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New Finding in the Chemistry of the Lower Oxides of Sulfur.

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New Findings in the Chemistry of the Lower Oxides of Sulfur [l]
BY PROF. DR. P. W. SCHENK AND DIPL.-CHEM. R. STEUDEL
ANORGANISCH-CHEMISCHES INSTITUT DER TECHNISCHEN UNIVERSITAT BERLIN (GERMANY)
Dedicated to Professor Erika Cremer on the occasion of her 65th birthday
Important advances have recently been made in the chemistry of the lower oxides of sulfur,
both in the preparative sphere and by the application of modern physical methods of investigation. In this paper the knowledge gained to date on the compounds SO, S202,s20,
S203, and the polysuljiur oxides is reviewed and critically appraised.
Definition
In the present paper, the term "lower o x i d e s of s u l fur" refers to oxides in which the oxidation number z of
suIfur is 2. Thus, although S2O3 is discussed briefly, it
does not belong to this group, particularly since the sulfur in this compound is evidently present in two very
different oxidation states. However, the term does encompass the oxides SO, S 2 0 2 , S 2 0 , and the polysulfur
oxides of general formula (S,O), in which n > 2. According to their properties, compounds containing hydrogen in addition to sulfur and oxygen should be
grouped together with the polysulfur oxides. These compounds will be referred to as polysulfane oxides.
SO (like S2) is present in an appreciable concentration
only at high temperatures. The enthalpy of formation of
sulfur monoxide is found by investigation of equilibria
such as those shown in Equations (b) and (c) [9, lo].
<
2 SO2
AH:
SOz f
AH:
SO -- SO3 at 1500°C
7'
=
(b)
+48.6 kcal/mole of SO
1/2
Sz
T'
2 SO at 1250 "C
(c)
+58.3 kcal/mole of SO2
A thermodynamic treatment of the Equilibria (c)-(e) is
given in the literature [l], but this calculation is based
'/z
sz + ' 1 2 0
AH:
=
2
so
(d)
i
*
-I 3.8 kcal/mole of SO
A. Sulfur Monoxide SO
I. Stability
The ultraviolet and microwave spectra of SO show that
the electronic ground state of this molecule, like that of
0 2 and S2, is a 3
c state [2-51. Unlike 0 2 , however, SO
is thermodynamically unstable, with regard to both its
theoretical dissociation into the elements and its actual
dissociation into sulfur and S02. The enthalpy of formation at 0 "K of SO from its elements in the standard
state is not yet known with certainty, but is approximately zero (AH; = +1.6 kcal/mole [6]).
The enthalpy of its decomposition [Equation (a)] is
AH; = -36.8 kcal/mole. This agrees with the fact that
so *
'12
Srhomb. f ' 1 2
so2
(a)
[ I ] For an earlier review, see P. W . Schenk, Chemiker-Ztg. 67,
251, 273 (1943).
f2] E. V. Martin, Physic. Rev. 41, 167 (1932).
[31 A. L. Myerson, F. R . Taylor, and P. L . Hanst, J . chem. Physics
26, 1309 (1957).
[4] M . Winnewisser, K . V. L. N . Sastry, and W . Cordy, Bull.
Amer. physic. SOC.9, 488 (1964); M . Winnewisser, K . V. L. N .
Sastry, R.L. Cook, and W. Cordy, J. chem. Physics 41, 1687(1964).
[ 5 ] A . B. Scott, J. Amer. chem. SOC.7 1 , 3145 (1949).
[6] All enthalpy values given in the presentrpaper are taken
from I71, or calculated from the data given there; see also [8].
402
on earlier values for the enthalpies of the reactions. The
results indicate that appreciable dissociation of SO2 into
S2 0 2 and into SO + 0 2 occurs only at very high temperatures. Only Equilibrium (d) is entirely displaced to
the right at room temperature [l]. More recent calculations [S] have indicated that there is 16 % SO present in
Equilibrium (c) at 1500 "C and 7.6 mm pressure.
+
Sulfur monoxide is found to be a true biradical which is
extremely reactive and decomposes immediately into sulfur or S 2 0 and SO2. In order to detect its occurrence as
an intermediate, it is therefore necessary to use mild
reaction conditions (vapor phase, low pressure) and
sensitive physical methods of measurement (see Table 1).
It is often possible to recognize the formation of SO indirectly by the detection of S 2 0 as a subsequent product.
[7] R. Steudel and P . W. Schenk, Z. physik. Chem. N F 43, 33
( I 964).
[8] R. Hagemann, C. R. hebd. Seances Acad. SCI.255, 899, 1102
(1962); Commissariat I'Energie Atomique (France) Rapp. N o .
2398 (1964).
[9] C. S t . Pierre and J . Chipman, J. Amer. chem. SOC.76, 4787
(1954).
[lo] E. W . Dewing and F. D. Richardson, Trans. Faraday SOC.
54, 679 (1958).
Angew. Chem. internot. Edit. I Vol. 4 (1965)
1 h'o. 5
11. Formation of SO
Ta b l e I. F o r m a t i o n a n d detection of sulfur monoxide.
I
Reaction
Detection of SO
Ref.
SO? [a]
U V photolysis
y-photolysis
U V s p ec tru m
deposition of S
112-141
SO? [b ]
glow discharge
H F discharge
M W discharge
SrO
ESR s p e c tru m
microwave s p ec tru n
L4.15- 171
so2
s h o c k waves
U V s p ec tru m
[Ill
SOz [c]
t h e rm a l dissocn
U V s p ec tru m SzO
t18-211
Material
so,
U V photolysis
U V s p ec tru m
SOX?
t h e rm a l dissocn. [d]
dehalogenation
szo
Sulfites
thermolysis
Sulfides
roasting If]
H2S
photolysis with Or
CS2, COS
COS
I
SO is formed from
SO2
in accordance with Equation (f).
so2
s10
[27-291
with a t o m ic oxygen
E S R s p ec tru m ,
mass s p e c tru m
[30,3 I I
photolysis o r
c o m b u s tio n with Oz
U V s p e c tru m , mass
spectrum, SzO
~3~27,
32,331
with a t o m ic oxygen
mass s p e c tru m
SOZ*
S 0 2 * + SO2
S u l fu r
c o m bustion with
Oz Ihl
Sulfur
combustion withCuO
s 2 0
f351
Sulfur
heating with SOz
electr. discharge [I]
szo
[15,36,371
Sulfur
(solid)
with a t o m ic oxygen,
H F o r glow disc h a r g e with Oz
m a s s s p e c tru m , SZO
microwave
s p e c tru m
Polysulfur
oxides a n d
polysulfane
oxides
thermolysis
SLO
C24.331
[40-421
[a1 Ultraviolet photolysis at 10-100 mni. y-Photolysis of liquid SOz
yields s u l fu r a n d SO3 a s e n d p ro d u cts f141.
