Some physical properties of technetium pentafluoride

J. inorg,nucl. Chem..Supplement1976. PergamonPress. Printedin GreatBritain

NOTES Some physical properties of technetium pentafluoride (Received 4 February 1974)

MUCHattention has been devoted during the past few years to the transition metal pentafluorides, particularly their vibrational spectra. Initially, spectra of NbFs[I] and TaFs[I,2] were interpreted in terms of monomers of D3, symmetry, mainly because these studies were not carried out over a sufficiently wide temperature range. Interpretations in terms of cis-fluorine bridged polymers[3] now appear fully justified, particularly as these are supported by Raman spectral4,5], molecular weight measurements [5] and molecular beam-mass spectrometric measurements[6] over wide temperature ranges. In view of this, assignments of the vibrational spectra of MoF5 in terms of monomers of D3, symmetry[7] should also be reviewed. Gas phase Raman spectra of MoF, as a function of temperature showed changes of relative peak intensities in the 700-750 cm ' region, indicating changes in monomer-polymerconcentrations [8]. The main features which characterize gas phase Raman spectra of genuine D3, monomers such as PFs[9], AsF,[10, 11] or VF5 [11] in the bond stretching region are one strong polarized band and another totally symmetric mode which is usually weak and only slightly polarized. For the cis-fluorine bridged polymer, the spectroscopically important features are the vibrations of the C2~ MF. residue together with the fluorine bridge modes [5]. In the Raman spectra, the former manifests itself as two strong polarized bands, corresponding to the axial and the equatorial symmetric stretching modes, while the latter are expected to be weak in intensity. The Raman spectrum of liquid TcFs, which we now report (Fig. I), is quite similar in appearance to those of the cis-fluorine bridged polymers which are characterized by two intense, strongly polarized bands. Technetium pentafluoride melts at 50°, but is readily obtained as a supercooled liquid at room temperature [12]. Raman spectra at temperatures between 80° and 100° do not differ substantially from those at room temperature, as is the case for MoF~. This does not necessarily indicate the presence of monomers only; rather it is expected that only at considerably higher temperatures these occur in appreciable concentrations [4, 5, 8]. The Raman spectra of liquid NbF~, TaFt, MoF~ and TcF~ are compared in Table 1. Assignments cannot be made in the absence of temperature dependent studies over a wide range. Because of the radioactive nature of technetium and the disproportionation of TcF, at higher temperatures[12], these were not attempted. Attempts to obtain

Raman spectra of solid TcF5 were not wholly successful, because the solid absorbed enough energy from the laser beam to cause local melting. For solid TcF5 at -50 °, the bond stretching region is dominated by three strong, sharp bands at 698, 750 and 775 cm ', in addition to several weak ones. In this respect, it resembles MoFs[7, 13] as well as NbF5 and TaFt[3]. Crystallographically, TcF5 belongs to a group of which VF~ is the prototype[14, 15] in which the units are linked in endless chains by cis-bridging via fluorine atoms. The similarity of the Raman spectra as well as other physical properties indicate that TcF~ is polymerized in the liquid as well. Technetium pentafluoride is only very sparingly soluble in anhydrous hydrogen fluoride, and attempts to obtain Raman spectra of such solutions were unsuccessful. It is thus similar to MoF~, which has been shown[16] to be a very much weaker fluoride ion acceptor (Lewis acid) than SbF~ and AsFs. We have also measured the magnetic susceptibility of TcFs. The results are given in Table 2. Diamagnetic core corrections have been included. If we assume that TcF5 obeys the Curie-Weiss law, we obtain a magnetic moment of 3.00 B.M. and a Weiss constant of 156°. The magnetic moment is thus close to the spin-only value for two non-bonding electrons and rather higher than that of the analogous ReF5 [17] (/~ - 2.5 B.M., 0 = 580°). The differences can partially be ascribed to smaller spin-orbit coupling for technetium. It is of interest that the spin-orbit coupling constant (~-450 cm ~') obtained for TcF, form Kotani's theory[18] corresponds approximately to that derived for the alkali hexafluorotechnetates (V) [19], but is lower than that predicted for octahedral Tc (V) by Figgis and Lewis [20]. The discrepancies may also be due to deviations from octahedral symmetry. Although nominally in TcF~ each central atom is surrounded by six fluorines, these do not constitute an octahedral environment, as there are considerable differences in the terminal and fluorine bridged Tc-F bond lengths. This makes detailed interpretations of the results difficult[19]. EXPERIMENTAL Technetium pentafluoride was prepared from the hexafluoride by the reduction with iodine in pentafluoride, according to IF 5

12+ 10 TcF~---~10 TcF~ + 2 IF,.

Table 1.Raman spectra(cm ') of liquidpentafluorides NbFs*(85 °, 120°)

TaFs*(100°, 120°)

MoF~t(25 °, 80°)

TcF~;(25 °)

763 s (pol) 726 w 680 m (pol)

755 vs (pod 712 vw 690 w (pol)

747 s (pol)

749 s (pol)

703 m (pol)

693 s (pod 669 w

256mw 227 w 180 w

251 m 222 w 184 w

440 w, br 231 w 201 w

*Refs.[2, 3]. fRef.[7]. SThis work. 231

282w 225 w 139 w

232

Notes I

I

I

I

f

693

749 >, ~P

E

B 1

I130

l

I

I

I

1

I

I

200

300

400

500

600

700

800

Wovenumber (cm-q

Fig. 1. Raman spectrum of liquid TcF~ at 25°. A--I( [1) B--I(±). Spectral slit width: 5 cm-'. Scan speed: 50 cm-1/min. Time constant: 0.25 sec. Laser power: 70 mW at 6471/~. at sample. Table 2. Magnetic susceptibility of TcF5 T(°K)

X(× 103)

g,,~r(B.M.)

80 98 125 148 197 224 249 273 300

4.59 4.33 4.03 3.72 3.20 2.97 2.73 2.63 2.41

1.72 1.85 2.03 2.11 2.25 2.32 2.34 2.41 2.41

Based on the amount of iodine taken, the yield of TcF5 was quantitative. The procedure was nearly identical to that used [15] for the synthesis of OsF5 and is preferable over the fluorination of technetium metal in which TcF5 was obtained as a by-product [12]. The sample for Raman spectroscopy was transferred in a dry box into a 1/'4in Teflon FEP tube which was heat sealed at one end and closed at the other end with a l/4in flare plug. The supercooled liquid sample did not solidify during the spectral measurements. In order to avoid radioactive contaminations, manipulations were carried out within a polyethylene glove bag introduced into a Vacuum-Atmospheres Corp. Drilab in a nitrogen atmosphere of less than 1 ppm water content. Raman spectra were recorded on a spectrometer described elsewhere [2 l]. Magnetic susceptibility measurements were carried out by the Faraday method on two separate samples (57 and 72 mg). Results of two series of measurements were in good agreement. Samples were contained in a machined Teflon TFE capsule with threaded closure. Corrections were applied for the diamagnetism of the capsule. Measurements were performed on an Alpha Model 1400 magnetic susceptibility system in conjunction with an Ainsworth model 1071 electronic balance (sensitivity: 3 gg).

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