Full Text
Chemistry
I. B. Baranovskii, O. N. Evstaf’eva, A. V. Babaeva
Pentacyanohalides of Tetravalent Platinum
(Presented by Academician I. I. Chernyaev on January 7, 1965)
A discussion of the data available in the literature on cyanides of divalent platinum led us to the conclusion that tetracyanoplatinite groups have a strong tendency, under certain conditions, to form additional bonds along the third coordinate, with a formal increase in the coordination number of divalent platinum \((^{12})\).
It was suggested that the bright colors of a number of compounds with \([\mathrm{Pt}(\mathrm{CN})_4]^{2-}\), which we had studied earlier \((^{1})\), are due to such an interaction.
For further study of this question, it was proposed to use, in reactions leading to brightly colored products, potassium hexacyanoplatinate instead of potassium tetracyanoplatinite. The octahedral environment of tetravalent platinum ruled out the possibility of additional interactions of the central atoms.
A comparison of the nature of formation, properties, color, and spectra of the corresponding compounds of di- and tetravalent platinum was apparently to provide some information about their structure.
The preparation of \(\mathrm{K}_2[\mathrm{Pt}(\mathrm{CN})_6]\) was carried out by the method of Chernyaev and Babkova \((^{2})\), by the reaction between \(\mathrm{K}_2[\mathrm{PtJ}_6]\) (synthesized according to Datta \((^{3})\)) and \(\mathrm{KCN}\) in the solid phase. However, it turned out that, according to the indicated procedure, in all cases the formation of \(\mathrm{K}_2[\mathrm{Pt}(\mathrm{CN})_5\mathrm{J}]\cdot \mathrm{H}_2\mathrm{O}\) proceeds in almost quantitative yield. We were unable to detect \(\mathrm{K}_2[\mathrm{Pt}(\mathrm{CN})_6]\) either in the solid phase or in solution.
Apparently, the method for preparing potassium hexacyanoplatinate proposed in \((^{2})\) is associated with certain details that were not specified by the authors. When the starting reagents were carefully dried and ground in a dry chamber, the reaction between \(\mathrm{K}_2[\mathrm{PtJ}_6]\) and \(\mathrm{KCN}\) did not proceed. Over the course of several days, no changes in the reaction mixture were observed, whereas in air the black mixture of \(\mathrm{K}_2[\mathrm{PtJ}_6]\) and \(\mathrm{KCN}\) rather rapidly became dark gray and, after some time, almost white.
To separate the reaction products, the reaction mass was dissolved in a minimal amount of water, the solution was filtered and evaporated in a vacuum desiccator over phosphorus pentoxide. The yellowish crystals that separated were filtered off and recrystallized in the same manner from a minimal amount of water. The crystals were selected, pressed with filter paper, and dried in air.
Found, %: Pt 35.13; 35.41; K 14.19; 14.26; C 11.11;
N 12.97; 12.41; J 22.01
\(\mathrm{K}_2[\mathrm{Pt}(\mathrm{CN})_5\mathrm{J}]\cdot \mathrm{H}_2\mathrm{O}\). Calculated, %: Pt 35.61; K 14.24; C 10.95;
N 12.77; J 23.18
The action of an excess of \(\mathrm{KCN}\) on \(\mathrm{K}_2[\mathrm{Pt}(\mathrm{CN})_5\mathrm{J}]\), both in the solid phase and in solution, also does not make it possible to replace the last iodine. \(\mathrm{K}_2[\mathrm{Pt}(\mathrm{CN})_5\mathrm{J}]\cdot \mathrm{H}_2\mathrm{O}\) consists of prismatic yellowish crystals with solubility in water of about 60%; it is also readily soluble in alcohol and acetone. In the region of \(\nu_{\mathrm{CN}}\) it gives two bands: \(2175\ \mathrm{cm}^{-1}\)—
intense, and 2199 cm\(^{-1}\)—of very weak intensity (Fig. 1).
