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Full text of "Molybdenum hexafluoride MoF6"

NASA TECHNICAL TRANSLATION NASA TT F-12,702 



CsJ 

o 

^ MOLYBDENUM HEXAFLUORIPE MoF 



6 
Maurice Carles 



< 



Translation of "L 'Hexaf luorure de Molybdene ^^o¥/^ 

Commissariat a L'Energie Ato/iique, Pierrelatte (France), 
Report CEA-BIB^124, pp. 1-25, Sept, I968. 



N 



70-1157^ 



(ACCESSION NJW3ER) 



45 



1PASE61 




-,.;ASA en OR TMX OR AJ NUMBtR) .**...«—. , ... , .^ 



NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 
Washington, D. C. 205^6 NOVEMBER, 1969 



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Classification Schedule 

1. Industrial Application of Isotopes and Radiation 

2. Biology and Medicine 

2.1 . General Biology 

2.2. Nuclear Indicators in Biology 

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11 



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111 



r 

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The C.E.A. bibliographies, starting from 1968, are available at the 
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IV 



NASA TT F- 12, 702 
MOLYBDENUM HEXAFLUORI DE MoF^ 

M, Carles 

ABSTRACT. MoF^ is generally prepared by direct synthesis 

of its components. The molecule is entirely symmetrical in 
structure; the metal atom is located in the center of an 
octahedron, the apexes of which are occupied by fluorine 
atoms. When cold, this compound appears as a crystallized 
white solid body. It melts around 17.5*^C and boils around 
35°C under air pressure. Very sensitive to hydrolysis and 
reduction, molybdenum hexafluoride forms addition compounds 
with sodium, potassium, rubidium, and cesium flour ides. It 
also forms complex compounds with nitrile and nitrosyl 
fluorides and nitrogen oxides. The reactivity of this 
symmetrical molecule shows the nonsaturation properties of 
molybdenum regarding coordination. Inversely, the high 
inertia of sulfur and seleni-m hexafluoride molecules, 
which are sirnilar, is to be noted. 

1 . Introduction. 

Molybdenum, of the 6a transition group, is one of the numerous metals or [V 
metalloids that yield a volatile hexavalent compound with fluorine. Let us 
mention among then MoF., WF , UF., PuF,, NpF., TeF^, R^F^* SF^, SeF^, OsF., 

etc. 

Many of these compounds have a similar structure and very close chemical 
properties. The strucutres of said formula XY^ molecules, and particularly 

the structure of MoF^, have been studied by many investigators. Let us men- 
tion here the remarkable synthesis of B. Weinstock [1] on the comparative prop- 
erties of these different hexaf luorides . 

Other studies have dealt with the complexes that said molecules are cap- 
able of forming with certain fluorides, in particular MoF. , UF^, and PuF^, 

these latter works being generally directed either to the sepatation of fission 
products obtained by volatilization of irradiated fuels or to the purification 
of UF^ by dry means. 

^Numbers in the margin indicated pagination in the foreign text. 



We express our appreciation to Mrs. Courtot and Mr. Simon, of the 
Document Service, for their val^uable assistance in the preparation of this 
paper. 

2. Preparations 

2.1. By Direct Action of Fluorine on the Metal 

The preparation of molybdenum hexafluoride was described for the first 
time by Ruff and collaborators [2, 3], who prepared it in 1907 by causing dry 
fluorine at about 60° to react directly with the powdered metal placed in a 
platinum tube. 

Fluorine can be diluted in a nitrogen current [4]. 

The reaction can also be carried out at a higher temperature (31S°C) in 
a nickel apparatus, where yield is then 78% [5]. 

Reaction can also take place in a copper tube [6]. The product obtain- 
ed, collected in a dry and grease-free glass trap, is purified by vacuum dis- 
tillation [6, 7]. It can be kept indefinitely in the glass vessel at room [Jl 
temperature without formation of SiF - [6] . 

2.2. By Action of Bromine and Chlorine Trifluoride on the Metal or the 
Corresponding Oxide. 

BrF reacts with the metal at 3S0°C to yield MoF [4], [8]. 

