STRUCTURAL CHANGES DURING THE TRANSITION OF COPPER(II) THIOCYANATE TO COPPER(I) THIOCYANATE

A. P. IL’IN, L. P. EREMIN, G. G. SAVEL’EV, N. F. KULIKOV

(Presented by the scientific seminar of the Department of General and Inorganic Chemistry)

The data available in the literature on the type of bonding of monovalent and divalent copper with thiocyanate groups are not sufficiently definite: some authors believe that the thiocyanate group is attached to copper through sulfur [1, 2], others through nitrogen [3]; at the same time, no distinction is made between the structures of thiocyanates of mono- and divalent copper. In this connection, in the present work a study was carried out of the structure of copper(II) and copper(I) thiocyanates using IR spectroscopy.

Copper(II) and copper(I) thiocyanates were prepared by mixing a 15% solution of copper sulfate and a 10% solution of ammonium thiocyanate. To obtain copper(II) thiocyanate, ammonium thiocyanate was taken in deficiency, and the black precipitate that formed was rapidly filtered off, thoroughly washed with distilled water to remove excess copper sulfate, and dried in a desiccator over phosphorus pentoxide. In the course of synthesis, copper(II) thiocyanate in the presence of water spontaneously and slowly converts into copper(I) thiocyanate [1, 2].

To prepare copper(I) thiocyanate, a small amount of sodium sulfite was introduced into the mixture of copper sulfate and ammonium thiocyanate in order to accelerate the reduction of copper(II) ions to copper(I). Ammonium thiocyanate was added until a colorless solution was obtained above the precipitate. The resulting mixture was heated to boiling and filtered through a dense paper filter. The white powder of copper(I) thiocyanate was dried on the filter at room temperature. Examination of the obtained copper(I) thiocyanate on a UEMV-100 V electron microscope by the carbon-replica method at a magnification of 13,000 showed that the copper thiocyanate has a crystalline structure: the dimensions of the crystal projections are less than 1 micron, and only individual ones have a size on the order of 1–1.5 microns.

The EPR spectra of the obtained crystals (EPR-2M spectrometer) indicate the presence of copper(II) ions \((10^{-3}—10^{-4}\%)\) in copper(I) thiocyanate.

IR spectra in the region \(4000—650\ \mathrm{cm}^{-1}\) were recorded on IKS-14A and IKS-22 spectrophotometers. The samples were prepared by the standard procedure by dispersion in crystalline potassium bromide. The results obtained are presented in Fig. 1.

In the IR spectrum of copper(I) thiocyanate, intense bands are observed at \(2151\ \mathrm{cm}^{-1}\) and \(740\ \mathrm{cm}^{-1}\), as well as weak bands at about \(2100\ \mathrm{cm}^{-1}\) and \(2083\ \mathrm{cm}^{-1}\).

Copper(II) thiocyanate in the low-frequency part of the spectrum has two absorption bands: \(807\ \mathrm{cm}^{-1}\) and \(760\ \mathrm{cm}^{-1}\). In the high-frequency region there is a band, broader than that for copper(I) thiocyanate, with an absorption maximum at

… at \(2177\ \mathrm{cm}^{-1}\) and absorption bands of low intensity at \(2125\), \(2100\), and \(2083\ \mathrm{cm}^{-1}\).

The spectra were interpreted on the basis of literature data [1, 3, 4, 5] on the frequencies of stretching vibrations of the C—N and C—S bonds in rhodano-group compounds of different structure. According to these data, the absorption bands obtained in the low-frequency region of the IR spectrum were assigned to stretching vibrations of the C—S bond, and those in the high-frequency region to vibrations of the C—N bond of the rhodano group.

It is known that the frequency of stretching vibrations of the C—N bond in a bridging rhodano group lies in the interval \(2180\)—\(2150\ \mathrm{cm}^{-1}\), and in a terminal group in the interval \(2120\)—\(2100\ \mathrm{cm}^{-1}\) [3, 4]. Since in the high-frequency region of the spectrum the principal absorption bands of copper(I) rhodanide, \(2151\ \mathrm{cm}^{-1}\), and copper(II) rhodanide, \(2177\ \mathrm{cm}^{-1}\), correspond to the bridging structure of the rhodano group, it follows that this structure is the principal one in both rhodanides.

Fig. 1. IR absorption spectra; 1—copper(II) rhodanide; 2—copper(I) rhodanide.

The absorption of rhodanides in the low-frequency region of the IR spectrum confirms this conclusion. The principal frequencies of vibrations of the C—S bond, about \(740\ \mathrm{cm}^{-1}\) in copper(I) rhodanide and \(760\ \mathrm{cm}^{-1}\) in copper(II) rhodanide, differ little from \(750\ \mathrm{cm}^{-1}\), the frequency of stretching vibrations of the C—S bond in the free rhodanide ion. In accordance with [4], such vibrations should be assigned to the C—S bond in a bridging rhodano group.

