Infrared spectrum and force field of tungsten hexachloride

Jouml of Molecular Structure. 36 (1977) l-6 @Eisevier Scientific Publishing Company, Amsterdam -

INFRARED SPECTRUM HEXACHLORIDE*

R. S. MCDOWELL, Los Alamos Scientific 87545 (U.S.A.)

Printed in ‘Ibe Netherlands

AND FORCE FIELD OF TUNGSTEN

R. C. KENNEDY, Laboratory,

L. B. ASPREY,

University

and R. J. SHERMAN

of California.

Los Atamos,

New

Mexrco

(Received 18 May 1976)

ABSTRACT Infrared spectra of W’CI, and W’CI,, prepared from 99.4% “Cl and 90.4% “Cl respectively, were recorded in CCI, and CS, solutions_ ‘Ihe isotope shifts in the Flu fundamentals Y, and Ye were used to determine the general quadratic symmetry force constants of WCl, , the force field obtained is quite different from several that have been previously reported using various approximation methods. Corresponding valence force constants are f, = 2.39, (-f,) = O-15 mdyn A-‘. These constants are comf, = 0.24, f,,’ = 0.16. and f, - f,,’ pared with those of WF,.

INTRODUCTION

Tungsten hexachloride is the best characterized of the few known neutral metal hexachlorides. The force constants of this molecule are of interest for comparison with those of the corresponding hexafluoride [l] ; in addition, its metal-chlorine force constants may be transferable to the interesting class of mixed halide complexes such as WF&l and WFJC12. WC16 molecules have octahedral (0, ) symmetry, rvi,, = A,(R) + Eg( R) + 2FiJIR) + F2JR) + F,,(inactive). The W-Cl bond length determined for the vapor by electron diffraction is 2.26 + O-02.4 [2] ; the value found in the solid by x-ray diffraction is 2.24 A [33 . Previously reported vibrational data include the Raman spectra of the vapor 141, solutions [ 53, and solid [6,7], and infrared spectra of solutions [8] and solid [7] _ In the F,, block the only available data have been the two fundamental frequencies, so force constant calculations have thus far been limited to Urey-Bradley [ 7,9, lo], orbital valence [l.O] ,extremal(Fa4 minimized) [ll, 121, and other [7, 91 approximations; no general quadratic force field has been reported. In the present work we have used the chlorine isotope shifts in the infraredactive fundamentals v3 and v4 as the additional data necessary to fix the Fi, force constmts. Because of the very low vapor pressure of WCl, at room temperature [I 31, we have taken the spectra in C-Cl, and CS2 solutions. *This work was supported by the U.S. Energy Research and Development Administration.

2 EXPERIMENTAL

Chlorine isotopes were purchased as NaCl from Oak Ridge National Laboratory; the isotopic analyses and resulting distributions of WCl, species are summarized in Table l_ The first step in the synthesis was the preparation of Cl, from NaCl. Typically, 2.0 mmol NaCl was mixed with 1.1 mmol KMnO, in a pyrex bulb; the bulb was evacuated and an excess of 12N HzS04 added. The resulting rapid reaction was completed in a few minutes, after which the bulb was submerged iu liquid nitrogen and pumped on to remove noncondensables_ It was then warmed to - 80°C and the chlorine cryopumped to a vessel where it was stored over PzOs. The infrared spectrum of the product showed no detectable impurities. To prepare WCl,, about 0.5 g powdered tungsten metal was first placed in a quartz tube, pumped on for several hours, and then heated to red heat (-700-800°C) in an atmosphere of purified hydrogen. The tube was evacuated and the metal then heated again to redness while being pumped. After this treatment, about 1.0 mm01 Clz was condensed on the metal at -196°C and the system again heated to -800°C. Dark blue-black crystals of WCle soon appeared on the upper portions of the tube_ After the consumption of >90% of the Clz, the system was evacuated and the desired amount of solvent (driedusing amolecularsieve)wascondensedin.The resulting deepredTABLE1 Abundances

of differentisotopicspeciesof WCl, (%)a

Species

NaturalWCI, M= 0.7577 N=0.2423

'Cl sample M = 0.9935 i 0.0006 N= 0.0066t 0.0005

wq WY!1 5"Cl

18.92 36.31

96.16 i 0.29 3.77 * 0.28

"Clsample M = 0.0964 T 0.0010 N= 0.9036 it0.0010 0.000 0.005

b

WX5&7& W"a ~q Ww:w:b W='Cl"Cl I W"Cf Mean6atomicmass

29.03 12.38 2.97 0.380 0.020

O-062* 0.001 0.000 0.000 0.000

0.009

0.106 1.32 9.29 34.84 64.43

* f i i

0.004 0.04 0.15 0.17 0.36

ofliganda(amu)d

35.453

34.982*

0.001

36.773 zt0.002

'If MT u andN* o(M-t N=1)aretbefractionalabundancesofthe"Cland~7Cfisotopes,respectively,the fractionalabundance oftheapeciesW'SCI,3'CI,(m-+ n = 6) is (6!/m!n!)MmN"[1* e I(m/M)-(n/N)I]. bThespeciesW'SCI,"CI,and~sCI,nCI,existascisandtm~ isomerswitbrelative~abundancesof4:l. CThespeciesW35Cl,r7Cl,existsas fat and mer isomerswithre]ativeabundanceso_f2:3. dThemagsesof'5(=1and'7CI-~eetakentobe rnM = 34.9688hand mN= 36.9659Oamu; the propererrorlimitsonthemeanligandi;tomic~ arereaciiJyshowntohe a(m~-ma).