Nikolaev [9, 10] 'ises either gaseous chlorine trifluoride, C1F«, which 

he causes to react with the metal powder in a quartz reactor or bromine tri- 
fluoride, BrF^, which he causes to react with the oxide MoO-. 

The use of BrF, is preferable in the latter case, because the reaction 
of GIF with the oxide is more violent. The action of BrF- can be controlled 
better. 

2.3. MoFx is Also Obtained by the Action of Anhydrous Hydrofluoric Acid on 
Molybdenum Pentachloride in the Presence of Air [?]• 

l.k. By Action of Sulphur Tetraf 1 uoride on the Corresponding Acid 

Excess SF. reacts with metallic oxides to yield a fluoride or an oxy- 

fluoride by substitution of two atoms of fluorine for one atom of oxygen. 
With temperatures ranging between 20 and 500^, depending on the case, MoF^ 

was obtained in this manner [11, 12]. 



2.5. MoF^ Has Been Prepared by Dl rect jFluorinatlon In a KF.2HF Electrolyte ' 
and Measured by Potentlometpy <{;l3il asi^wel 1 as According to the Reaction ! 

1 

Mo ♦ 6NOF . 3 HF > MoF^ ♦ 6 NO ♦ 18 HP (M) 

3. Methods of Analysis in the Gaseous Phase 

3.1. Gas-Liquid Chromatography 

This method enables us to measure gaseous MoF, in a mixture with another 

o 

gas. The coltuims used are polyfluoromonochlorethylene granules impregnated 

with oil of the same nature. It is thus possible to measure MoF^ in UF^ [14] 

or to measure MoF^ and W in CIF^ [15]. 

3.2. Infrared Spectrophotometry /3 

As the MoF^ absorption spectrum presents very sensitive infrared absorp- 
tion bands (reference given in the structure chapter) it is possible in many 
cases, to measure traces of MoF^ in a gas with the aid of a spectrophotometer 

covering the 2.5 - 20 micron range anH an analysis cell adapted to fluorinated 
elements [16, 17]. 

3.3. M35S Spect rome t ry 

It is possible to measure MoF^ traces in natural UF^ using a mass spect- 
rometer with which the peaks corresponding to ions ^^oF^ and ^^^UFt are 
compared. 

An automatic apparatus based on this principle was developed by the Oak 
Ridge gaseous diffusion plant [18]. 

3.^. Atomic Absorption 

Gaseous uranium hexafluoride is sent to a premix heater and the molyb- 
denum is determined from a standard curve obtained from synthetic mixtures of 
uranium hexafluoride and molybdenum hexafluoride. The field of optimal con- 
centration ranges between 10 and 50 ppm of molybdenum by weight in relation to 
uranium. Ultimate detection is 2 ppm [94]. 

4. Molecular Structure 

Numerous investigators have studied the structure of the regular octa- 
hedral molecules of which molybdenum hexafluoride is a part. The metal atom 
is situated in the center of an octahedron whose summits are occupied by 
fluorine atoms. This structure is deduced from the study :)f the infrared 



! 

[-spectra of the Raman spectra, of electron diffraction, as well as from the 
I nuclear magnetic resonance. 

, '^.1. Infrared Spectrum of the Gas 

Only the basic V^ and V. bands are active in the infrared. They are 



observed in the following frequencies expressed in cm 



-1 



Bands 


(1») 


.References 
1 [20] 


(ID 


^ 


742 


741 


741 


^4 


U9 


• 

MO 


ac4 


^ 


040) 


(234) 


(I»0) 



/4 



Band V^ is inactive in the infrared. The values 

given in this table were calculated. (See also 
[22] and [23]). 

k.2, Raman Spectra 

The basic bands active in the Raman spectrum for the gas and the liquid 
are: 



Bands 



(24) Uq. 



References 
(20) liq. 



r 



[21] gas 



796 • 



-I 



741 liq. 