[ b l Electric discharges in SO2 a t 0.01-1.0 m m . Particularly high SO
c o n c e n t ra t i o n s ( u p to 100 %) a r e o b ta in ed if s u lfu r is als o present i n
t h e discharge tube, in acc o rd an ce with Equilibrium (c) [171.
[cl T h e r m a l dissociation of SO2 either a t a Nern s t r o d o r in a luminous
a r c in t h e presence of s u l f u r vapor.
Id1 T h e d e c o m p o s i t i o n tem p e ratu re s of t h e thionyl halides a n d pseudohalides decrase in t h e o rd er S O C l z .> S O B r z 1 SO(CN)? \ SO(SCN)2
(221
[el Detection of SzO by f o r m a t i o n of sulfide with Ag9 s o lu tio n (251.
[ f l At t e m p e ra t u re s of u p to 4 3 5 O C a n d oxygen pressures of 0.1-4.0
m m , pyrites a n d zinc sulfide yield o n ly SO2 a n d metal oxide on roasting.
SrO is o b t a i n e d in small quantities o n ly f r o m AgzS [231.
[gl Detection of SZO b y its ultraviolet s p e c tru m .
[ h l W i t h p u r e oxygen a t 10-40 m m
[il T h e f o r m a t i o n of SO m a y be quantitative. T h e g as o b ta in ed consists
or a stoichiometric ( I : I ) mixture of SzO a n d SO2 [IS,19,27,36,37,401.
[ I 11 A . G. Gaydon, G. H. Kimbell, and H . B. Palmer, Proc. Roy.
SOC.(London), Ser. A 276, 461 (1963).
(121 R . G. W. Norrish and G . A . Oldershaw, Proc. Roy. SOC.
(London), Ser. A 249, 498 (1958).
1131 P. Warneck, F. F. Marmo, and J . 0 . Sullivan, J. chem.
Physics 40, I 132 (1964).
[ 141 W. G. Rothschild, J. Amer. chem. SOC.86, I307 ( 1 964).
[ I S ] P. W . Schenk, Z. anorg. allg. Chern. 211, 150 (1933).
1161 J . M . Daniels and P . B. Dorain, J. chem. Physics 40, 1160
(I 964).
1171 P. W . Schenk and W. Holst, 2. anorg. allg. Chem. 319, 337
(1963).
1181 P . W . Schenk and H . Platz, Z. anorg. allg. Chem. 222, 177
(1935).
[I91 P. W . S<hrnk and H.Triebe1, Z . anorg. allg. Chem. 229, 305
( 1936).
[20] G. Punnetier, C. R. hebd. Seances Acad. Sci. 228,478 (1949).
Vol. 4 119651 1 N o . 5
so + 0
(f)
so -t 0
(g)
+ S O + SO3
(h)
--f
SO is formed from SOC12, SOBr2, S O ( C N ) 2 , and
SO(SCN)z by thermal decomposition in accordance
with Equation (i), or by dehalogenation of thionyl chloride or bromide with silver or antimony [e.g. Equation
(k)] at 100-400 "C and about 1 mm pressure.
sox2
SOC12+2Ag
1
->
This reaction requires about 131 kcal/mole [7], i.e. it is
effected only by high-energy radiation, an electric discharge, or high temperatures. After the activation process
S O 2 + S02*, Reactions (g) and (h) are possible [I I].
U V spectrum. mass
spectrum, SzO
Angew. Chem. iriternnt. Edit.
Ways in which sulfur monoxide is formed as an intermediate are listed in Table 1 , together with methods for
its detection.
+
s o + x2
+ SO+2AgCl
(1)
(k)
[21] T. M . Sugden, E. M . Bulewicz, and A. Denierdache: Proceedings Internat. Symposium on Chemical Reactions in the Lower
and Upper Atmosphere, San Francisco, Calif., 1961, p. 89;
Chem. Abstr. 56, 13583 (1962).
I221 P. W . Schenk and H. Bloching, Chem. Ber. 92, 2333 (1959).
[23] P. W. Schenk and R. Steudel, unpublished experiments.
[24] P. W . Schenk and H . Platr, Z . anorg. allg. Chem. 215, I13
( 19 33).
(251 C. C. Bisi and A . Clevici, Gazz. chim. ital. 93, 1444 (1963);
Chem. Abstr. 60, I I602 (1964).
[26] N . P . Diyev et al., Trudy Inst. Met. Akad. Nauk SSSR,
Ural, Filial Sbornik Rubot I , 17 (1957); Chem. Abstr. 53,21085
( I 959).
I271 J . J . McGarvey and W . D. McGrath, Proc. Roy. SOC.(London), Ser. A 278, 490 (1963).
[28] D . G. H . Marsden, Canad. J. Chem. 41, 2607 (1963).
[29] N . M . Emunuel, Dokl. Akad. Nauk S S S R 59, 1137 (1948);
Chem. Abstr. 42, 7142 (1948); G. Markovich and N. M . Emanuel,
Zhur. fiz. Khim. 21, 1251, 1259 (1947); Chem. Abstr. 42, 5313
(1948); N . N. Semenov: Some Problems of Chemical Kinetics
and Reactions. Pergamon Press, London 1959.
