MÖSSBAUER INVESTIGATION OF SOME WATER ADDUCTS OF ANTIMONY PENTACHLORIDE

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MÖSSBAUER INVESTIGATION OF SOME WATER

ADDUCTS OF ANTIMONY PENTACHLORIDE

J. Resce, J. Stevens

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 12, Tome 37, Ddcembre 1976, page C6-531

M~SSBAUER

INVESTIGATION

OF

SOME WATER ADDUCTS

OF

ANTIMONY PENTACHLORIDE

J. L. RESCE and J. G. STEVENS University of North Carolina at Asheville, U. S. A.

Rhum6.

-

Les spectres Mossbauer de quatre pentachlorure d'antimoine hydrates SbC15, nHnO (n = 1,2,3,4), ont Btb obtenus avec l'isotope 121Sb. Les valeurs de l'isomer shift par rapport

?InSb sont comprises entre 3,3 et 4,l mm/s. L'effet quadrupolaire est de i

-

7,6 ; 8,3 ;

-

11,2 et - 10,2 mm/s pour le monohydrate et tetrahydrate respectivement. Dans le monohydrate les HzO occupent les six sites de la symktrie octaedrique. Dans le dihydrate les seconds Hz0 sont relies aux premiers par liaison hydrogkne. Ces deux rbultats sont coherents avec les rksultats obtenus precbdemment par spectrometric infrarouge et resonance magnetique nucleaire. Les troisiemes HzO dans le trihydrate sont partiellement lits aux atomes d'antimoine, apparaissant ainsi comme d'un 7' ligand. Les quatrikmes Hz0 dans le tetrahydrate sont probablement lies aux troisikmes par liaison hydrogkne de la m6me manikre que les seconds.

Abstract.

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121Sb Mossbauer spectra of four water adducts of antimony pentachloride, SbCls.nHz0 (n = 1, 2, 3, 4) have been obtained. The isomer shifts, range from 3.3 to 4.1 mm/s relative to InSb. The quadrupole coupling constants are - 7.6 mm/s, - 8.3, - 11.2 and

-

10.2 for the monohydrate, dihydrate, trihydrate and tetrahydrate, respectively. The monohydrate has octahedral symmetry with the water occupying the sixth position. The second water in the dihydrate is hydrogen bonded to the first. Both of these findings are consistent with previous IR and NMR studies. The third water of the trihydrate forms a partial bond to the antimony, thus appearing as a seventh ligand. The fourth water of the tetrahydrate is probably hydrogen bonded to the third, similar to what takes place with the second water.

1. Introduction.

-

The four hydrates of antimony pentachloride, i. e. SbC15.nH20 (n = 1, 2, 3, 4),

have been studied by various methods, yet no overall structural scheme for the placement of the waters exists. The monohydrate (SbCI,

.

H 2 0 ) presents no difficulty, however, since the first water easily bonds to the Lewis acid, SbCI,, forming an electron-pair donor-acceptor octahedral complex. PMR studies by

L. Bernander and G. Olofsson [I] report hydrogen bonding between the first and second water in the dihydrate (SbC1,.2 H20). The appearance of only a single narrow peak for both hydrates was explained by fast proton exchange ; electron density about the protons decreased as a second water was added. Both this work and a calorimetric study by G. Olofsson and I. Olofsson [2] suggests that adduct formation enhances the acidity of the water protons thus leading to the formation of ternary complexes such as the dihydrate through hydrogen bonding. A. Schmidt and

R. Ortwein [3] reached the same conclusions with I R and Raman studies of these two adducts and C,, symmetry was assigned to the monohydrate. The positions of the third and fourth waters in the trihy- drate (SbCl,. 3 H 2 0 ) and tetrahydrate (SbCl, .4 H,O)

respectively, still remained uncertain. A liquid-solid phase equilibria study by G. Picotin and P. Viste [4] point out that the dihydrate decomposes in the solid state at - 23 OC. This and the melting point of the trihydrate are in disagreement with the findings by Schmidt and Ortwein. Our present work discusses the lZ1Sb Mossbauer spectra of the four hydrates and an incorporation of all significant data reported on the series thus far is used to propose structures of these hydrates.

2. Experimental.

-

Preparation of the SbCl,

.

nH,O (n = 1, 2, 3, 4) series in our laboratory followed the procedure outlined by Schmidt 131, who himself also

supplied additional samples. Elemental analyses of his samples was reported in his work. Verification of our samples was made by comparing the melting points to those of Schmidt's. Special care was taken to prevent moisture in the air from attacking the hydrates thus causing decomposition into SbOCl and other Sb(V) oxides. Due to the highly exothermic nature of the reaction, the temperature of the reactants had to be rigorously maintained below about 15 OC to prevent side reactions. Samples were mixed with boron trini-

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C6-532 J. L. RESCE AND J. G. STEVENS

tride as an inert filler and encased in lucite holders. Extreme care was taken in every stage of the experi- ment to prevent decomposition and all handling of the hydrates was carried out under dry nitrogen environment in a glove box. The absorber thickness, in each case, was 25 mg/cm2 natural antimony.

