1112 J. Chem. SOC. (A), 1971 Weak Complexes of Sulphur and Selenium. Part 1. Complex Species of SO2, SOC12,and S02C12 with Chloride, Bromide, and Iodide Ions By A. Salama, S. B. Salama, M. Sobeir, and Saad Wasif,' Department of Chemistry, Faculty of Science, University of Khartoum, Khartoum, Sudan The formation of 1 : 1 complex species between SO,, SOCI,, SO,CI,, and CI-, Br-, and I- is reported. The stability constants of the S0,X- species were determined in acetonitrile. dimethyl sulphoxide. and water. The stability constants of SOCI,,X- and SO,CI,,X- were determined in acetonitrile. The standard enthalpies of formation point to weak association of a charge-transfertype. The nature of association between the complexing components is discussed in relation to the acid-base characters of such components. WITECKOWA and WITOK investigated the reaction between SO, and iodine in the gas phase and in aqueous solutions spectrophotometrically and kinetically. They suggested that the interaction between HI and SO, in aqueous solutions is due to dipole-dipole interaction. Burke and Smith studied the molecular complexes between HF and SO, by i.r. methods. Jander and Tuerk3 studied the adduct of iodine with H,S in di- chloroethylene at -95 "C. The low enthalpy of form- ation (AH" = -7.59 kcal mol-l) was taken as an indica- tion of the charge-transfer nature of the adduct form- ation. They also prepared SO,,I, and SO,,I- complexes. Burow studied the solvate formation between SO, and C1-, Br-, and I- in liquid sulphur dioxide. Gutmann isolated a number of adducts of SOCI, and SO,CI,. Sandhu et aL6 discussed the tendency of sul- phuryl chloride (SO,Cl,) to form adducts with Lewis acids and bases. We have extended our earlier work on the tendency of sulphur compounds to form weak complexes with halide ions. Our work on selenium compounds will be discussed separately. EXPERIMENTAL Detection of Complex Species in Solution.-Figure 1 shows the absorbance peaks of mixtures of SO, with A, tetra- methylammonium iodide ; B, tetraethylammonium bro- mide, and C, tetramethylammonium chloride in aceto- nitrile. Table 1 gives Lx. values. Similar peaks were obtained in dimethyl sulphoxide, nitromethane, water (only for SO,), and other solvents. The spectra of SO, with halide ions confirm work by Jander and Seel * and their co-workers. The same spectra were obtained when alkali- metal, trimethylsulphonium, and trimethylphosphonium halides were used. This confirmed that the new spectra resulted from the interaction of the halide ions with SO,, SOCl,, and SO,Cl,. Stoicheiouzetvy.- Job's @ and Asmus's lo methods were adopted in studying the stoicheiometry of the complex species of sulphur compounds with halide ions. The former gave the empirical formula of the complex and the 1 S. Witeckowa and T. Witok, Zeszyty nauk. Politech. lodz., Chem. Spoz., 1955, 1, 73; Roczniki Chem., 1957, 31, 437; S. Witeckowa, ibid., p. 395. T. G. Burke and D. F. Smith, J. Mol. Spectroscopy, 1959, 3, 381; J. Jander and G. Tuerk, Chem. Ber., 1962, 95, 881, 2314; Angew. Chem., 1963, 75, 792. D. F. Burow, personal communication. 5 V. Gutmann, Quart. Rev., 1956, 10, 451; ~lfonalsh., 1964, 85, 395, 404. latter its molecular formula. Since Job's method proves conclusively the existence of a 1 : 1 species, the results on Asmus's method will not be reported. 2.0 '4 4 1.2 A/nm FIGURE 1 Spectra of complex halide species in acetonitrile (A, €3, C see text) *TABLE 1 hm,/nm of mixtures in acetonitrile c1- Br- I- 280 292 320 380 293 322 382 so2 S02C12 275 293 322 375 SOCI, 280 Evaluation of Stability Constants.-(a) Grafihical method. The complex species are formed between a polar molecule (SO,, SOCl,, and SO,Cl,) and a halide ion (Cl-, Br-, and I-). Solvents of a certain polarity were needed to enable the electrolyte to dissolve. Sulphur dioxide species were studied in acetonitrile, dimethyl sulphoxide, and water, but the other species were studied in acetonitrile. The quater- nary halides used have no absorbance in the operating spec- tral range 250-400 nm but SO,, SOCI,, and SO,Cl, absorb in the range 275-280 nm, near the peak for the chloride ion species and, not far from that for the bromide species. The absorbances of the iodide species are at 375-382 nm, and are therefore due to the complex species only. For the bromide and the chloride species equation (1) is applicable to the absorbances at Amax. of the complex species. In this situation the spectral data can conveniently be treated dabs = dcomplex + dSulphur compound (1) 6 A. Singh and S. S. Sandhu, J. Indian Chem. Soc., 1962, 39, 115; S. S. Sandhu et al., ibid., 1960, 37, 299. 