[30] C. C. McDonald, J. chein. Physics 39, 2587 (1963).
[3 I ] C. Liuti, S. Dondes, and P . Harteck: Abstract 40T, Division
of Physical Chemistry, American Chemical Society, New York,
Sept. 1963.
[32] J . Akriche, J. Chim. physique Physico-Chim. biol. 60, 732
(1963).
[33] P . W . Schenk, 2. anorg. allg. Chem. 220, 268 (1934).
[34] J . 0. Sullivan and P. Warneck, Ber. Bunsenges. physik.
Chem. 69, 7 ( 1 965).
1351 A. R. V. Murthy, Nature (London) 193, 773 (1962); S. R.
Satyanarayana and A . R. V. Murthy, Z . anorg. allg. Chem. 330,
245 (1964); Proc. Indian Acad. Sci., Sect. A 59, 263 (1964).
I361 H . Cordes aiid P. W . Schenk, Z . anorg. allg. Chem. 214, 33
(1933).
[37] H . Cordes and P. W. Schenk, Z . Elektrochem. angew. physik. Chem. 39, 594 (1933).
1381 P. W. Schenk, 2. physik. Chem. B 5 1 , 1 1 3 (1942).
I391 F. X . Powelland D . R . Lide, J. chem. Physics41, 1413 (1964).
[40] P. W. Schenk, Z . anorg. allg. Chem. 233, 385 (1937).
[41] P. W . Schenk and W . Kretschmer, Angew. Chem. 7 4 , 695
(1962); Angew. Chem. internat. Edit. I , 550 (1962).
[42] P . W . Schenk, 2. anorg. allg. Chern. 248, 297 (1941).
403
SO and SO2 are formed on decomposition of the sulfites (MS03) of sc, y , La, etcc.in a current of nitrogen
at 300-8OO0C:
SOz i- M S 0 3 -+ SO t MSO4
2 SO2 t 3 M S 0 3
(I)
-+ SzO t 3 M S 0 4
(m)
Roasting of sulfides at 700-1000 "C in a current of nitrogen containing oxygen also yields S 2 0 1261, which is undoubtedly derived from SO formed as an intermediate.
When a mixture of metal oxides is heated with sulfur in
vacuo, disulfur monoxide is formed via sulfur monoxide:
2CuO-i
s
--f
CUSf
so
(n)
The disulfur monoxide can be detected as such [35].
High concentration of SO are obtained in mixtures of
sulfur vapor and SO2 at a Nernst rod, in a glow disharge,
or in a high-voltage luminous arc at pressures of 0.1 to
100 mm. The SO can be detected as its subsequent reaction product SzO.
SO i s evolved from solid polysulfur oxides and polysulfane oxides on dry distillation in vucuo, possibly in accordance with Equation (0).
-s-s-s-
8
-+
so +
2 Srhomb.
There was no preparative method available around 1940 to
solve this question. Although several publications suggested
that the product was probably S202 [29,45,47a] (cf. the criticism by Schenk 146,47bl),Meschi and Myers later showed by
mass spectroscopy that it was in fact an equimolar mixture of
S20 and SO2 1481.
Thus, data given in the earlier literature on the -preparation,
isolation, and properties of a stable gas SO or S202 are erroneous, and actually apply to the s20,so2 mixture [491.
Owing to the rapidity of the decomposition of sulfur
monoxide, practically nothing is known of its chemical
properties, apart from a few facts regarding the decomposition itself. The reaction is known to proceed in accordance with the overall Equation (p) because of the
constant composition of the gas (S:O = 1:l) [17], the
mean molecular weight of about 70 (calculated value ==
72) 1421, and the fact that the intensities of the ,320 and
S O 2 peaks in the mass spectrum are equal [31].
The disproportionation of SO proceeds extremely rapidly, even at pressures under 1 mm; the half-life of the
compound is about 2 msec [34]. Attempts to freeze out
SO immediately after its formation have so far led only
to the S20ISO2 mixture [49a].
IV. Mechanism of the Decomposition of SO
(0)
The high rate of decomposition of SO rules out a threebody collision [Equation (p)]. The first step is probably
a dimerization:
111. Properties of SO
In order to investigate the decomposition of SO, the
compound was prepared by the following methods:
photolysis of S O 2 1121, photolysis of an HzS/Oz mixture
[27,28], combustion of CS2 and COS with 0 2 or atomic
oxygen [3,27,32,34]. The reaction conditions entailed
temperatures between 20 and 300O0C and total pressures of 0.1 to 100 mm. The SO could no longer be detected after about 1-10 msec.
Sulfur monoxide decomposes spontaneously in accordance with Equation (a) to form sulfur and sulfur dioxide.
At temperatures below 100°C and pressures below
100 mm, however, it is possible to detect several intermediate products. Thus, an equimolar mixture of S20
and SO,, which was long thought to be SO itself, is formed directly by disproportionation of SO [Equation (p)].
2 so
--f
S20rSO2
(9)
In considering possible mechanisms for the rapid decomposition of (S0)2, the Reactions (r) and (s) occasionally proposed can be rejected, since both are endothermic. The only other possibility would be Reaction (t).
(S0)z
AH:
--f
2
(S0)z
AH:
--f
--
SzO i0
(r)
+33.0 kcal/mole
szt 0
2
(s )
-1-27.5kcal/mol
-+
S02t
s
(9
(P)
Disulfur monoxide S 2 0 was first prepared as a mixture with
SO2 in 1933 [15,24,36]. On the basis of the methods of synthesis (action of glow discharge on SO2; dehalogenation of
SOX2) and the composition of the gas mixture ( S : O = l:l),
the new oxide was assigned the formula SO. However, molecular-weight determinations in the vapor phase gave a value
of about 70 [42] instead of 48, as expected for SO, and the ratio of the original to the final volume on decomposition of the
gas into sulfur and SO2 [Equation (a)] was not 2:1, but 4:3
[18,42,43]. These results could not be reconciled with the
simple formula SO and led to the assumption that part of the
gas was either irreversibly dimerized (2 SO + S202) or disproportionated in accordance with Equation (p) [42,44].
