Alan H. Cowley University of Texas Austin
The Structures and Reactions of the Phosphorus Sulfides
The reaction between the elements phosphorus and sulfur was first noted more than two hundred years ago (I). During a century or so following this discovery, chemical individuality was claimed for no less than fourteen phosphorussulfur compositions ranging from P4Sto PZSI2. This chaotic state of affairs persisted into the beginning of the twentieth century when Alfred Stock and co-workers pointed out for the first time that only three compounds could be prepared by the thermal reaction of phosphorus and sulfur. The formulas Pa8, PIS,, and PISlowhich Stock arrived a t on the basis of careful analyses and molecular weight determinations have been confirmed by subsequent X-ray crystallographic studies. It is not greatly to Stock's discredit, however, that he failed to discover P,Ss the fourth member of the presently recognized compounds, since its thermal stability is less than the other three and this would preclude its detection in a phosphorus-sulfur phase diagram. In addition to the four crystalline phosphorus sulfides, there also appear to be polymeric combinations of the elements, and some evidence has been presented for a phase lying within the compositionlimits PpSs and P&P. The phosphorus sulfides are an interesting class of compounds from the dual standpoints of their unusual structures and the types of reactions which they undergo. The reactions and properties of the phosphorus sulfides have been reviewed previously (S), but since that article was published prior to the X-ray diiraction studies, it seems appropriate to discuss the topic in the light of the additional structural information. Nomenclature
The various names which have been used in connection with the four crystallime phosphorus sulfides are shown below: PAS3 tetraphosphorus trisulfide, phosphorus tetritatrisulfide, or phosphorus segquitlulfide P,Sr tetr8,phosphorus pentasulfide, or phosphorus tetritspenta-
.. ....-.
anlfiA!= . .
P,Si
P,%
termpl.osphorua heptnsulfide, phosphorus tetritaheptasAlirle, or phamplwrw heptaaulfide tetmplcos~hururdreasullide, or pl~naphoruspentltaulfide
In each case the first name given is the one used in the present article. Preparation and Propelties
The compounds PaSa, PB7, and Pas10can be prepared (3) by h a t i n g stoichiometric quantities of red phosphorus and powdered sulfur in either evacuated sealed tubes or under an inert atmosphere. Purification of PlS3 and P4Slocan be effected by recrystallization from carbon disulfide, but Pa,, because of its low solubility (see Table I), is usually purified by solvent extraction 530 / Journol o f Chemical Edumtion
of the more soluble impurities. Tetraphosphorus pentasuKde, the least well known of the phosphorus sulfides, cannot be made by direct union of the elements because of its low thermal stability. I t is best prepared photochemically (4) by exposing a carbon disulfide solution of stoichiometric quantities of PISs and sulfur plus a trace of iodine to diffuse daylight for several days. Other reactions which have led to the formation of phosphorus sulfides are the reaction of gaseous phosphine with elemental sulfur (5), the photochemical reaction of sulfur and PJ4 (6) and the successive addition of sulfur to P4S3to give the higher - sulfides PISl and P.Sl0 (7). Vapor density measurements (Table 1) suggest the following sequence of decreasing thermal stability: P,Sz
-
PrS,
> P,S,O> P,Ss
The range of thermal stability is from Pas,which decom~osesa t its meltincr ~ o i n of t 170°C M. ~. ..., to P4S8 and P~S? which do not show appreciable decomposition up to 700°C. Although P4S10is extensively decomposed a-few degrees above its boiling point the reaction is apparently reversible because it is possible to purify the compound by distillation a t atmospheric pressure. This dissociation may involve the production of free radicals since a metastable green solid is formed (8) when P&o vapor is rapidly condensed onto a cold surface. Reports of a compound of overall composition P& have persisted in the literature for some time. A recent attempt to settle the question (9)confirmed an earlier report (10) that PCla undergoes thiosolvolysis in liquid hydrogen sulfide to give a product of composition P&.oo. However, although the infrared spectrum of this material differed slightly from the other wellcharacterized phosphorus sulfides, the X-ray powder pattern showed it to be mainly Pi&, and purification procedures such as recrystallization or extraction led only to the isolation of PpSs or P4S7. A new phase which is stable between 200' and 250°C has been obtained (11) from phosphorus-sulfur melts in the composition range PaS5to P8e.p. Cooling the phase to room temperature caused a slow separation of PISr and recrystallization, vacuum sublimation, or extraction resulted in the isolation of P4S5and P&. The phase is apparently of variable composition since X-ray examination showed that the d spacings increased reproducihly with decreasing sulfur content. Thus, variability of composition, diminished range of thermal stability and sensitivity to solvents would suggest that it is impossible to obtain a compound of composition P& (0;~~s~). Polymeric phosphorus sulfides have been produced (19, 18) hy incorporating phosphorus into sulfur melts.