\(\mu_{1000}^{25} = 252\ \Omega^{-1}\cdot\text{cm}^2\), which corresponds to a ternary electrolyte. The cesium salt \(\mathrm{Cs_2[Pt(CN)_5J]}\) was also obtained.
Found, %: C 8.79; 8.21; N 9.98; 9.43
Calculated, %: C 8.36; N 9.74
On prolonged heating of a concentrated aqueous solution of potassium pentacyanoiodoplatinate, slight reduction is observed (the appearance of \(\mathrm{K_2[Pt(CN)_4]}\)). Heating with concentrated alkali (40%) leads to complete reduction with the formation of potassium tetracyanoplatinite. \(\mathrm{K_2[Pt(CN)_5J]}\) can also be obtained by the reaction between \(\mathrm{K_2[Pt(CN)_4J_2]}\) and KCN in the solid phase.
Fig. 12
When potassium cyanide acts on chloro- and bromoplatinates, both in the solid phase and in solution, formation of the corresponding pentacyanohalide of Pt(IV) likewise takes place, the yield decreasing from iodide to chloride (sodium chloroplatinate reacts with potassium cyanide in the solid phase very slowly), and correspondingly in the same direction the yield of potassium tetracyanoplatinite increases. The chloride compound was isolated as the cesium salt \(\mathrm{Cs_2[Pt(CN)_5Cl]}\). To a 20% solution of \(\mathrm{H_2PtCl_6}\) was added a 20% solution of sodium cyanide (6 moles of NaCN per mole of salt), and the solution was evaporated over phosphorus pentoxide. Crystals of tetracyanoplatinite were filtered off, and cesium chloride was added to the mother liquor. The precipitated crystals of the cesium salt were recrystallized with heating from a minimum amount of water.
\(\mathrm{Cs_2[Pt(CN)_5Cl]}\). Found, %: Pt 31.03; C 9.31; 9.89; N 11.53; 11.06; Cl 5.12
Calculated, %: Pt 31.18; C 9.58; N 11.19; Cl 5.67
\(\mathrm{Cs_2[Pt(CN)_5Cl]}\)—fine prismatic white crystals, \(\nu_{\mathrm{CN}}\) 2173 cm\(^{-1}\)—a very intense band; 2182 cm\(^{-1}\)—a band of very weak intensity. \(\mathrm{K_2[Pt(CN)_5Br]}\) (\(\nu_{\mathrm{CN}}\) 2205 cm\(^{-1}\)—a very weak band and 2182 cm\(^{-1}\)—very intense) could not be isolated in pure form, since its solubility is very close to the solubility of potassium tetracyanoplatinite, which is also formed in the reaction process.
Preparation by the reactions indicated above,
\[ [\mathrm{PtX_6}]^{2-} + \mathrm{CN^-} \to [\mathrm{Pt(CN)_5X}]^{2-}, \]
of the pentacyanide as the main product, the complete absence of traces of hexacyanoplatinate in the solution, and also a number of other facts—for example, the constancy with time of the electrical conductivity values of \(\mathrm{[Pt(CH_3NH_2)_2CNCl_3]}\) (\(\mu = 2\ \Omega^{-1}\cdot\text{cm}^2\)) (4) and \(\mathrm{K_2[Pt(CN)_5J]}\), indicating the absence of hydrolysis; the impossibility of substituting the latter iodine by the reactions:
\[ \left. \begin{array}{l} [\mathrm{Pt(NH_3)_2J_4}]\\ [\mathrm{Pt(NH_3)_2(CN)_2J_2}] \end{array} \right\} + \mathrm{KCN} \to [\mathrm{Pt(NH_3)_2J(CN)_3}] \tag{5} \]
indicate a small trans influence of the cyano group in compounds of tetravalent platinum.