[43] E A. Evans, A. B. Scott, and J . L . Huston, J. Amer. chem.
SOC.74, 5525 (1952).
[44] B. S. Rao, Proc. Indian Acad. Sci., Sect. A 10, 491 (1939);
Chem. Abstr. 34, 3198 (1940).
404
(SO12
This assumption is supported by the detection of (SO)2.
Because of the low heat of reaction involved, this first
step can proceed by a two-body collision (see Section B).
(S0)Z
3so
+
The enthalpy of Reaction (t) is practically zero. However, even if this reaction were feasible on energetic
grounds, it does not lead to an SzO/SO2 mixture unless
[45] A. Yakovleva and V. Kondratyev, Acta physrcochim. S S S R
2, 241 (1940); Y . Kondratyevn and W . Kondratyev, Zhur. fiz.
Khim. 14, 1528 (1940).
1461 P . W . Schenk, Z. physik. Chem. B 52, 295 (1942).
[47a] A . V. Jones, J. chem. Physics 18, 1263 (1950).
[47b] P . W . Schenk, Z . anorg. allg. Chem. 270, 301 (1952).
[48] D.J . Meschi and R. J . Myers, J. Amer. chem. SOC.78, 6220
(1956).
[49] The publications In question include: [ I ] , [3], [15], [181,
[191, [241, P61, [291, V31, [361, P71, P81, W1, I42L 1431, [451,
[461, [47al, W b l , [561, P91, @ I ] , W I , [641, W1, P11.
[49a] Recently a yellow substance has been isolated at low
temperatures which is probably condensed SO ( R . Sfeudel, Ph.
D. Thesis, Technische Universitat Berlin, 1965).
Angew. Chem. internat. Edit.
Vol. 4 (1965) / No. 5
the sulfur reacts quantitatively in accordance with Equation (u). This is however improbable. Reaction (u) is
s + so
- f
AH:;
szo
(4
90.5 kcal/molc
According to Equilibrium (x), ( S 0 ) 2 can be expected to
appear only in the mass spectrum of nascent SO, since
one second is long enough for complete decomposition
of the SO 18,481.
only conceivable as a wall reaction or as a three-body
collision reaction. Thus, Reaction (t) can also be ruled
out.
C . Disulfur Monoxide SzO
There is therefore no alternative but to assume that
( S 0 ) 2 in equilibrium with monomeric SO has a sufficiently long life time to enable it to react in accordance
with Equation (v).
I. Stability of S 2 0
(SO)? +
so
AH:
i-
SZ0-k
so2
The microwave spectrum of S20 (2) indicates that this
molecule has a structure similar to that of SO2 (I) [53].
(v)
97.8 kcal/mole
This reaction is supported by the observation that, if an
excess of sulfur vapor is present during the decomposition of sulfur monoxide, the disproportionation is partially suppressed in favor of Reaction (w):
(S0)Z -t- S? -f 2 szo
AH: = -79.4 kcal/mole
Here the SzO is obtained in 85% purity [17].
Table 2. Dissociation energies Do, bond lengths r, force
constants k , a n d bond orders n in 0 2 , SO, and S I .
S-0
S-S
I
118.0
123.5
101.5
1
I
1.208
1.481
1.893
1
1
11.39
7.94
4.99
I
1
1.45
1.98
2.00
B. Dimeric Sulfur Monoxide (SO)z
It is only recently that a molecule S20z has been discovered in mass-spectroscopic studies on the reaction of
atomic oxygen with sulfur, HzS, and COS. In addition
to the SO band, the mass spectrum of the reaction product also contains peaks due to S20, S 0 2 , and S202 in
intensity ratios of 1 :1:0.04. The peak at mass 96 is assigned to S202 because another peak is observed at mass
98, and these peaks correspond in their intensity ratio to
half the natural 32S/34S ratio [31].
Although no details are available regarding the nature
of the new compound, it can be assumed to be a molecule similar to dimeric 0 2 , i. e. 0 4 . It is therefore preferable to use the formula (S0)z. Owing to the low enthalpy
of formation (from themonomers) of suchdimers(AH: =
--0.13 kcal/mole for O4), they can result from two-body
collisions [Equation (x)], if the excess energy is stored in
internal degrees of freedom.
2 so
s
o
118"
/2/
e (S0)l
The dipole moments of the bonds were calculated to be
= 0.30 D and pso = 1.58 D; the resultant molecular moment is pszo= 1.47 D, the individual moments
being opposite in sign [53]. The resonance structure ( 2 c )
can therefore contribute little to the ground state, and
the bond order n of the S-S bond should be close to 2.
Using MofSititt's formulae [Sl], Cigukre [54] obtained a
value of n 1.80 for the S - 0 bond.
pss
2
Ref.
1
l.884i/-\l.4655(
S
(W)
Comparison of the dissociation energies Do [7] (Table2)
shows that the bond strength in sulfur monoxide is
greater than that in 0 2 and in S2. Table 2 also includes
the interatomic distances r, the force constants k, and
the bond orders n calculated from these data.
0-0
S
.aazX
0
0
119.5" / I /
I .432x/-\l
(x)
The standard enthalpy of formation of S20 is calculated
from the sharp predissociation limit to be AH; = - 22.7
kcal/mole. The dissociation energy of the S-S bond is
Do(OS-S) = 90.5 kcal/mole; that of the S-0 bond is
D,(SS-0) = 112.5 kcal/mole 171. Recently, the enthalpy
of formation of S 2 0 was calculated from the appearance
potential of SO@from S 2 0 and from equilibrium measurements on sulfur vapor/SOz mixtures to be AH; =
-9.6 kcal/mole [8]. Thus, SzO is thermodynamically
stable, although it tends to polymerize.
On heating, disulfur monoxide decomposes into sulfur
and SO2 [Equation (y)]. Gravimetric determination of
the sulfur formed is used for quantitative analysis of
2 SZo
3 Srhomb
so2
AH: - -12.5 kcal/mole of S 2 0
--f
(Y)
S 2 0 (polysulfur oxide is formed first, but is completely
decomposed by heating above 100 "C) [40].