741 ♦ 1 



641 cm 



645 Uq. 



643 4^5 



316 cm 



-1 



331 Uq. 



uncertain 



^.3. Absorption Spectrum in the Ultraviolet 

The UV absorption spectrum were studied by Tanner and Duncan [24]; they 
found the following a values for the absorption coefficient: 



MoFr Pressure 



6 


Torr 


o 


en* 


• 

A 




ssa 


o.og 


35 524 


(2815) 




7.07 


6.8 


41 484 






a. 51 


18 


43 280 






0.S4 


57,6 


48 356 






0.34 


61.4 


48 852 






O.M 


68.4 


48 358 






0,S4 


83.6 


48 702 






0.S4 


127 


50 226 






0,34 


176 


50 761 


(1870) 


Jr. 


Note: C( 


Dmmas Indica 


ite decimal points. 1 



/5 



The coefficient a is defined according to the following formula: 

Tr. Note: Cornmas indicate decimal points • 

La I / lo • a lo 



where I/To is the luminous energy fraction transmitted by a cell, lo in length 
expressed in cm, filled with gas at a pressure of 760 Torr measured at 0°C. 

4.4. Nuclear Magnetic Resonance Speccrum 

The high-resolution magnetic resonance spectra of the fluorines of the 
liquid hexaf luorides , MoF^, WF^, UP., contain essentially a single line in 

agreement with a regular octahedral structure. For MoF^ this is a weak and 

distant field line at 56.4 MHz, with 15,910 ± 40 Hz in relation to the mono- 

fluorotrichlorethane taken as reference, Rigny [25]. The relaxation time of 
the molecule measured at 2.8, 16 and 56.4 MHz is equal to: 



Tj • 0,85 ♦ 0,1 • kZrC (25) 



Blinc and collaborators also studied the NMR spectra and the relaxation 
time as a function of temperature and of the magnetic field [26J. 

4.5. Atomic Distances 

All of these studies agree in defining a regular octahedral structure of 
Oh symmetry group--a structure common to numerous molecules of the XY^ formula. 

The length of the metal-fluorine link is equal to: 



Mo 



r • l.MA^ (17) (18) 
P • 1.840 A (84) 



^.6. Force Constants Z^ 

The force constants calculated and expressed in millidynes/A are equal to: 



k' 


HM 


rad 


f 


l» --!#•• 


rH 


r"H 


M# • - r'd#) 


BM. 


4.8875 
4.71 


O.IS38 
0.844 


0.1561 


0.890 


0.0810 


-0. 104S 


•0. 91118 


0.4846 


(89) 
(90) 



fd 



Jr. Note: Commas indicate decimal points, 
being the Mo-F fo?.'ce constant. 



fdd and f'dd the adjacent and non-radjaccnt link interaction constants. 

f0 the deformation constant. 

The other constants being interaction link - link and link - deformation 
constants. 

See also [27] and [31] to [39]. 

For electron diffraction see [40]. 

For the Bastiansen-Morino effect see [41]. 

For the Jahn-Teller effect see [42, 73]. 

5. Physical Properties 

When cold MoF^ is a snowy crystallized solid; it melts at 17.5** and boils 
at 35'C at atmospheric pressure [43]. (Figure 1). 



5.1. The physical constants are: 

Units 



Values 



/7 



References 



Molecular Mass 



Melting !>oint 



6oi 1 ing Point at 
760 mm Hg 



''C 
**C 
"K 
*C 
*C 



209,93 

17.5 
17 
290,76 
36,0 
35" 



(A6) 

(^3) (^6) (47) 
(23) (44) (45) (48) 

(57) 
(2) (44) (49) (48> 

(45) (47) 



Units 



Values 



Reference 



Vapor Tension 

for the sol id 
For the liquid 



p 


in 


nvn 


Hg 


t 


• 

in 


»c 




p 


• 
in 


nun 


Hg 


T 


• 
in 


»K 




P 


in 


rum 


Hg 



T in "K 



Densi ty of the sol id 

Calculated by x 

diffraction 
Calculated by I ine 

displacement 



Density of the liquid 
Measure by pycnotneter 

Magnetic susceptibility 



Solid transition 
temperature 



Valid from 291 to 320**K 

2.88 at •t-S^'C (calculated) 

3.27 at -36*C(calculated) 
3.5I9 at 77. 1 6* K (measured) 

3.5I9 at 77-l6°K(calculated) 
3.393 at 173.83*K(measured) 
3.393 ** 173.83°K(calculated) 

2.5A4 at 294.33°K(measured) 
2.5^^ at 29^. 33** K (calculated) 

2.491 at 307.08*K(measured) 

2.492 at 307.08*K(calcu>ated) 

Xg - - (0,124 ± 0,005) 10-6 

Xs = -26.10-6 e 25*C 

263,6 (-9,6*'C) 

263,48 1 0, 02 

-9.8» 

-8,7 



Note: Commas indicate decimal points. 




