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The Molecular Structure of Selenium Dichloride, SeCl2, Determined by Gas Electron Diffraction

Liv Fernholt, Arne Haaland*, and Ragnhild Seip

Department of Chemistry, University of Oslo, Box 1033, Blindern, Oslo 3, Norway

Rüdiger Kniep and Lutz Korte Institut für Anorganische Chemie und Strukturchemie der Universität Düsseldorf, Universitätsstraße 1, D-4000 Düsseldorf Z. Naturforsch. 38b, 1072-1073 (1983); eingegangen am 19. Mai 1983 Molecular Structure, Selenium(II) Chloride, Gas Electron Diffraction

The electron diffraction pattern of the vapor from a sample of SeCU has been recorded with a reservoir and nozzle temperature of about 175 °C. The gas jet was found to consist of SeCl2 (80%) and Cl2 (20%). The bond distance in SeCl2 is ra(Se-Cl) = 2.157(3) Ä, the valence angle <ClSeCl = 99.6(5)°.

Two of the three known selenium chlorides, Se2Cl2

and SeCU, are stable in the solid phase but not in the gas phase, while the third, SeCl2, is stable in the gas phase but does not appear to exist in the solid phase [1]: Se2Cl2 forms a molecular solid which melts incongruently at about —48 °C [2, 3]. On evaporation it dissociates partly if not completely to Se(^) and SeCl2(g). SeCU forms cubane-like tetra-mers in the solid phase [2, 4]. The melting point (in a closed system) is about 306 °C. On evaporation it dissociates completely according to

SeCl4(s) SeCl2(g) + Cl2(g). [5, 6] The molecular structures of Se2Cl2 and tetrameric SeCU have recently been determined by X-ray crystallography [2-4].

The SeCl2 molecule has been studied by gas phase Raman spectroscopy [6] and by He(I) photoelectron spectroscopy [7, 8]. We now report the molecular structure determined by gas electron diffraction.

A sample of SeCU was synthesized as described elsewhere [2], Electron diffraction of the vapor was recorded on Balzers Eldigraph KDG-2 with nozzle and reservoir temperatures of about 175 °C. Ex-posures were made with nozzle-to-plate distances of about 50 and 25 cm. The data were processed by standard procedures. Six 50 cm plates yielded an average modified molecular intensity curve ex-tending from s = 2.00 to 14.75 Ä - 1 with increment 0.125 A-i. Four 25 cm plates yielded an average intensity curve extending from s = 5.00 to 29.00 Ä - 1

* Reprint requests to Dr. A. Haaland. 0340-5087/83/0900-1072/5 01.00/0

with increment 0.25 Ä -1. Atomic scattering factors were calculated from atomic potentials [9] by the partial wave method [10].

An RD curve obtained by Fourier inversion of the experimental intensity is shown in Fig. 1. The composite peak at about r = 2.1 A consists of peaks representing the bond distances Cl-Cl and Se-Cl in Cl2 and SeCl2 respectively, the peak at 3.3 A repre-sents the nonbonded C1-- C1 distance in SeCl2. Note that the RD curve contains no peaks in the range 3.8 to 5.0 Ä indicating the presence of measurable quantities of Se2Cl2 or SeCU.

RD

aRD

2 3 U r/Ä 5

Fig. 1. (Above): Experimental RD curve for the SeCl2/Cl2 gas mixture eminating from the nozzle. Artificial damping constant k = 0.002 Ä2 ; (Below): difference between the experimental curve and the theoretical curve calculated for best model (Table I).

L. Fernholt et al. • The Molecular Structure of Selenium Dichloride 1073

The bond distance and the root-mean-square vibrational amplitude of the CI2 molecule were fixed at the values determined by Shibata, r g = 1.993 Ä and ^=0.051 Ä, respectively [11]. The bond dis-tance, valence angle and R.M. S. vibrational ampli-tudes of SeCl2 as well as the mole fractions of SeCk and CI2 in the gas jet were refined by least-squares calculations on the intensity data with a program originally written by H. M. Seip [12]. The best para-meter values are listed in Table I. The estimated standard deviations have been multiplied by a factor of two to compensate for data correlation and expanded to include a scale uncertainty of 0.1%.