In the vapor phase disulfur monoxide is stable for a few
hours to a few days, but only at pressures of about 1 mm
[40]. Its molecular weight corresponds to the simple formula SzO [42]. Its decomposition is accelerated by heating and proceeds quantitatively in one minute at 180 "C
[36]. Free S20 is not stable in the condensed phase; polysulfur oxides are always formed.
Very sensitive methods are available for qualitative detection of S20. When gas containing SzO is cooled to
[50] H . Siehert, 2. anorg. allg. Chem. 273, 170 (1953).
[5 11 W . Mojfirt, Proc. Roy. SOC.(London), Sect. A 200,409 ( 1950).
I521 A . F. Wells: Structural Inorganic Chemistry. Clarendon
Press, Oxford 1962, p. 414.
Angew. Chem. internut. Edit. 1 Vol. 4 (196s)
1 No. 5
1531 D.J. Meschi and R. J . Myers, J. molecular Spectroscopy 3,
405 ( I 959).
[54] P. A. Gigusre, J. physic. Chem. 64, 190 (1960).
405
-195.8 “C (liquid nitrogen), an orange to deep red condensate is obtained. In this way it is possible to detect as
little as 5-10% SzO in a gas mixture; the color deepens
with increasing SzO content. The red condensate undergoes a characteristic color change to yellow on warming
to between -150°C and -100 “ C [23,55]; at room temperature a solid, deep yellow residue of polysulfur oxide,
together with SOz, is all that remains. When the quantity of S 2 0 present is small, the polysulfur oxide is found
in the cold trap as a yellow to whitish-yellow ring; this
provides a very sensitive indication of the presence of
SzO if a large quantity of gas has been condensed [19].
An unambiguous and extremely sensitive method of detection involves ultraviolet spectroscopy, which permits
thedetection of S20 at partial pressures as low as lO-3mm
in a cell 100cm long. The most intense bands lie between 2800 and 3200 A [15,40,47a,56].
Jones [47a] discovered two absorption regions at 679
and 1165 cm-1 in the infrared spectrum of S20 (which
he thought to be S20z). These absorptions can be assigned to the S-S and S - 0 valence vibrations, respectively.
The deformation vibration absorbs at 387 cm-1 [53].
11. Formation of S20
Disulfur monoxide can be obtained by two routes:
1. From sulfur monoxide, by disproportionation or by
treatment with sulfur [Equations (z) and (oc), respect ively1.
(SO)>+ so
+
(S0)Z i- sz
+ 2
szo+ so2
szo
(2)
(d
2. By direct synthesis [Equations (@j-(&j].
In Syntheses (y) and (E), the gaseous component (thionyl
chloride or disulfur dichloridej is passed at about 1 mm
over the heated metal sulfides (e.g. Ag2S or SbzS3) or
oxides (e.g. AgzO, Sb2O3, or CdO). Some of the SzO decomposes on the metal sulfide to form SO;! [59]. If both
of the reactants are gaseous, the gas mixture is allowed
to pass rapidly at about 1 mm through a furnace and is
subsequently condensed in a cold trap.
111. Properties of S 2 0
The most striking property of disulfur monoxide is its
ease of polymerization, which prevents the establishment of phase equilibria, and hence also the characterization of the compound by its melting or boiling point.
The deep red condensate described above (Section C. I)
probably owes its color to congealed monomeric S20.
Like SO2, which has a similar structure, disulfur monoxide reacts with trimethylamine to form a crystalline donor-acceptor complex (3), which is in equilibrium with
its components 1601. The yellow solid (3) condenses out
0 .. ..
(CH3)zN: + SzO P ( C H 3 ) 3 N - S = 0
:S:
6
-
0
..
(CH3)sN-S-O:@
!I
:S:
(3J
from a mixture of the components at 1 mm and -30 “C
and is stable at this temperature for a few days. It can be
sublimed in vacuo above -27 “C. Spectroscopic evidence
shows the presence of free SzO in the vapor; the complex
therefore represents S20 which is “stabilized” in the
solid state. The complex (3) gives an intensely yellow
solution in anhydrous inert organic solvents (CHC13 or
C S 3 but decomposes at room temperature to yield sulfur, SOz, and trimethylamine, owing to the cleavage of
the S-S bonds by amine nitrogen [23]. Disulfur monoxide also appears to form a stable solid complex with triphenylphosphine [61]. BF3 and S20 (like BF3 and S02)
yield only very unstable adducts [60].
Disulfur monoxide does not react with nitrogen [ 181, and
reacts with oxygen only after ignition or at elevated temperatures [37,62]; however, it reacts immediately with
chlorine to form SOC12 and SzC12 [I].
Water vapor decomposes S20 with deposition of sulfur
and formation of H2S; however, the decomposition is
not so fast as to prevent the detection of S20 in any reaction in which it is formed together with Hz0 [23,24,
28,631.
Introduction of an SzO/SOz mixture into ice-water leads
to the formation of sulfur, hydrogen sulfide, and sulfur
dioxide, while sulfide, sulfite, and thiosulfate are formed
in alkaline sohtions. Surprisingly, no polythionic acids
can be detected [641.
Potassium iodide solutions are oxidized by S20 to iodine
with simultaneous formation of sulfur and hydrogen
sulfide [64].
D. Polysulfur Oxides a n d Polysulfane Oxides
Reactions (p), (a), and (E) do not proceed to completion.
Disulfur monoxide has so far been obtained only
together with SOz, but according to Reaction (7) at a
purity of up to 97 % 1581.
These names are used to denote products which always
contain sulfur and oxygen, but which may also contain
hydrogen, depending on their mode of preparation. They
are generally mixtures of more or less polymeric compounds, which have not yet been extensively studied.
[ 5 5 ] R. Steudef, Diploma Thesis, Freie Universitat Berlin, 1963.