3

brown solutions of WCl, were transferred to evacuated infrared cells. Care had to be exercised throughout the above procedure to exclude air and water vapor from the system, or appreciable amounts of WOCl, were formed; this compound has a strong infrared peak at about 380 cm-’ [S, 141 that can interfere with the measurement of the WCl, absorption near 365 cm-‘. Infrared spectra were recorded on a Perkin-Elmer Model 180 double-beam grating spectrometer that was calibrated using the IUPAC tables [ 15,16]_ Cell windows were of CsI for all bands. We originally tied polyethylene windows for the v, region, but experienced difficulty with an apparent slow reaction of the WQ: the solution evolved a gas and became pale yellow, a dark solid was deposited on the windows, and v3 disappeared over the course of several hours. This reaction did not occur when using CsI windows, and although 160 cm-’ is at the extreme edge of the useful range of this window material, we were able to secure satisfactory spectra by using thin (1 mm) windows with compensating windows in the reference beam, and scanning at high gti and very slow speed. The entire infrared spectrum was monitored for possible adsorptions due to oxyfluorides, and also to WCIS 1171, since there is the possibility that WCl, can he reduced to the pentachloride in organic solvents [6]. For all spectra used in the final measurements, such impurity bands were either absent or of such low intensity that they caused nr) interference_ The measured frequencies are given in Table 2. FORCE

CONSTANTS

The product ratio v3v4(W37C16)/v~v4(W35C16), using the frequencies of Table 2, is 0.9640 + 0.0007, iu satisfactory agreement with the theoretical ratio of 0.9642. Table 3(A) summarizes the determination of the off-diagonal symmetry force constant FM_ The isotope frequency shifts for the two fundamentals yield mutually consistent values for this constant. The symmetry force constants of WCl, are presented in Table 3(B)_ For the Raman-active fundamentals vI, vl, and us, we have used in these calculations what we consider to be the most reliable frequencies from previous Raman studies [4-7]_ (There has been some disagreement on the frequency TABLE

2

Infrared

absorption

frequencies

Band

solvent

natural

v, + y, VI +v. “+ + “4 “3

ala, CS, CCL

V.

CCL

767 569 466 366 163

(cm-’ WC2

) of WCl, solutions JsCl sample

“Cl sample

TX--“Cl

366.8 IS+.0

369.3 161.4

7.5 * 0.1 2.6 + 0.1

shift

4 TABLE

3

Force constants of WCl, (mdyn A-?) * (A} Determination of F,., from the isotope frequency shifts W’Y3 -W3’Cl shift in v w%ib-w%16 . d shift in Y3 Fina&ahX?~ (B) Symmetry force eonstantac F*, -fr+-4frr+frr‘

F,=f,--2f,+f,’ F,, =fr--fm’ Fu = fa + 2f,, - Yam” - fan”’ F, = 2(f,, - fr,“) F,, = fa - !Jfaa’ + faa” Fa = fa - 2f& + 2f,,” - faa’”

(C)

0.331 i 0.025 0.314 f 0.066 0.328 i 0.023 3.51 2.07 2.24 0.208 0.328 0.142

f * * 2 + *

2.39 0.24 0.16 0.148 0.164

* 0.03 -, 0.02 f 0.03 LIZ 0.011 f 0.011

0.10

0.09 0.07 0.01 0.006 0.023

0.009

f 0.04

Valenceforce constants :r

(c&z intfzaction) frr’ (trims interaction)

f, - faa’ t-f,) f TQ- fra”

p1 mdyn A-" = 100 N m-l. See the text for a discussion of error limits. bAverage weighted according to the inverse squares of the estimated errors. CAsuming v, = 410 2 5. vz = 315 f 5, v, =165* 5andv,=lOO* 20cm-‘.