9^ M «0 
Molar % UFg 



Figure 1* MoF^ - UF^ 

Sol id-Liquid Equi 1 i- 
brium Diagram. 



^.2. Crystalline Structure 

Molybdenum hexafluoride crystallizes at room temperature following the 
body cente"«^ed cubic system; the lattice constant is equal to 



/8 



at + lO^^C 
at + S'^C 



a ■ 6.aS j:0«01 A (55) 
a « 6.23 ^0.04 A 'SO) 



Below the transition point (-9*6°) the orthorhombic structure is obtained. 
The lattice constants are then 



at -20°C 



at -se'^c 



a « 9.65 ^0,02 A 
b - 6.68 ^C, 03 A 
c 5.05 -«- 0,02 A 



a - 9.61 ^0.02 A 
b • 8.75 ♦0.02 A 
c - 5.07 ^0.02 A 



(55) 



5.3. Thermodynanic Constants 



Units 



Values 



Reference 



Liquid formation enthalpy 

at 298J6^!<, AH'f 

Gas formation enthalpy, 

H**f 
F *ee liquid formation energy 

at 298.16*'K; AGM 
Standard enthropy of gas, S^ 

Standard enthropy of liquid, S* 
Transition enthalpy o^ solid; 

Triple point fusion heat 
Vaporization heat, aH''293 



Subl imation heat, AH' 



298 



Kcal/Mole 



Kcal/iDole 

Kcal/zDole 
cal/Mole deg. 



cal/Mole deg. 

cal/Molft 

cal/Mole 

cal/MolA 

cal/Mote 



- 390.9 

- 388.6 

- 392.2 

- 390.9 

372.3- +0.022 

- 363.1 

- 361.2 

83.77 + 0,1 
79.76 
80.60 
79,7 +0.6 

62.06 + 0.06 
CO.! ;60.6 

X933.2 + 2.0 
1960 

1034.2 + 1.0 
1060 

6000 
6630 + 25 

8500 




Tr. Note: Cowrmas indicate decimal points. 



The specific heats and thermodynamic functions of gaseous MoF^ at a 

pressure of 1 atmosphere calculated at different temperatures yield the fol- 
lowing values : 



/9 



TTC 


^P' .1 
cal. mole 


caLmoto 


IT. h: 

Cld.lBOl* '^ 


. (c» - voir 


250 


26.425 


76.4S4 


4 201.6 


59.648 


273.15 


27. 544 


78.844 


4 826.S 


61.174 


298.15 


28.61C 


81,303 


5 528.8 


62.760 


300 


28,683 


81.481 


5 581.8 


62.875 


350 


30.415 


86.039 


7 061.1 


65.864 


400 


31.743 


90.190 


11616.5 


68.649 


500 


33.565 


97.486 


11 889 


73.707 


600 


34. 700 


103.714 


15 307 


78.202 


700 


35.441 


109,122 


18 816 


82.241 


800 


35.948 


113.890 


32 387 


85.905 


900 


36. 307 


118.145 


26 001 


89.255 


1 COO 


36.571 


121.985 


29 646 


92.340 



Tr. Note: Conmds indicate decimal points. 

See also [27] for temperatures comprised between 100 and 1,500°K, [51] 
for temperatures comprised between 5 and 350^K as well as [60] to [63]. 

5.^. UF, - MoF, Liquid-Solid Equilibrium Diagram 

The UF^ - MoF^ liquid-solid equilibrium diagram was studied by Trevorrow 

and collaborators [55], Two solid solutions coexist at low temperatures, one 
30% molar MoF^ in UF^ and the other 13% molar UF^ in MoF^, see Figure 1. 