[56] H . Cordes, Z . Physik 105, 251 (1937).
[57] P. W. Schenk, R. Steudel, and M . Topert, 2. Naturforsch.
19b, 535 (1964).
[581 P . W. Sehenk and R . Steudel, Angew. Chem. 76, 97 (1964);
Angew. Chem. internat. Edit. 3, 61 (1964).
[591 B. S. Rao and M . R. A . Roo, Current Sci. 12, 323 (1943).
[60] P . W . Schenk and R . Steudel, Angew. Chem. 75, 793 (1963);
Angew. Chem. internat. Edit. 2, 685 (1963).
[6I] K.-D. Wiebusch, Ph. D. Thesis, Universitat Heidelberg, 1948.
1621 Y . Kondratyeva and W. Kondratyev, Dokl. Acad. Sci. SSSR
31, 128 (1941); Chem. Zbl. 1943, 1246.
[63] M . Tupert, Diploma Thesis, Freie Universitat Berlin, 1964.
[64] P . W. Schenk, 2. anorg. allg. Chem. 265, 169 (1951).
406
Angew. Chern. internat. Edit.
/
Vol. 4 (1965)
Xo. 5
1. Polysulfur Oxides
“Pure” polysulfur oxides are formed during the polymerization of disulfur monoxide, which is always associated with partial disproportionation (“degradative polymerization”) [40].
In the polysulfur oxides, the uniformity of the bond order
along the sulfur chain is upset. The triply bonded sulfur atoms
attain a stable configuration with 12 valence electrons which
prevents the formation of two double bonds to the two adjacent sulfur atoms. This explains the ease of thermolysis of
polysulfur oxides; the reaction probably involves elimination
of SO [cf. Scheme (A)].
0
-3-S-S-
c)
-$-$-$-
II
:o:
..
:O:
This decomposition reaction always occurs when the
S 2 0 concentration is raised above a critical value by
condensation of the gas at -196°C and subsequent
thawing, by introduction of the gas into organic solvents, or by raising the pressure in the vapor phase.
I t may be assumed that polymerization proceeds by
opening of the S-S double bonds, and that SO2 is eliminated when the 0s groups of two S20 molecules come
together, e.g. as shown in Equation (q).
s=s+ s=s + s=s
& a
b
+
s=s-s-s-s
+ so2
s
(q)
However, owing to the polarity of the S 20 molecule,
polymerization is also possible with similarly oriented
molecules [Equation (%)I. Thus Reaction
can yield
(a)
polysulfur oxides with the composition S : 0 - 2:1 (cf.
the composition S:O = 3:l calculated on the basis of
Equation (-q) [40,42,55]). The remaining S-S double
bond in the polysulfur oxides is probably delocalized, as
in x-sulfur [67].
A characteristic feature with all polysulfur oxides is the
course of their thermal decomposition in vacuo, which
results in regeneration of disulfur monoxide (maximum
50 mole S20) [40], probably in accordance with Equations (x) and (p).
(S,O),
-->
xS0
+ (n-1)xS
(1)
In compounds containing sulfur chains, the S atoms are not
simply linked by single bonds; instead, the “lone” electron
pairs also participate, with occupation of the bonding d-orbitals [52,68-711. This can be expressed by the formulae (40)
and (4b).
Q
!
(A)
0 0
_. 0 0 c, =S-$;S=
=s=s-s=
,? :&:
:o:
“ k,
~
I . Preparation of’Polysulfur Oxides by Frec>zingout S20
On thawing, the red condensate of disulfur monoxide
(cf. Section C. I) becomes deep yellow between -1 50 and
-100 “C.This color change is regarded as a sign of polymerization. At room temperature, a solid, yellow polysulfur oxide is obtained, the gas phase then consisting of
SO;! together with some S 2 0 [23,40]. Thus, condensed
S20 cannot be vaporized without decomposition. After
the gas has been pumped off, the stoichionietric composition of the polysulfur oxide is found to be approximately S30 [42,65]. The substance is amorphous to Xrays [I], and its molecular weight is unknown. Only a
small portion dissolves in carbon disulfide, giving a deep
yellow solution [l ,721.
Murtliy [65] claims that warming condensed S 2 0 to -30 ‘C
leads to pure, orange-red S20, if the sulfur dioxide which is
still present from the preparation of the S 2 0 is removed by
purging with nitrogen. However, this color could not be reproduced at -3OOC [23,55]. It is doubtful whether the SO2
could be completely removed from the yellow polymer in this
2:l is probably merely fortuiway. The composition S:O :
tous 1661 and furnishes n o proof of the presence of pure disulfur monoxide.
2. Preparation of’Polysulfur Oxides by Dissolution of’s20
When a mixture of S20 and SO2 is led into cooled
(--2OoC) chloroform or methyl phenyl ether, the S20
immediately polymerizes. The yellow solution can be
freed from SO2 by purging with nitrogen. The dissolved
material has a molecular weight of 210- 380 or higher
and a composition S:O = 2.2:l-5.6:l; it is therefore a
polysulfur oxide [42,44,731.
Nevertheless Mrrrthv described the reactions of these solutions
with water and piperidine as “reactions of SzO” [74,751.
Solutions of polysulfur oxides oxidize hydrogen iodide
stoichiometrically to iodine, in accordance with Equation (p). This provides a convenient titrimetric method
Sulfur chains, such as those in T;-sulfur [671, and S2 and S20,
with almost pure S - S double bonds and electron deficiencies [681 at the terminal sulfur atoms, exhibit a strong
tendency to polymerize.
[651 A . R . V. Miirfhy, IUPAC Colloquium, Division of Inorganic Chemistry, Miinster (Germany) 1954, Verlag Chemie,
Weinheim/Bergstr. 1955, p. 141; A . R . V. M u r f h y , Proc. Indian
Acad. Sci., Sect. A 36, 388 (1952).
[66] P. W. Schenk, 2. anorg. allg. Chem. 285, 297 (1956).
[671 P. W . Schenk and U . Thuirwi/er, 2. anorg. allg. Chem. 3/5,
271 (1962).