The Mossbauer spectra were obtained with the source and absorber at liquid nitrogen temperature. An Austin Science Associates, Inc. constant acceleration spectrometer (model S3) was used with a xenon- methane proportional counter and a Nuclear Data 2200 multichannel analyzer. The source was Sn as the alloy Ni,,Sn,B,. In order to improve the count rate, two single channel analyzers were employed, one set on the lZ1Sb Mossbauer escape peak (9 keV) and the other on the photoelectric peak. Each run lasted from two to six days. Velocity and zero channel calibration was determined from the six line room temperature spectra of a standard a-iron foil. The results were analyzed on an IBM 370, model 165 computer utilizing an iterative least-squares procedure [5]. Each spectrum was fit to an eight line pure quadrupole interaction and finally, where necessary for an improved fit, to a twelve line quadrupole interaction including asymmetry (y). The relative line positions and intensities of these interactions are given by Shenoy and Dunlop [6]. The ratio of quadrupole moments of the excited state to, the ground state was taken to be 1.32 [7]. After a preliminary investigation of linewidths, a value of 2.35 mm/s was found to be consistent throughout the series. The pure quadrupole and asymmetry programs were then altered so as to constrain the linewidth to 2.35 mm/s in order to reduce the error in the asymmetry and quadrupole coupling constant. A correction was made on the quadrupole coupling constant and asymmetry parameters due to the influence of absorber thickness [8].

3. Results and Discussion.

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The Mossbauer spectrum of each hydrate displays a single peak whose linewidths are all near 2.35 mm/s, which is very dose to the natural linewidth for antimony (2.10 mm/s) (7). This verifies the existence of only one Sb(V) lattice site. The isomer shift values, relative to InSb, were all quite similar, ranging from

+ 3.3 to

+

4.1 mm/s. Asymmetry was observed in the trihydrate and tetrahydrate.

The large increase in the isomer shift when the first water is added to SbC1, points to the presence of an Sb-0 bond formed at the sixth octahedral position. This increase is caused by a reduction in 5s electron density at the antimony nucleus due to the electron withdrawing effect of the electronegative water. The quadrupole coupling constant nearly doubles, substantiating this theory. Further isomer shift changes in the series are small in comparison since there are no new bonding sites or available electron pairs on the antimony. Only minor changes take place with the

ISOMER S H I F T Cmm/s)

FIG. 1.

-

Mossbauer absorption spectrum of SbC15.3 H z 0 at

77 K relative to InSb.

Mossbauer Parameters of Antimony Pentachloride Hydrates at 77 K

8

(*I

Compound (mm/s) (mm/s> e2 qQ 4

(*) All isomer shifts are relative to InSb.

addition of the second water reinforcing previous theories that it is hydrogen bonded to the first. This second water would then not directly affect the 5s electron density or the efg at the antimony nucleus. The relatively large decrease in quadrupole interaction with the addition of the third water of the trihydrate infers some type of bonding with the antimony itself. This decrease results from an elongation of the efg along the vertical axis indicating that the third water attacks along this axis, probably from below and opposite the position of the first. The resulting distortion from octahedral symmetry due to the seventh ligand gives rise to the large asymmetry observed. Only minor changes in the Mossbauer parameters are observed between the trihydrate and tetrahydrate. This is to be expected if the fourth water attaches to the third, via hydrogen bonds, in a manner similar to the attachment of the second water to the first. The asymmetry is of course still present since this fourth water cannot restore the octahedral symmetry.

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M~SSBAUER INVESTIGATION O F WATER ADDUCTS OF ANTIMONY PENTACHLORIDE C6-533

FIG. 2.

-

Structural diagrams of the four hydrates of antimony pentachloride : A) SbC15. H20 (Monohydrate), B) SbC15.2 H z 0 (Dihydrate), C) SbC15.3 H z 0 (Trihydrate). D) SbC15.4 H z 0

(Tetrahydrate).

drupole moment is positive. Thus the efg is drawn out along the vertical axis as a result of the first water molecule. The monohydrate is then an MAB, octahedral complex. The dihydrate is quite similar except that the second water is hydrogen bonded to the first. In the trihydrate, the third water approaches the antimony as a seventh ligand along the same axis as the first water but probably opposite to it. The actual type of bonding that takes place is uncertain, however, since there is no significant change in isomer shift, we can only infer the existence of a weak partial bond which serves to interact with the efg. This seventh ligand gives rise to octahedral distortion which is observed in the large asymmetry parameter. The tetrahydrate has Mossbauer parameters similar to the trihydrate, so the fourth water probably hydrogen bonds to the third as does the second to the first.

Certainly the differences between these hydrates are very subtle. In the near future the hydrates will be investigated again so as to yield improved data on isomer shifts. A more exacting interpretation of the

isomer shifts will then be possible.

References

[I] BERNANDER, L. and OLOFSSON, G., Acta, Chem. Scand. 27 [6] SHENOY, G. and DUNLAP, B., Nucl. Instrum. Methods, (1973) 1034. 71 (1969) 285.

[2] OLOFSSON, G. and OLOFSSON, I., Tetrahedron 29 (1973) 1171. _[7]_{S T E ~ N S ,}_J._{and STEVENS,}_{V., MiiSSbauer Data Effect Index} [3] ORTWEIN, R. and SCHMIDT, A., Z. Anorg. Allg. Chem. 408 _{IF1 (Plenum, New York) 1974.}

(1974) 42.

[4] PrconN, G . and VISTE, P., Bull. Sot. Chim. Fr. 7-8 (1974) L8] SHENOY, G . and FRIEDT, ''3 Nut*' Instrum' Methods

1291. (1974) 573.

[5] RUBY, S., KALVIUS, G., SNYDER, R. and BEARD, G., Phys. [9] BOWEN, L., STEVENS, J. and LONG, G., J. Chem. Phys. 51