7 J. Jander and H. Mesech, 2. phys. Chem., 1937, A, 183, 121, 137. 8 F. Seel and L. Riehl, 2. anorg. Chem., 1965, 282, 293; Angew. Chem., 1955, 67, 32. 9 P. Job, Ann. Chim., 1928, 10, 113. 10 E. Asmus, Analyt. Chem., 1960, 178, 104. Downloaded by University of Massachusetts - Amherst on 17 October 2012 Published on 01 January 1971 on http://pubs.rsc.org | doi:10.1039/J19710001112 View Online / Journal Homepage / Table of Contents for this issue
Transcript
1112 J. Chem. SOC. (A), 1971
Weak Complexes of Sulphur and Selenium. Part 1. Complex Species of SO2, SOC12, and S02C12 with Chloride, Bromide, and Iodide Ions
By A. Salama, S. B. Salama, M. Sobeir, and Saad Wasif,' Department of Chemistry, Faculty of Science, University of Khartoum, Khartoum, Sudan
The formation of 1 : 1 complex species between SO,, SOCI,, SO,CI,, and CI-, Br-, and I - is reported. The stability constants of the S0,X- species were determined in acetonitrile. dimethyl sulphoxide. and water. The stability constants of SOCI,,X- and SO,CI,,X- were determined in acetonitrile. The standard enthalpies of formation point to weak association of a charge-transfer type. The nature of association between the complexing components is discussed in relation to the acid-base characters of such components.
WITECKOWA and WITOK investigated the reaction between SO, and iodine in the gas phase and in aqueous solutions spectrophotometrically and kinetically. They suggested that the interaction between HI and SO, in aqueous solutions is due to dipole-dipole interaction. Burke and Smith studied the molecular complexes between HF and SO, by i.r. methods. Jander and Tuerk3 studied the adduct of iodine with H,S in di- chloroethylene at -95 "C. The low enthalpy of form- ation (AH" = -7.59 kcal mol-l) was taken as an indica- tion of the charge-transfer nature of the adduct form- ation. They also prepared SO,,I, and SO,,I- complexes. Burow studied the solvate formation between SO, and C1-, Br-, and I- in liquid sulphur dioxide.
Gutmann isolated a number of adducts of SOCI, and SO,CI,. Sandhu et aL6 discussed the tendency of sul- phuryl chloride (SO,Cl,) to form adducts with Lewis acids and bases. We have extended our earlier work on the tendency of sulphur compounds to form weak complexes with halide ions. Our work on selenium compounds will be discussed separately.
EXPERIMENTAL
Detection of Complex Species in Solution.-Figure 1 shows the absorbance peaks of mixtures of SO, with A, tetra- methylammonium iodide ; B, tetraethylammonium bro- mide, and C, tetramethylammonium chloride in aceto- nitrile. Table 1 gives Lx. values. Similar peaks were obtained in dimethyl sulphoxide, nitromethane, water (only for SO,), and other solvents. The spectra of SO, with halide ions confirm work by Jander and Seel * and their co-workers. The same spectra were obtained when alkali- metal, trimethylsulphonium, and trimethylphosphonium halides were used. This confirmed that the new spectra resulted from the interaction of the halide ions with SO,, SOCl,, and SO,Cl,.
Stoicheiouzetvy.- Job's @ and Asmus's lo methods were adopted in studying the stoicheiometry of the complex species of sulphur compounds with halide ions. The former gave the empirical formula of the complex and the
1 S. Witeckowa and T. Witok, Zeszyty nauk . Politech. lodz., Chem. Spoz., 1955, 1, 73; Roczniki Chem., 1957, 31, 437; S. Witeckowa, ibid., p. 395.
T. G. Burke and D. F. Smith, J . Mol . Spectroscopy, 1959, 3, 381;
J. Jander and G. Tuerk, Chem. Ber., 1962, 95, 881, 2314; Angew. Chem., 1963, 75, 792.
D. F. Burow, personal communication. 5 V. Gutmann, Quart. Rev., 1956, 10, 451; ~l fonalsh . , 1964,
85, 395, 404.
latter its molecular formula. Since Job's method proves conclusively the existence of a 1 : 1 species, the results on Asmus's method will not be reported.