--
Table 1.
Some Physical Properties of the Phosphorus Sulfides P,%
Color Crystalline form Melting point (OC) Boiling point (760 mm, "C) Density (17'C) Solubilityin CSl (g/100g CSs) Molecular we1gbtb (700°C) (800°C) (9OO0C)
PIS&
yellow arthorhombic~ 173-174.5 407-8 2.03 76.9
... . .. ...
219 202 182 179
n n n n 0-, c ,----
PIS,
yellow monoclinic 17W220 (dec.) dec. 2.17 (25%)
...
.. .
P*Sm
almost white monoclinic 305-10 523 219 0.029
yellow triclinic 286-90 513-5 2.09 0.22
337 202 179 167
185 144 136 133
P4Sahas two crystalline modifications (transition point = 39 11°C). The low-temperature modification is orthorhombic (81).
* by gas density measurements (18).
molecule has four types of average bond angle: S-P-S (99.4O), P--S-P (103'), S-P-P (103.1") and P-P-P (60.0'). Of varticnlar interest are the 60' bond angles in the straided three-membered ring P2PJP4. As with the P4molecule this poses the question as to which bonding orbitals the phosphorus atom utilizes in arriving a t these small hond angles. Arnold (924) has
The viscosity of these melts reaches a maximum in the range &30% phosphorus, hut as more phosphorus is added the maxlmum vanishes, leading to the conclusion that the long chain polymers have disappeared and that the liquid consists of smaller species such as the crystalline phosphorus sulfides. A partially crystalline polymeric phosphorus monosulfide has been prepared (14) by the action of magnesium on PSC13,and polymeric polysulfides of general formula (PS,). are reported (15) to result from the reaction of either SClz or SzClz with thiophosphoric acid.
-
22 PSBrs 22 HsPS4
+ 32 Mg 2(PS). + 31: MgBn + 32 &CIS+ 2(PS7), + 6x HCI
Structures and Bonding
The quartet of phosphorus atoms persists in the four crystalline phosphorus sulfides, hence they can be regarded as being derived from elemental phosphorus by the progressive replacement of P-P bonds with P S - P bridges (Fig. 1). Such a replacement leads to a gradual decrease in overall strain in going from PISs to P& as shown by the increase in average bond angles of the hasal planes of these molecules (Table 2). Since electron delocalization has been shown to occur (17) in N4S4and Asps4(the cage-like sulfides of nitrogen and arsenic), it is logical to inquire whether there is any evidence of rr-bonding in the sulfides of phosphorus. Using arguments based on bond lengths, Van Wazer (18) has concluded that there is essentially no T-bonding in the endocyclic P-S or P-P bonds and that only the exocyclic P-S bonds have hond orders greater than unity. Some structural aspects of the individual phosphorus sulfides are discussed next. PISs. From a structural standpoint, P4S3 has received more attention than the other phosphorus sulfides. Both X-ray (19, 20, 21) and electron diffraction (22,23) agree on the cage-like structure shown in Figure lb; hence there appears to be no change in conformation in going from the crystalline solid to the vapor. The Table 2.