Table I. Mole fraction and molecular structure of SeCl2 in the molecular beam.

r./A i\k

* (SeCl2) = 0.79(4) Se-Cl 2.157(3) 0.061(3) C l - C l 3.295(13) 0.132(9) < ClSeCla 99.6(5)°

z (Cl2) = 0.21(4) Cl-Cl 1.993^ 0.051*

a Not corrected for shrinkage; b ref. [11].

The low mole fraction of CI2 was initially some-thing of a surprise. Refinements on data from single plates indicated, however, that the amount of CI2

present in the gas jet decreased in the course of the experiment. We assume that most of the CI2 formed escaped before we began to record the scattering pattern.

The bond distance in SeCl2 is indistinguishable from the terminal Se-Cl bond distances in the SeCU tetramer [2, 4], but significantly shorter than in crystalline Se2Cl2 where the mean Se-Cl bond distance is 2.204 Ä [3]. At the same time the valence angle in SeCh is significantly smaller than the valence angle at Se in Se2Cl2, < SeSeCl = 104.3° (mean value). The same differences have been noted between SC12 and S2C12 [3].

In a very early electron diffraction investigation of the vapor from solid SeCLi, Lister and Sutton concluded that the SeCL molecule is (distorted) tetrahedral with a (mean) SeCl bond distance of 2.13 ± 0.04 Ä [13]. The possibility that SeCl4 dis-sociates in the gas phase was not considered. This study, which has found its way into the literature, must be regarded as invalid. A later study by Akishin, Spiridonov and Mishulima concluded that the sample had undergone partial or complete decomposition, most probably to give SeCl2 and CI2. The degree of dissociation could not be determined, however, and the average SeCl bond distance of the "molecules present in the vapor" given as 2.18 ± 0.02 Ä [14]. This result, though less accurate, is consistent with ours.

We are grateful to the Norwegian Research Coun-cil for Science and the Humanities (NAVF) and to the Fonds der Chemischen Industrie for financial support.

[1] Gmelins Handbuch der Anorganischen Chemie, 10 B, Die Verbindungen des Selens, 8th Ed., Clausthal-Zellerfeld, 1949, p. 118ff.

[2] P. Born, R. Kniep, D. Mootz, M. Hein, and B. Krebs, Z. Naturforsch. 36b, 1516 (1981).

[3] R. Kniep, L. Körte, and D. Mootz, Z. Natur-forsch. 38b, 1 (1983).

[4] R. Kniep, L. Körte, and D. Mootz, Z. Natur-forsch. 36b, 1660 (1981).

[5] D. M. Yost and C. E. Kischer, J. Am. Chem. Soc. 52, 4680 (1930).

[6] G. A. Ozin and A. V. Voet, Chem. Commun. 1970, 896.

[7] D. M. de Leeuw, R. Mooyman, and C. A. de Lange, Chem. Phys. 38, 21 (1979).

[8] E. Nagy-Felsobuki and J. B. Peel, J. Chem. Soc. Faraday II 76, 148 (1980).

[9] T. G. Strand and R. A. Bonham, J. Chem. Phys. 47, 2599 (1967).

[10] A. C. Yates, Comput. Phys. Commun. 2, 175 (1971).

[11] S. Shibata, J. Chem. Phys. 67, 2256 (1963). [12] B. Andersen, H. M. Seip, T. G. Strand, and R.

St0levik, Acta Chem. Scand. 23, 3224 (1969). [13] M. W. Lister and L. E. Sutton, Trans. Faraday

Soc. 37, 393 (1941). [14] A. P. Akishin, V. P. Spiridonov, and R. A.

Mishulina, Vestn. Mosk. Univ. Ser. II, Khim. 17, 23 (1962); C. A. 57, 8166c (1962). See also comments in L. V. Vilkov, N. G. Rambidi, and V. P. Spiridonov, J. Struct. Chem. (Engl.) 8, 715 (1967).