The shift of the frequency of stretching vibrations of the C—S bond, as compared with the free rhodanide ion, toward lower values is associated with the fact that the rhodano group is attached to the metal atom through sulfur, and toward higher frequency values through nitrogen [4]. On this basis, it may be assumed that in the bridging structure of copper(I) rhodanide, which has stretching vibrations of the C—S bond at \(740\ \mathrm{cm}^{-1}\), the copper atom is more strongly bound to sulfur, whereas in the bridging structure of copper(II) rhodanide, with stretching vibrations of the C—S bond at about \(760\ \mathrm{cm}^{-1}\), the copper atom is more strongly bound to nitrogen.

At the same time, in copper(I) and copper(II) rhodanides there is a certain amount of nonbridging rhodano groups. The weak absorption band at \(2083\ \mathrm{cm}^{-1}\) in both rhodanides coincides with the stretching vibrations of the C—N bond in potassium rhodanide, which has an ionic structure. This provides grounds for the conclusion that a small amount of free rhodanide ions is present in the copper rhodanides.

In the copper rhodanides there is one more common absorption band at about \(2100\ \mathrm{cm}^{-1}\), which lies in the frequency region of stretching vibrations of the C—N bond in terminal rhodano groups [3, 4, 6].

The absorption frequency at \(807\ \mathrm{cm}^{-1}\) in copper(II) rhodanide should be assigned to stretching vibrations of the C—S bond. Since in this case the frequency of stretching vibrations of the C—S bond lies substantially higher than…

in the free thiocyanate ion, the bonding of these thiocyanate groups through sulfur is excluded [8]. Thus, the absorption at \(2100\ \mathrm{cm}^{-1}\) should be assigned to terminal thiocyanate groups bonded to the copper atom through nitrogen.

The absorption band at \(2125\ \mathrm{cm}^{-1}\) in copper(II) thiocyanate was assigned to vibrations of the C—N bond in a terminal thiocyanate group attached to the copper atom through sulfur, since it coincides with the absorption in monovalent gold thiocyanate [7].

The values of the observed absorption bands in the IR spectra of copper thiocyanates and their assignments are presented in Table 1.

Table 1

Frequencies \(\nu(\mathrm{CN})\) and \(\nu(\mathrm{CS})\) in the IR spectra of copper(II) and copper(I) thiocyanates

The process of transition of copper(II) thiocyanate to copper(I) thiocyanate apparently proceeds through rupture of the bridging bonds (\(2177\) and \(760\ \mathrm{cm}^{-1}\)), formation of terminal thiocyanate groups (\(2100\) and \(807\ \mathrm{cm}^{-1}\)), rupture of the copper—nitrogen bond and formation of a copper—sulfur bond with simultaneous reduction to copper(I) (\(2125\ \mathrm{cm}^{-1}\)). Only then is the bridging structure of copper(I) thiocyanate formed (\(2151\) and \(740\ \mathrm{cm}^{-1}\)). It is possible that at some one of these stages the transition occurs through an ionic structure of the thiocyanate groups (\(2083\ \mathrm{cm}^{-1}\)).

1. Copper thiocyanates have a bridging structure: in copper(II) thiocyanate the thiocyanate groups are more strongly bonded to copper through nitrogen, whereas in copper(I) thiocyanate they are bonded through sulfur.

2. The spontaneous transition of copper(II) thiocyanate to copper(I) thiocyanate is accompanied by a change of the principal bond from Cu—N to Cu—S; moreover, probably at the initial stage copper(I) thiocyanate does not have a bridging structure.

3. In copper(I) thiocyanate, a certain amount of copper(II) ions is stabilized, preserving the Cu—N bond.

LITERATURE

1. B. V. Nekrasov, Fundamentals of General Chemistry, vol. I, 527 (1973).

2. G. Remy, Course of Inorganic Chemistry, vol. I, 505 (1963).

3. K. Nakamoto, Infrared Spectra of Inorganic and Coordination Compounds. Moscow, “Mir,” 1966.

4. Yu. Ya. Kharitonov, G. V. Tsintsadze, M. A. Porai-Koshits. Dokl. AN SSSR, 160, 6, 1351 (1965).

5. Yu. Ya. Kharitonov, G. V. Tsintsadze, A. Yu. Tsivadze. ZhNKh, 15, 5, 1196 (1970).

6. A. N. Sergeeva, L. N. Kogut. ZhNKh, 18, 8, 2281 (1973).

7. J. L. Burmeister, J. B. Melpolder. J. Chem. Soc. Chem. Commun., 17, 613 (1973).