Figure 1.
Structures of the phosphorus svlflder
suggested that the valence state of phosphorus in P4 is (3p)(3d)%,but as pointed out by Moffitt (25) the promotion of two electrons without a compensating increase in valence is not very plausible on energetic grounds. Furthermore, it would be expected that the 3d radius of phosphorus is larger than that of 3p, and yet the P-P separations of both P4 (26) and P4S3are quite normal.
Structural Data on the Phosphorus Sulfides
Molecule
No. of molecules per unit cell
Space group
Skeletal P-P bond Length (A)
Skeletal P P S Exocyclic P=S bond length ( d ) bond length ( d )
(21) (31)
p& P4Ss
8 2
Pmnb PI,
2.235*0.005 2.21 +0.025
(3% 33) (393
Pas7 P481
4 2
P,, PI
2.35*0.01 none
2.09010.005 2.08to2.19 +0.025 2.08*0.01 2.0810.01
Reference
none 1 . 9 4 1 0.02 1.9510.02 1.95 2 ~ 0 . 0 1 5
Average bond angle of basal plane 60.0 86.9 103.0 109.6
Volume 41, Number 10, October 1964
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531
It is more likely that the phosphorus orbitals used in P4 and the three-membered phosphorus ring of P4Sa are predominantly 3p in type and that the bonds are "bent" in the sense that the lines of maximum charge density do not coincide with the directions of attachment of the phosphorus atoms. Since all the bond lengths correspond to u-bond distances there appears to be little a-bonding in P83. Van Wazer has estimated (18) that there is 0.1 a bond/# bond in the P-P bonds and no T-bond'mg in the P S bonds. The NMR spectrum of P4S3(27) has been found to be in complete agreement with the structure as determined by X-ray or electron diffraction. Two major peaks are produced (Fig. 2) owing to phosphorus atoms in two different chemical environments. Peak 1,a 1-3-3-1 quartet, corresponds to the apex phosphorus atom; peak 2, a 1-1doublet with three times the total area of peak 1, is assigned to the three phosphorus atoms in the basal plane. As with the P4 molecule (28), it is found that strained bonds give rise to a large positive chemical shift (+I20 ppm). Likewise, the relatively large spin-spin splitting (86 cps) appears to be associated with the existence of strained bonds rather than with a special capacity of sulfur for transmitting interactions between phosphorus atoms. The chemical shift of the apex phosphorus atom (-71 ppm) is in the usual range for triple connection to sulfur atoms.
existence of eight different types of bond angle and the abnormal length of the P-P bond. The latter is especially interesting because in twelve other compounds investigated the P-P bond length was found to be insensitive to either the valence state of the phosphorus atom or to the nature of the attached groups. As with PBb the exocyclic P S bonds are the only ones where a-bonding is significant (0.45 a bond/u bond). P&. I n crystalline P4Slo(32) each phosphorus atom is tetrahedrally surrounded by sulfur atoms in a structure which is very similar to that of P,O,O. The range of bond angles is small (107-113°) and the two types of P S bond length correspond to 0-0.1 s bond/# bond in the skeletal P S bonds, and 0.45 s bond/u bond in the exocyclic P S bonds. Overall there is much less a-bonding in this molecule than in P4010. Reactions
An extensive literature search would reveal a large number of patents (34) relating to the use of P4SIO in the preparation of lubricant oil additives and flotation reagents. Although the commercial importance of these formulations should not be underestimated, they have not contributed to an understanding of the reactions of the phosphorus sulfides and are not considered in this article. More attention is given to those reactions which have led to well characterized products. Crystallime products of general formula (RPS& have been obtained from the reaction of aliphatic (36) or aromatic (36) hydrocarbons with Paslo withm specific temperature ranges. However, the two papers diiered on the structure of the product (RPSz). Thus, for R = cyclohexene the structure
-100 - 8 0 - 6 0 - 4 0 -20 0 t 2 0 t 4 0 t 6 0 f 8 0 t100t120 + I 4 0 Shift from peokfor 85% H3P04, rneosured in p.p.m. of magnetic field Figure 2. "P NMRrpoctrvm of Pb8.t 7140 Gauss and 12.3 M r (Repmdussd with permission (271.