1681 1.Coidmzu, Angew. Chem. 69, 77 (1957).
A n g e w . Cliein. internot. Edit.
1 VoL 4 (1965) 1 No. 5
(S,O),
-7
2 x Hl
--f
n x S
x12
T
xHzO
(p.)
[69] L . Pauling: The Nature of the Chemical Bond. 3rd edit.,
Cornell Univ. Press, lthaca 1960.
[70] M . Schmidt, dsterr. Chemiker-Ztg. 64, 236 (1963).
[71] F. Fehir and H. Miinzer, Chem. Ber. 96, 1131 (1963).
I721 W. Holst, Ph. D. Thesis, Freie Universitdt Berlin, 1960.
1731 M. Goehring and K.-D. Wiebusch, 2. anorg. allg. Chem. 257,
227 (1948).
[74] K.Shffrodfrand.4. R . V. Miwthy,J. Indianlnst.Sci.44,49( 1962).
[75] C.G. R . Nnir and A. R. V . Murthy. Canad. J. Chem. 41, 898
(1963).
407
for the volumetric determination of the oxidation state
173 761.
3. Preparation of PolysuQur Oxides by Compression
vf Gaseous S20
Gaseous disulfur monoxide is stable for a few hours to a
few days, but only at about 1 rnm [40]; on compression
of the gas to about 40 mm, polysulfur oxide separates
out as an aerosol and as a film on the wall of the vessel
[36]. However, gaseous SzO can be handled at higher
pressures when diluted with nitrogen or sulfur dioxide,
provided its partial pressure remains low [I].
In our opinion, the initial product of the Wackenroder
reaction is (as has long been thought [79]) thiosulfurous
acid, which exists in equilibrium with H2S and H2SO3
[24,41,64, SO]. The unsymmetrical form of thiosulfurous
H2S
+ H z S O ~ +:
HzS202
+ H2O
(v)
acid, HO-SO-SH, is initially formed and slowly rearranges to the symmetrical form HO-SS-OH. This then
condenses with sulfurous acid to form tetrathionic acid
(S), which is the first and principal product of the
Wackenroder reaction [81].
HO-SS - O H
+ 2 HzS03
+ H2S406
+ 2 H2O
(5)
(5)
11. Polysulfane Oxides (Polysulfur Oxides Containing
Hydrogen)
1. Preparation of Polysulfane Oxides from SO2 and H2S
H2S and SO;! react at room temperature and below in the
presence of traces of water to form polysulfane oxides.
When the moist gases are mixed, the polysulfane oxide
is deposited on the walls of the vessel [ a ] ; if the gases
are led into cooled organic solvents, they form solutions
of polysulfane oxides, from which the latter can be precipitated [73].
The most important of these polymers is “Wackenroder’s
sulfur”; it is precipitated when a vigorous current of
H2S is led into ice-water saturated with SO2. After filtration, washing, and drying at 10-2 mm and O”C, the
approximate composition of the yellow, plastic product
is S:O:H e 5:l:l.l [41,63]. The substance can be
stored at -20 “C for several days without decomposing.
On thermolysis in vucuo, S02, S20, and H20 are liberated, possibly with some HzS, leaving a residue of sulfur.
It is not improbable that some of the hydrogen here is
bound in the form of occluded water [63,77].
The yellow solutions obtained by dissolving H2S and
SO2 in organic solvents tend to become turbid and
to fluorescence, and must be stabilized by drying over
phosphorus pentoxide, since they otherwise decompose to form SO2 and sulfur. After removal of residual
synthesis gases with a filter pump, the composition was S:O = 4:1-8:l. The molecular weight of one
product was found to be 360 [73]. As expected, the dissolved polysulfane oxide disproportionates on heating
to form sulfur and SO2 [73]. The yellow solutions oxidize hydrazoic acid to nitrogen and liberate an equivalent amount of iodine from a solution of hydrogen iodide
in formic acid [73].
The reaction of H2S and SO2 in cc14 at - 10 to - 15 “C
yields a polysulfane oxide with a particularly high oxygen
content. This product is obtained as an intensely yellow
precipitate with a composition corresponding closely to
the formula H(S20),SH; this formula can be derived
from Equations (v) and (x) [78].
[76] H . Sfanim and K.-D. Wiebusch, Naturwissenschaften 32, 42
(1944).
[77] W. Kretschmer, DiplomaThesis, FreieUniversitiit Berlin, 1962.
1781 R. Ludwig, Diploma Thesis, Technische Universitat Berlin,
1965.
408
When H2S is led rapidly into the SO2 solution, some of
the unsymmetrical thiosulfurous acid condenses to form
the deep yellow polysulfane oxide, which precipitates
out (Wackenroder’s sulfur) [see Scheme XI.
~ I S H+ HO-S-OH + HSH + HO- -OH
-
a
HS-S-OH
-%
H=S
ii,
(r)
+ HS-
I)
HS-?-S-?-SH
0 0
+ 2 H20
The ideal composition of polysulfane oxide, H(S20),SH,
is never quite attained because of the sensitivity of the
substance towards water. The oxygen content of polysulfane oxide increases as the quantity of water present
decreases. The fluffy powder is stable for several days at
-20 “C. No monomeric S20 can be detected during the
Wackenroder reaction [24].
2. Preparation of Polysulfane Oxidesfrom SOCI;!and H2S
Liquid thionyl chloride reacts with H2S at room temperature in accordance with Scheme (p).
HSH + Cl-S-CI + HSH
0
- 4 HCI
+ Cl-S-C1 + HSH
a
H-S-S-S-S-S-H
a::
The yellow,plastic reaction productwas formerly thought
to be sulfur, but has been identified as a polysulfane
oxide by thermolysis in vacuo and spectroscopic detection
of the S20 formed [57]. If H2S is passed into a solution
of SOC12 in absolute ether at -10 OC, polysulfane oxide
again precipitates out as a light powder. After drying in
vacuo, the composition of a product obtained in this
manner was found to correspond approximately to the
formula H2S1405. Thus the terminal atoms of the
chains are hydrogen, and not chlorine [78].