5.5. Solubility in Uranium Hexafluoride and Liquid Hydrofluoric Acid 

Mears and collaborators studied the solubility of different fluorides in 
liquid UF at 70**C [65] for purposes of recovering UF^ by distillation. The 

solubility of MoF^ is more than 22.5% by weight at 38*^0. The only point of 

liquid-vapor equilibrium established by Mears indicates that the behavior of 
the solution is close to that of an ideal solution. 



Trevorrow [68], on the other hand, finds a marked deviation from ideality /lO 
but the four points established show a great dispersion of results. 



Frlec and Hyman studied the solubility of different hexafluorides in 
liquid hydrofluoric acid [68]. They obtained the following values at room 
temperature 



MoF^ 



WF, 



VF^ 



Molea f 1 OOOgd'HF 



1.50 

3.14 
0.49 



Tr. Note: Commas 
indicate decimal points 



R«f. 



(66) 
(66) 

(67) 



r 



MoF, 



WF^ 



UF, 



% in Weight 
by HF 



18,5+1 
: 1 . 52 + 1.5 
0,98 + 0. 5 



Tr. Note: Commas 
indicate decimal points 




The$e authors also measured the 
electric conductivity of these solutions 
[66]. Nikolaev and collaborators also 
studied said solubilities [97, 98] at 

5.6. Recovery of \}F^ Polluted by Moly- 

bdenum Hexafluoride by Distilla- 
tion. 

Several papers deal with the distil- 
lation of UF, - MoF^ mixtures. In ad- 
6 6 

dition to the values of the separation 
factor measured by Mears [65] and by 
Trevorrow [68] we find a description of a 
distilling column [70]. 



6. 



Reference [69] also deals with the distillation of said mixtures. 
Chemical Properties 



As seen in the introduction, molybdenum hexafluoride presents numerous 
chemical analogies with other hexafluorides, especially with those of tungsten 
and uranium, which we shall examine. 

6.1. Reactions by Addition of MoF,, WF^ and UF, to Certain Flourldes 

The molybdenum, tupgsten and uranixim hexafluorides combine with certain 
alkaline fluorides giving alkaline fluoro-molybdates, txmgstates and uranates 
where the transition metal is in the oxidation six state. Said fluorine salts 
yield by decomposition or reduction complexes in wliich the metal has a valence 
5 or 4. The following are obtained, in particular with MoF^: 

'from gaseous MoF — MoF + NaF yield Na MoF-, Na^MoFg [71, 72, 90, 91j . 

M2M0F with (M = K, Rb, Cs) [8j are also obtained, 
-in solution in IF^ £74] 

. Rb MoF^ 
. CsMoF^ . 



/ll 



10 



Several authors studied the dissociation pressure of the MoF, - NaF 
complexes. They find: 



at 100" 


0.4 to 4.6 mm Hg 


[78] 


at 150" 


8.4 to 27 mm Hg 


[78] 


at 200" 


64 mm Hg 


[78] 



Katz obtained the following formulas: 

-for the vapor tension of MoF^ of the equilibrium MoF + 2 NaF ^^ Na^MoF^ 

log P mm Hg = 8.27 ± 0.07 -(2.87 x 10^)/T„ I71]. 

-for the vapor tension of MoF, of the equilibrium Na^MoF^ + ^^^a ^'^ 
^"^ 2 NaMoF^ log P mm Hg = 7.29 ± 0.03 -(1,83 x lO"^) Tj^ I72j. 

See also [79, 80] and [81] which deal with MoF^ - NaF complexes. 

-while manganese fluoride, MnF^, reacts with UF, and WF , it does not 
with MoF^ even at 330**C [75]. 

-a U. S. Patent [76] indicates that magnesium fluoride can selectively 
absorb MoF. present in trace form in UF,, together with other volatile 

fluorides. Another paper [77] indicates that said absorption is low at 
250^ F. 

-MoF. in solution in CIF- also forms complexes with cesium and ammonium 
fluorides. Cs(MoFp and NH^(MoF^) [88] are obtained. UF^ and WF yield 
analogous compounds . 

-MoF., finally, forms an addition con5)ound with sulphur tetrafluoride, /12 
SF . at 40®C. The solid obtained is intense yellow in color, it decomposes 
at a higher temperature [89]. 