of y5, but a value near 165 cm’-” now seems convincingly established [4,73 .) The values chosen are g-hen in the footnote to Table 3, and have been assigned uncerkinties of -I 5 cm-’ to reflect the scatter in frequencies reported by different investigators_ There is yet no experimental evidence for the position of u6, since the difference band from which Evans and Lo [S] obtained v6 = 97 cm-’ was probably due to WOCL 171, but it is expected to lie near 100 cm-’ [‘7,8]. We have used this value, with a generous uncertainty of * 20 cm-‘, to estimate the force constant FG6. The error Limits in Table 3(B) arise from these uncertainties in the frequencies, except for the Fzu block (FSk cr;T, F&, where they r&e& the probable &rrorin the deviation of .Fx4. There is no point in a~rnp~ to make ~~o~~i~ corrections with the available data, but in practice the use of uncorrected frequencies will result in errors of only a few percent in F1 I, Fz2, aud F,,, and no significant error in the other F values [ 18 1. VibrationA amplitudes and shrinkage effects calculated from the force Constants of Table 3 are presented in Table 4. The alternate solution for the F,, block is 3’33= 0.910, F+,= 0.873, FS4 = 0.660 mdyn A-‘, which reproduces the frequency shifts as well as does the force &Id of Table’ 3. These alternate constants would result in a w--cI amplitude +f 0.080 A at 300 K and 0,096 A at 450 K, sufficiently different from those-of-Table 4, so that the two force fields could be easily distinguished by an ~~~n~tio~

5 TABLE

4

Root-mean-squarevibrationalamplitudesandshrinkageeffectsin WCX,(A)" T-OK u(W--cI) u(Cl. - - Cl, short) u(Cl- - - CI, long) 6(Cl--- Cl,short) 6(Cl.-- Cl,long) ‘Calculated 0.02A [2].

0.0405? 0.0680 L 0.0528 i 0.0005 5 0.0022i

T=300K 0.0003 0.0019 0.0003 0.0001 0.0002

0.0499 0.1123 0.0646 0.0013 0.0075

T=450K + i 2 2 f

0.0007 0.0090 0.0007 0.0003 0.0018

for a mean chlorine mass of 35.46 amu and a W-Cl

0.0582 0.1353 0.0753 0.0019 0.0110

2 0.0008 + 0.0111 f 0.0009 + 0.0005 -c0.0026

bond length of 2.26 %

determination of this vibrational amplitude_ Unfortunately, Ewens and Lister [2] did not estimate the amplitudes in their early study of WCl,; however, the alternate force field is physically unreasonable and can be rejected on the ground that it interchanges the normal coordinate designations of the fundamentals, making v3 essentially a bending mode. Valence force constants calculated from the final set of symmetry force constants are given in Table 3(C) . DISCUSSION

Two approximate methods of estimating valence force constants have been applied to WC& [7,9,11,12] _ In the often-used extremal approximation, the correct F,, force field is assumed to be that for which Fe4 isminimized. For WCl, this corresponds to FsJ = 0.10 mdyn A-‘, compared to the correct value of 0.33 mdyn A-‘. Van Bronswyck et al. [ 71, considering a series of MCI,- and MC&‘- ions and WCl, itself, proposed the relation f,’ = (4/3)f, in order to obtain f, from frequency data for a single isotopic series. (Their “generalized valence force field” has faQ” = f&“’ = fm” = 0 and faa = faa’ in addition to the constraint on f,‘, and thus is not a GVFF in the usual sense of that term.) This assumption is equivalent to requiring that Fs3 = (lOF,, - F,,)/9.Apart from the fact that there is no physical justification for this, it yields, for WC16, Fag = 0.85 or -0.02 mdyn A-’ instead of 0.33 mdyn A-‘, and thus is even less satisfactory than the extremal approximation. The primary stretching force constant f, is roughly one-half that of WF, [I], which is reasonable considering the greater tungsten-halogen distance (2.26 A vs. 1.83 A) and decreased ligand electronegativity in WCI, . The cis interaction constants f, are about the same in the two molecules, while the tmns interaction constant f,’ is much smaller in WC16 (0.16 vs. 0.42 mdyn A:‘). The positive stretching interaction force constants can be explained by changes in electronegativity during the course of a molecular vibration [ 191-

Thatthere is some influence from non-bonded Cl - - - Cl repulsion can be plausibly argued, firstly from the fact that f, > f,‘, as with SF6 1201 but in contrast to both WF6 [l] and MoF6 [18], and secondly from the large stretch-bend interaction constant f,o - fro”, which exceeds that of either WF6 or MoF,. Non-bonded repulsion alone, however, would predict f,’ y 0, as with SF6 1201, so other effects such as eleclxonegativity changes must be operatjve. As with the hexafluorides, the large positive value off, eliminates orbital following as an important factor [19]_ The primary bending constant f,, is small enough to indicate that the effect of ligand repulsions on the bending modes is only minor; so does the interaction constant faa - faa” = (1/4)(F.,4 - F& = 0.03 r~_0.01, which would be negative if Cl - - - Cl repulsions were significant 1191. Previous studies [18, 191 have indicated that in the me&l hexafluorides no one simple bonding picture dominates, and apparently the same is true of WC16. We are currently investigating both UC16 and mixed halide complexes of tungsten, and the general quadratic force constants of these molecules may help to clarify some of these effects.

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