The vibrational spectrum of P4S3has been interpreted (29) on the basis of a C3, configuration, and the nine expected Raman frequencies (4A1 5E) have been assigned. Force constant calculations indicated that both the P S and P-P bonds are weaker than expected. The P-P force constant was found to be particularly low compared with the Gordy's rule prediction, which is in agreement with the idea that the P-P bonds are bent. The dipole moment (0.81 D) and parachor of P4S3are also in agreement with the cage-like structure (30). Pi%. The structure of this molecule, as obtained by X-ray diffraction ($I), is shown in F i r e Ic. The average bond angles in the basal plane P&P3P4 (86.9') indicate that the molecule is still slightly strained, but much less so than P4S3. There is a significant diierence in length between the exocyclic P S bond and the three endocyclic P i 3 bonds (Table 2). On the basis of bond lengths it has been estimated that the exocyclic P S bond has 0.5 T bond/u bond, and that there is essentially no x-bonding in the rest of the structure. Par. The two outstanding features which emerged in the X-ray diffraction study of P4S7 (32, 33) were the
+
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Journal o f Chemical Education
was proposed, whereas for R = phenyl, o-xylyl, anisyl, phenetyl, napthyl, or 2-isopropylnapthyl the dimer S
R-P
I/
+- S+S P-R +
-4
was suggested on the basis that a four-membered P S - P S ring is unlikely. The phosphorus sulfides Paslo, P&, and P4S3have been found to react with Grignard reagents. Thus, depending on the temperature and ratio of RMgX to P4& one, two, or three alkyl groups can be attached to each phosphorus atom (37, 38) by a mechanism which presumably involves electrophilic attack of the phosphorus atoms by carbonium ions followed by P S bond cleavage.
+
PISLO 8 RMgX P&
-
S
II
4 RlPSMgX
+ 12 RMgX
-
+ 2 MgXa + 2 MgS
acid
4 RePSSH (2) 4 RP(S)
+ 6 MgX? + 6 MgS
(3)
Reactions (I), (2), and (3) must be considered idealized because large quantities of RP( :S) (SH) (OH) were isolated in the preparation of R2PSSH, and yields in general were poor due to byproduct formation. Grignard reagents react with PPS3(39) in approximation with the equation P,&
-
+ 9 RMgX + 3 H.0
3 RIPH
+ RIP + 3 MgXs + 2 MgS + 3 Mg(0H)X
Ni(NHa)+2. From the relationship between the number of P-S--P bridges in P& and P4S5and the total number of ammonia molecules that react it appears that all the P S - P bridges except one are cleaved by NH2- ions. The higher phosphorus sulfides undergo more extensive cleavage than the lower ones. The equation for the over-all reaction of P4S7with liquid ammonia (61) is
These products can be regarded as arising from two diierent ways of cleaving the P4S7 molecule (dotted lines indicate cleavage).
The 3: 1 ratio of RzPH to R3P would suggest that the R3P comes from the apex phosphorus atom and the W H form the basal plane (Fig. lb). Compounds containing X-H bonds (X = 0, S, or N) react with P4Sloby elimination of HzS. Thus, alcohols and phenols (40, 41, 48, 43) give predominantly secondary dithiophosphates, P,S,,
+ 8 ROH
-
4 (ROXPSSH
+ 2 HIS
and aromatic thiols (44) give a mixture of triaryl tetrathiophosphates, (RS)PS, and aryl trithiometaphosphates, RSPS2. P,&o
+ 8 RSH
-
2 (RS)SS
+ 2 RSPSB+ 4 &S
Anomalous reactions have been reported for t-hutyl and tamyl alcohols (46). Although these alcohols undergo normal reactions with P&o a t 45'C they are dehydrated to the corresponding alkenes a t higher temperatures. Alcohols also react with PASsand P4S7to give products of composition (RO)zPSH, (R0)2PSSH, and (R0)zPSSR (46). Primary amines react with P4Slo(47) to yield either thiophosphoric disanides, (RNH)&'SSH, or thiophosphoric triamides, (RNH)ePS, depending on the reactant ratio and the temperature.