Proceeding from the small trans effect of the cyano group, it is difficult to explain the formation of \([\mathrm{Pt}(\mathrm{CN})_5\mathrm{J}]^{2-}\) and \([\mathrm{Pt}(\mathrm{NH}_3)_2\mathrm{J}(\mathrm{CN})_3]\) by simple replacement of iodine by a cyano group in compounds of Pt(IV). Indeed, in that case one would have to ascribe to the cyano groups that first entered the inner sphere of Pt(IV) a strong trans effect, in order to explain the formation of the coordinate arrangement CN—Pt—CN, and then, when the molecule contained 3 or 5 cyano groups, they would have to be regarded as weakly trans-directing. This contradiction is readily removed if a different mechanism is assumed for the formation of the indicated compounds.
It has been shown (6) that the ions \([\mathrm{PtX}_6]^{2-}\) in solutions \((\mathrm{X} — \mathrm{Cl}, \mathrm{Br}, \mathrm{J})\) behave as oxidizing agents toward compounds of divalent platinum.
It is evident that \(\mathrm{K}_2[\mathrm{PtX}_6]\) in an alkaline medium always contains some quantities of \([\mathrm{PtX}_4]^{2-}\). In the presence of an excess of KCN, any tetrahalogenoplatinite instantaneously forms \(\mathrm{K}_2[\mathrm{Pt}(\mathrm{CN})_4]\), owing to the very large trans effect of the cyano group in compounds of divalent platinum. In the presence of \(\mathrm{K}_2[\mathrm{PtX}_6]\), the oxidation of \(\mathrm{K}_2[\mathrm{Pt}(\mathrm{CN})_4]\) takes place and certain quantities of \(\mathrm{K}_2[\mathrm{Pt}(\mathrm{CN})_4\mathrm{X}_2]\) are formed; with excess KCN this gives \(\mathrm{K}_2[\mathrm{Pt}(\mathrm{CN})_5\mathrm{X}]\), since the halogen on the coordinate X—Pt—X is labilized. The small trans effect of CN in Pt(IV) does not make possible the formation of \([\mathrm{Pt}(\mathrm{CN})_6]^{2-}\). In this respect the behavior of the cyano group completely repeats the behavior of the nitro group in Pt(IV) complexes. Thus, when nitrite ions act on \([\mathrm{PtX}_6]^{2-}\) \((\mathrm{X} — \mathrm{Cl}, \mathrm{Br})\), mainly \([\mathrm{Pt}(\mathrm{NO}_2)_5\mathrm{X}]^{2-}\) (and not hexanitrite) is formed (7). The identical mechanism of these reactions is due to the fact that the cyano group and the nitro group in Pt(II) compounds possess a strong trans effect, whereas in Pt(IV) complexes they possess a weak one.
The stability in solution of \([\mathrm{Pt}(\mathrm{CN})_5\mathrm{X}]^{2-}\) shifts the equilibrium of the system toward its formation. The change in reaction yields as a function of the halogen is probably connected with the different oxidizing ability of the hexahalogenoplatinates and the different trans effect of the halogens when they are replaced on the coordinates X—Pt—X by a cyano group.
In the case of iodine such replacement must occur rapidly; in the case of chlorine, it is considerably hindered. This accounts for the small yield of pentacyanochloroplatinate and the formation, in the case of \([\mathrm{PtCl}_6]^{2-}\), of large quantities of \(\mathrm{K}_2[\mathrm{Pt}(\mathrm{CN})_4]\).
Apparently, the oxidation–reduction process
\[ \mathrm{K}_2[\mathrm{PtX}_6] + \mathrm{K}_2[\mathrm{Pt}(\mathrm{CN})_4] \rightleftharpoons \mathrm{K}_2[\mathrm{Pt}(\mathrm{CN})_4\mathrm{X}_2] + \mathrm{K}_2[\mathrm{PtX}_4] \]
proceeds through intermediate bridged complexes, since the pentacyanohalogenoplatinates are formed both in solution and in the solid state; and in the latter case it is especially difficult to explain the almost quantitative yield of \(\mathrm{K}_2[\mathrm{Pt}(\mathrm{CN})_5\mathrm{J}]\), if it is assumed that the oxidizing agent is molecular iodine, which is produced upon reduction of \(\mathrm{K}_2[\mathrm{PtJ}_6]\) in an alkaline medium.