[79] R. Foerster and A . Hornig, Z. anorg. allg. Chem. 125, 86
(1922).
[80] H . Sfamm and M . Goehring, Naturwissenschaften 27, 317
(1939).
[ S l l R . Krumer, Ph. D. Thesis, Technische L’niversitgt Berlin,
1964.
Angew. Chem. internat. Edit. 1 VoI. 4 (1965) 1 No. 5
3. Pwpavation ofPolysulfane Oxides from S2Cl2 and H20
When S2C12, preferably dissolved in CC4, is hydrolysed
with ice-water, polysulfane oxide is formed, probably
via thiosulfurous acid [Equation (o)],as in the Wackenroder reaction [57,63].
The analogy with the Wackenroder reaction would be
particularly striking if the disulfur dichloride could react
as thiothionyl chloride-(6), as shown in Equation (7).
C1
HOH
+
"S,
CI
+
S~(OH)+
Z 2 HC1
(T)
HOH
(6 1
The syntheses of S 2 0 from S2CI2 vapor suggest that the
isomer (6) does perhaps exist in equilibrium with the dichlorodisulfane Cl-S-S-C1 detected. This is also supported by other reactions of S2C12 [82]. In the case of
S2F2, both isomers are actually obtained and have been
characterized by their infrared and microwave spectra
[83,84].
E. Disulfur Trioxide
S203
reactive [ 8 5 ] . This reaction and the discovery that S203
synthesized from radioactive elemental sulfur and SO3
yields only inactive SO3 on degradation support the
structural formula (7) [86].
111. Properties of S20)
Disulfur trioxide gradually assumes a light brown color
and is stable only at -80 "C under CO2, and even then
for only a few hours [85]. It is insoluble in liquid sulfur
trioxide, but dissolves in fuming sulfuric acid to give a
deep blue solution. Similar blue solutions are also obtained on direct addition of sulfur to fuming sulfuric
acid.
Depending on the SO3 content of these solutions, the ESR
spectrum shows the presence of one of two different free sulfur
radicals, namely a blue compound at high SO3 contents and a
brown substance at low so3 contents [87]. It has been concluded from comparison of the visible, ultraviolet, and ESR
spectra of the solutions of sulfur in oleum that the blue paramagnetic substance is monomeric S2O3, which slowly polymerizes to colorless (S2O3),. The solutions do become colorless on standing, but the blue color can be regenerated by
gentle warming [881.
The brown radical present in solutions with low so3 contents
is thought to be similar to the radicals present in molten sulfur
[89], being a stage intermediate between S2O3 and the sulfur
precipitated on dilution of sulfur-oleum solutions.
The only products of hydrolysis of S2O3, apart from
sulfur, are H2SO4 and H 2 S O 3 in a ratio of about 5 : 1 .
The sulfurous acid results from reduction of SO3 by
elemental sulfur [SS].
I. Formation of S 2 0 3
Disulfur trioxide is a blue-green solid formed in a vigorous reaction when sulfur is introduced into liquid sulfur
trioxide. After a few hours, the product can be separated
from the excess S O 3 by decantation; residual traces of
S O 3 are removed in a current of COz [ 8 5 ] or with a filter
pump, care being taken to exclude moisture [77].
11. Constitution of SzO,
Disulfur trioxide is thought to exist as a polymeric substance (7) [85] formed by insertion of sulfur into the
electron deficiencies of the Lewis acid SO3.
Decomposition of solid disulfur trioxide at temperatures
of up to 150°C yields sulfur, S02, and S O 3 [85]. It was
thought earlier that SO was formed as an intermediate,
and decomposed in accordance with Equation (a) [90].
However,attempts to detect SOby ultraviolet spectroscopy
as its derivative S20 yielded no experimental evidence
for the intermediate formation of lower oxides of sulfur
[24]. The earlier results were confirmed recently after
evidence had meanwhile been given to the contrary [91];
the ,520 spectrum was not observed, and n o polymeric
salfur oxide residue was found on evaporation of the
condensed gases 1771.
It may therefore be assumed that the decomposition
proceeds according to Equation (9).and that the ex-
szo,
+
s + so3
(q)
tremely reactive sulfur produced reduces some of the
to SO2 [Reaction (%)I.
S2O3
171
2
The sulfur acts here as an electron donor and can consequently be displaced by stronger donors. For example,
S203 is decomposed by pyridine, with formation of the
adduct pyridineS03 and sulfur; the sulfur is extremely
[821 Cf. Gmelins Handbuch der anorganischenchemie, Sulfur B,
Section 3. Verlag Chemie, WeinheimiBergstr. 1963, p. 1765.
IS31 F. See/ and R. Budenz, Chimia 17, 355 (1963); F. See1 and
H. D. Golitz, 2. anorg. allg. Chem. 327, 32 (1964).
1841 R . L . Kuc-kowski, J. Amcr. chem. SOC.85. 3617 (1963).
[85] R. A p p e l and M . Coehring, 2. anorg. allg. Chem. 265, 312
( I 95 I).
Angew. C h i n . internut. Edit.
/
Vul. 4 (1965)
1 Nu. 5
s20,
+
s-:-3 so1
(%)
Received: December 7th. 1964
[ A 441/219 IEl
G e r m a n version: Angew. Chem. 77. 437 (1965)
Translated by Express Translation Service, L o n d o n
[861 R . A p p e l , Naturwissenschaften 40, 509 (1953).
[87] D. J . E. Ingram and M . C . R . Symons, J. chem. SOC. (London) 1957. 2437.
[88] M . C.R. Symons, J. chem. SOC.(London) 1957, 2440.
[89] D. M . Gardner and G. K . Fraenkel, J. Amer. chem. SOC. 78,
3279 (1956).
[90] L . Wiihler and 0. Wegwitz, 2. anorg. ally. Chem. 213, 129
(1933).
[91] A. R. V . Murtlry, Nature (London) 168, 475 (1951).
409
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