With the secondary aliphatic amines (48) it was only possible to isolate thiophosphoric monoamides, FhNP(:S)(SH)z. However, it was found that the monoamides would react with excess secondary amine to give mixed thiophosphoric diamides, which could in turn be converted into triamides identical with those obtainable from the reaction of primary amines with P4Slo. &NP( :S)(SH),
R
PN\P(:S)SH RHN/
+ 2 &NH
-
+ 3 RINH
RIN\
P(:S)SH RHN /
-
(RHNhPS
Some fifty years earlier Stock (62, 63) had found that P4Sloreacts analogously with liquid ammonia.
+ RsN + HZ3
+ 2 RsN + Hd3
The reactions of the phosphorus sulfides with liquid ammonia have been described in an interesting series of papers by Behrens and co-workers. At -33'C, P4S3 (49) and P4S5 (60) react to give compounds of overall composition P4S3.4NH3and P4Ss.6NHJ respectively. These con~poundshave been formulated as ammonium salts (NH4)2[PpSa(NH2)2]and (NH& p4S5(NH2)3]on the basis that the ammonium ions can be replaced by other cations such as C ~ ( N H , ) S and +~
The phosphorus sulfides range in hydrolytic stability from P4S3,which reacts only slowly with cold dilute HC1 or H$04, to P4S7which is hydrolyzed by atmospheric moisture. The sequence of hydrolytic stability appears to be PlS3 > P4Sm> P a 7 (4). All the phosphorus sulfides are hydrolyzed by aqueous alhlies to give products which include hydrogen, hydrogen sulfide, phosphine, hypophosphorous acid, phosphorous acid, and phosphoric acid. There is considerable disagreement (3,.6, 64) on the exact distribution of phosphorus atoms among the various hydrolysis products Volume 4 1 , Number 10, October 1964
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533
(Table 3). Recently (66) it has been reported that it is possible to isolate oxyacids containing P-P bonds by oxidative hydrolysis of P&, PIS5,and P4S7. Table 3. Percentage Distribution of Phosphorus in the Alkaline Hydrolysis Products of the Phosvhorus Sulfides
-Treadwell and Beeli (4)--Pernert Pas8 PS, P&, P,S, PHI HIPO~ H8P08 HZOa
5 15 75 0
3 2 38 57
0 10 0 80
3 38 49 6
and Brown (2)P'S, P,S,o 0 24 22 51
0 0 0 100
Excess iodine reacts with P4Sa to produce PIa and P4SI (4). However, reaction of stoichiometric quantities of PlS3and iodine in CSI a t room temperature (56) results in an interesting phosphorus thioiodide, P4Sa12, the structure of which (57) is closely related to that of P&. It can be seen that in going from P4Sa(Fig. lb) to P8aL (Fig. 3) that the Pa-P4 bond is broken, and P, and S3 change positions. Since these ehanges take place a t room temperature it has been inferred (52?,53) that the bonding in P4S3is quite mobile.
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AND
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--
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AND
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Sfructure of P<.
LECHER, H. z., GREENWOOD, R. A,, WBITEROUS~ K. C.,
Tetraphosphorus decasnlfide is a familiar reagent in organic chemistry for converting OH, C=O, COOH, P=O, etc. into their corresponding sulfur analog^, and for preparing thiophene derivatives (58) from 1,4difunctional compounds such as 1,44i-esters, 1,4diketones, and snccinic anhydride. Acknowledgment
The author wishes to thank Professors George W. Watt and Harry H. Sisler for their helpful comments on this manuscript. Literature Cited
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Journal of Chemicol Educofion
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