6.2. Reaction of MoF^, WF^ and UF^ with the Nitrile and Nitrosyl Fluorides 
NO2F, NOF and Nitrogen Oxides 

These three hexafluorides react with gaseous NO2F or NOF [81', 83, 85] to 
yield solid addition compounds of the form 

NO F • MF. where M can be Mo, W, U 
x o 

X can be equal to I or 2. 

11 



Infrared data indicates that said compounds are all in ion form. 

NO + and MF - 

-The gaseous nitric oxide, NO, reacts with MoF, and UF^ to yield the 

o o 

solid ion compounds [83, 84, 87]. 

NO* (MoF. ) " and NO* (UF^) ' . 

O D 

The same compounds are obtained by causing mitrosyl fluoride to react 
with MoFg and UF^. On the other hand, no reaction is observed between WF and 

NO. 

-Gaseous nitrogen dioxide, NO2 reacts with UF, [82, 85] to yield the 

solid confound NO-UF^, but no reaction occurs under the same conditions 

with MoF, and WF,. 
o o 

Gaseous liquid peroxide NO reacts finally with MoF, and WF to give, 

respectively, nitrosyl pentafluoromolybdate and pent afluorotungst ate, white 
solid confounds [86]. 

2 MoFj ♦ 3 Nj O^ » 2NOM0OF5 * NOjF ♦ NOF + NjGj 

Nj O^ Uqaid 
2Wr,*3N,0, » '*,05W,Fjj*NO,F*Nj05 

The second con^jound decomposes at between 100-200* to yield: 

UFg reacts with liquid N^O^ yielding solid NOUF, 

Gaseous N20^ excess reacts with MoF, and WF. to yield nitrile fluoride and the 
nitrosyl salt [86] . 



12 



^^6 (g) * "^2^, (g, * N««^OF^ ^^, . NO^F ,^, 

UF, likewiise reacts under the same conciltions to yield NOUF,. 

-Nitrous oxide N^O yields no reaction with MoF^ , WF, and UF, [84, 87]. 

2 600 

6.3. Reaction with Nitrosyl Chloride, NOCl 

NOCl reacts with MoF, and UF, to yield the nitrosyl salt NOMoF or NOUF^, 
on the other hand no reaction is observed with WF^ [82, 83]. 

NOCl , , * MF, . . » NOMF, , . * J CI 

(g) 6 (g) 6 (g) 2 2 (g) 

M being Mo or U. 

6.4. Oxidation-Reduction Reaction 

Molybdenum hexafluoride behaves like a fluorinating agent with numerous 
halides. The following reactions are obtained [92, 93] 

PF andiioF » MoF^andPFj 

Cs^andMoFg ► M0F5. (CF,J, S^nd.s 

WF^andMoFg ♦ MoF^and^Fj 

It may thus be stated that molybdenum hexafluoride is an oxidizing agent 
with respect to tungsten fluoride. It is known, moreover, that UF6 is in it- 
self an oxidizing agent with respect to molybdenum sub -fluorides, sis we have 
the reaction: 

UFg tt M0F5 » MoKgandUf4 (93) (or UF- in excess of UF,) 

The following classification is thus obtained: 

'*" ^'^6 UF^ being the greatest oxidizing agent of the 

'*■ ^°^\ three hexaf luorides . 

3-. WFg 



/14 



13 



H6.5. Exchange Reaction with Ha) ides 

i- 

' Molybaenum hexafluoride reacts with the following compounds to yield 
either molybdenum pentafluoride or a molybdenum halogen fluoride [92], 

PCI yields ^<^2^^3^6' ^^^5 ^^ ^^5 ^^^ MoCl^, PF- and PCK in excess 

of MoF^) 

AsCl- yields Mo.Xl^F^* and AsF. 

SbCl^ yields Mo^Cl^F^, and SbCl2F^ 
TiCl^ yields ^'^2^^Z^6 ^^ ^^^4 ^^^ MoCl^, TiF^ and Cl^ in excess of 

MoF^) 

C CI4 yields Mo2Cl2F^, CCl^F, CCI2F2 and CCIF^ 

SiCl^ yields Mo2Cl2F^ and SiF^ 

BCl^ yields Mo2Cl2F^, BF^, BCIF2 and BCI2F (or MoCl^, BF^, BCIF2 and 

BCl F in excess of MoF^) 
PBr2 yields MoBr., PF«, PF. and Br2. 