Attempts to isolate or detect the intermediate complexes (from IR spectra) were unsuccessful.
The formation of bridged complexes of Pt(II) and Pt(IV) has long been observed. Work in recent years on the study of reaction mechanisms makes it possible to assume that many reactions of inner-sphere substitution in Pt(IV) compounds proceed through the formation of active complexes of Pt(II) and Pt(IV).
It has been shown (8) that the processes of isotopic exchange of chlorine in all the investigated aminahalogenides of Pt(IV) do not occur if there are no conditions for the formation of intermediate complexes of Pt(II) and Pt(IV).
A. A. Grinberg (9) established the absence of hydrolysis of \([\mathrm{Pt}(\mathrm{NH}_3)_2\mathrm{Cl}_4]\) in the dark without additions of \([\mathrm{Pt}(\mathrm{NH}_3)_2\mathrm{Cl}_2]\), which suggests the participation of intermediate complexes of Pt(II) and Pt(IV) in reactions of chlorine substitution by water.
In a number of cases, in particular for trans-dibromodiammine and trans-tetrabromodiammine platinum, such an intermediate complex can be isolated; its structure has been established \(^{(10,11)}\). Apparently, the necessary conditions for isolating such intermediate complexes must be the following: either the solubility of the molecular compound is much lower than the solubility of its individual parts, or the solubilities of the starting complexes are approximately the same.
For comparison of the reducing ability of tetracyanodiiodo- and pentacyanoiodoplatinates, polarograms were obtained for \(1 \cdot 10^{-3}\ M\) solutions of \(K_2[\mathrm{Pt}(\mathrm{CN})_5\mathrm{J}]\) and \(K_2[\mathrm{Pt}(\mathrm{CN})_4\mathrm{J}_2]\) with a background of 0.1 N \(NaClO_4\) solution.
The half-wave potential of \(K_2[\mathrm{Pt}(\mathrm{CN})_4\mathrm{J}_2]\) is \(0.07\) V, and that of \(K_2[\mathrm{Pt}(\mathrm{CN})_5\mathrm{J}]\) is \(0.21\) V, i.e., the entry of the fifth cyano group into the complex substantially increases its resistance to reduction.
Institute of General and Inorganic Chemistry
named after N. S. Kurnakov
Academy of Sciences of the USSR
Received
21 XII 1964
CITED LITERATURE
- O. N. Evstaf’eva, I. B. Baranovskii, A. V. Babaeva, ZhNKh, 10, No. 1, 28 (1965).
- I. I. Chernyaev, A. V. Babkov, DAN, 152, 882 (1963).
- R. L. Datta, J. Am. Chem. Soc., 35, 1186 (1913).
- I. I. Chernyaev, T. N. Leonova, ZhNKh, 9, 2079 (1964).
- I. I. Chernyaev, A. V. Babkov, ZhNKh, 10, No. 4 (1965).
- A. A. Grinberg, F. M. Filinov, Izv. AN SSSR, OKhN, 1941, No. 3, 361.
- I. I. Chernyaev, L. A. Nazarova, A. S. Morozova, ZhNKh, 6, No. 2, 283 (1961).
- F. Basolo, H. L. Morris, R. G. Pearson, Disc. Farad. Soc., No. 29, 80 (1960).
- A. A. Grinberg, A. A. Korableva, ZhNKh, 9, No. 10, 2313 (1964).
- C. Brosset, Arkiv. Kem. Min. Geol., 25, A, 19 (1948).
- D. Hell, P. P. Williams, Acta crystallogr., 11, 624 (1958).
- O. N. Evstaf’eva, I. B. Baranovskii, A. V. Babaeva, ZhNKh (in press).