseudohalides, let us compare the solubilities of the corresponding salts in silver nitrate solutions with a concentration of 3 mol/l. The following values are obtained: AgCNSe $8.4 \cdot 10^{-3}$; AgJ $9.4 \cdot 10^{-3}$ (3); AgCNS $2.6 \cdot 10^{-3}$ (3) mol/l. The solubility of AgJ and AgCNS was determined at 25° (3). Thus, in its ability to form complexes with silver, the CNSe ion is closer to the $\mathrm{J}^{-}$ ion than to $\mathrm{CNS}^{-}$, $\mathrm{Cl}^{-}$, $\mathrm{Br}^{-}$. By the solubility of AgX salts in 3M $\mathrm{AgNO}_{3}$ solution, the halide and pseudohalide ions may be arranged in the series

$$ \mathrm{J}^{-} > \mathrm{CNSe}^{-} > \mathrm{Cl}^{-} > \mathrm{CNS}^{-} > \mathrm{Br}^{-}. $$

We also attempted to isolate salts containing cationic complexes in their composition. For this purpose, aqueous solutions of $\mathrm{AgNO}_{3}$ and $\mathrm{AgClO}_{4}$ (approximately 3 mol/l) were prepared. The solution was heated to 60–70°, then freshly prepared AgCNSe was added to saturation. The excess AgCNSe was rapidly separated, and the filtrate was left for crystallization. From the nitrate solution, white, slightly brownish, very small crystals stable in air were obtained. They are decomposed by water with liberation of AgCNSe; they dissolve without decomposition in $\mathrm{AgNO}_{3}$ solutions; on heating to 118–120° they decompose.

To determine the composition of the salt obtained, a sample was treated with hot water; the AgCNSe that separated was filtered off, washed with water and alcohol, and dried at 100°. Silver was determined in the filtrate by titration with 0.01 N NaCl.

Found, %: $\mathrm{AgNO}_{3}$ 61.2; 61.3; 61.0; AgCNSe 37.6; 38.7; 39.1
$(\mathrm{Ag}_{3}\mathrm{CNSe})(\mathrm{NO}_{3})_{2}$. Calculated, %: $\mathrm{AgNO}_{3}$ 61.5; AgCNSe 38.5

From a solution of $\mathrm{AgClO_4}$, upon saturation with $\mathrm{AgCNSe}$, white needle-like crystals were isolated; they are stable in air and do not decompose when heated to $150^\circ$. At a higher temperature the salt obtained from solutions of $\mathrm{AgClO_4}$ decomposes with an explosion. It also decomposes upon dissolution in water. The composition of the salt corresponds to the formula $(\mathrm{Ag_2CNSe})\,\mathrm{ClO_4}$.

\[ \begin{aligned} &\text{Found, \%: } && \mathrm{AgClO_4}\ 49.20;\ 49.23;\quad \mathrm{AgCNSe}\ 49.90;\ 49.70 \\ &(\mathrm{Ag_2CNSe})\,\mathrm{ClO_4}.\ \text{Calculated, \%: } && \mathrm{AgClO_4}\ 49.23;\quad \mathrm{AgCNSe}\ 50.77 \end{aligned} \]

Thus, the synthesis of molecular compounds, like the solubility data, makes it possible to draw a conclusion about the influence of nonspherical ions on the interaction between $\mathrm{CNSe^-}$ and $\mathrm{Ag^+}$ ions.

Kyiv State University
named after T. G. Shevchenko

Received
9 I 1961

CITED LITERATURE

$^{1}$ V. A. Kistyakovskii, Zs. phys. Chem., 6, 97 (1890).
$^{2}$ V. A. Kistyakovskii, ZhRFKhO, 33, 480 (1901).
$^{3}$ K. Hellwig, Zs. anorg. Chem., 25, 157 (1900).
$^{4}$ S. V. Gorbachev, ZhOKh, 4, issue 10, 1327 (1934).
$^{5}$ I. A. Kablukov, Zs. phys. Chem., 65, 121 (1909).
$^{6}$ Handbook of Technical Encyclopedias, 7, Moscow, 1931.
$^{7}$ K. N. Lieser, Zs. anorg. u. allgem. Chem., 292, 114 (1957).
$^{8}$ B. V. Nekrasov, Course of General Chemistry, Moscow, 1954, p. 817.
$^{9}$ K. B. Yatsimirskii, DAN, 77, 819 (1951).
$^{10}$ H. Hahn, V. Viohl, Zs. anal. Chem., 149, 50 (1956).
$^{11}$ J. Leden, Zs. phys. Chem., A 188, 160 (1941).
$^{12}$ G. S. Cave, D. N. Hume, J. Am. Chem. Soc., 75, 2893 (1953).
$^{13}$ Chemist’s Handbook, 3, Moscow, 1952.