On the other hand MoF^ yields no reaction with the following compounds 
[92, 95] ^ 



AsFg, SoF^, BiF . 



6.6. Hydrolysis Reacti'^n 



Pure water hydrolyzes molybdenum hexafluoride into molybdic and hydro- 
fluoric acids without reduction (Ruff and Ascher) [2]. Nikolaev and collab- 
orators studied this hydrolysis reaction [97, 98] 

and determined the equilibrium constant at -5°C. 

K • 3.10' 

They get, under the same conditions, 6 x 10^ for WF^ and 9 x 10^ for UF,. IIL 

Nikolaev and collaborators likewise studied the MoF^ - HF - H^O system 
[100]; the following phases were brought out: 



14 



HjMoOjFj. 1.5 H,0 
Hjf.oOjFj. H,0 

"2 ^*^4 



In thb MoF^ hydro fluorhydric acid sy start the following phases appear 
[101]: ^ 



"2 ^^3^2 • ^2^ 



and 2 2 4 

6.7- Relative Acidity of Molybdenum Hexafluoride 

The relative acidity of an entire series of fluorides, wherexn MoF, and 
WF^ has been detected by their solubility in HF and by the ability of their 
solution to dissolve the fluorides classified in order of basicity [102]. 

6.8. Reduction of Molybdenum Hexafluoride 

Ogle and Smith [103] studied the reduction of MoF^ at 25° by the follow- 
ing gaseous con5)ounds: 

HCl 
HBr 
HI 
CO 

The only solid product obtained is molybdenum pentafluoride MoF.. 

The reduction of MoF^ by hydrogen is used in the fabrication of metal 

objects [104] and powders [115]. Deposition is effected in several stages at 
temperatures ranging between 400 - 800^. A similar technique is used to 
manufacture binary NK) - W alloys by reduction of a mixture of molybdenixm and 
tungsten hexafluorides [105]. Metallic molybdenum can likewise be recovered 
from MoF^ reduced on a heated metal plate [106], or even by electro-deposition 

[107]. Corrosion of nickel and its alloys by MoF^ in a mixture with F^ has 

been the object of a special study [99, 108]. 

MoF^ may also be reduced by carbon [109]. CF. is obtained above 1,000**, /16 
as well as small amounts of heavier fluorocarbons. C^F. is obtained at the 
electric arc temperature. 

With CF2, MoF. is also reduced and yields CF. [110]. 

15 



6.9. Different Uses of Molybdenum Hex^fluoride 

MoF. is used as a molybdenum hallde ^in the manufacture of additives for 

6 r 

lubricatinig mineral oils [111> 112]. 

MoF^ is also used to accelerate, on the surface, the fluoridizing of 
o 

aluminum objects by fluorine [113]. 

FiMlly, MoF^ is used in the preparation of an additive to prevent pi?ra- • 
site coloring of electrotypes used in lithography [114]. 

VII. Conclusions 

Molybdenum hexafluoride is a volatile flouride with a symmetrical octa- 
hedral structure like many of the XY^ formula molecules. 

It crystallizes at room temperature in a body-centered cubic form. It 
exhibits, moreover, a low -temperature transition point below which it crystall- 
izes in the orthorhombic system. 

Melting at 17.5 and boiling at 25*^0 at atmospheric pressure its vapors 
hydrolize in contact with moist air. Like other hfexafluorides, it gives add- 
itional compounds with sodium, potassium, rubidiim and cesium fluorides as 
well as with mitrile or nitrosyl fluorides, and with nitrogen oxides. It also 
gives place to different oxidation -reduction reactions with numerous halides. 

The sensitivity of this symmetrical molecule to hydrolysis and to re- 
duction thus reveals its non-saturated nature, from the point-of-view of the 
coordination of metallic molybdenum whose greatest coordinance is S, which con- 
trasts with the high inertia value of similar sulphur or selenii:an hexafluoride 
molecules. 

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Translated for the National Aeronautics and Space Administration under con- 
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21061. 



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