2.0 '4 4 1.2
A/nm FIGURE 1 Spectra of complex halide species in acetonitrile
(A, €3, C see text)
*TABLE 1 hm,/nm of mixtures in acetonitrile
c1- Br- I- 280 292 320 380
293 322 382 so2 S02C12 275 293 322 375 SOCI, 280
Evaluation of Stability Constants.-(a) Grafihical method. The complex species are formed between a polar molecule (SO,, SOCl,, and SO,Cl,) and a halide ion (Cl-, Br-, and I-). Solvents of a certain polarity were needed to enable the electrolyte to dissolve. Sulphur dioxide species were studied in acetonitrile, dimethyl sulphoxide, and water, but the other species were studied in acetonitrile. The quater- nary halides used have no absorbance in the operating spec- tral range 250-400 nm but SO,, SOCI,, and SO,Cl, absorb in the range 275-280 nm, near the peak for the chloride ion species and, not far from that for the bromide species. The absorbances of the iodide species are at 375-382 nm, and are therefore due to the complex species only. For the bromide and the chloride species equation (1) is applicable to the absorbances at Amax. of the complex species. In this situation the spectral data can conveniently be treated
dabs = dcomplex + dSulphur compound (1) 6 A. Singh and S. S. Sandhu, J . Indian Chem. Soc., 1962, 39,
115; S. S. Sandhu et al., ibid., 1960, 37, 299. 7 J. Jander and H. Mesech, 2. phys. Chem., 1937, A , 183,
121, 137. 8 F. Seel and L. Riehl, 2. anorg. Chem., 1965, 282, 293;
Angew. Chem., 1955, 67, 32. 9 P. Job, Ann. Chim., 1928, 10, 113. 10 E. Asmus, Analyt . Chem., 1960, 178, 104.
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graphically by equation (2),11 where E, is the observed
extinction of the experimental mixture, equal to d,,bs/[SOz],-, and similarly for any sulphur compound such as SOCl,. The mixtures had the composition [S compound] : [Halide ion] in the range 1 : 5 to 1 : 200; E A is the extinction co- efficient of the sulphur compound, which will be called the acceptor, a t lmx of the complex species investigated; E, is the extinction coefficient of the complex species (all E in 1 mol-1 cm-1) ; K, is the stability constant of the complex species in 1 mol-l, and cD is the molar concentration of the halide ion, which will be called the donor. Equation (2) implies that the plot of l/(EB - Ea) against l / c ~ should give a straight line whose intercept is l/(Ec - EA) and whose slope is l/K,(E, - EA). Equation (2) is applicable to all complexes where the peaks of the acceptor and the complex species are near those of the complex species, as for the chlorides and bromides. For the iodide species equation (2) takes the form suggested by Benesi and Hildebrand,12 used as in (3). E,, K,, and cD have the same meaning as before
[S compound],, . Z/d,b, = l/lr',E,cD + l /Ec (3)
and I, the light path length, was 1 cm. complex species are in Tables 2-13.
The data for all the
TABLE 2 Evaluation of K, for SO,,I- species in acetonitrile Soh tion 1 2 3 4 5
1O3[I-],/M 2.411 3.214 4.088 4.821 5.625 1 0 4 W 2 1 0 / ~ 8.20 8-20 8.20 8.20 8.20 doba a t 25 "c 0.124 0.161 0.195 0-225 0.260 dabs a t 30 "c 0.115 0-148 0.178 0.210 0.235 dabs a t 35 "c 0.110 0.135 0.162 0.190 0.220
105c, /~ a t 25 "C 6-82 8-85 10-73 12.38 14.30 Kc a t 26 "C 38.71 38.70 38-46 37.85 38.55 105c,/~ a t 30 "C 6.33 8.14 9-79 11-55 12.93 K, a t 30 "C 35.58 35-22 34-59 34.86 34-05 ~ O % , / M a t 36 "C 7-43 8.91 10.45 12-10 K, a t 35 "C 31-70 31.02 31.00 31-46
TABLE 3 Evaluation of K , for SO,,Br- species in acetonitrile Solution 1 2 3 4 5
1 03[Br-] o / ~ 1.102 1.417 1-732 2.047 2.362
dabs a t 10 c 0.255 0.310 0.350 0.418 0.432 doba at 20 "c 0.202 0.250 0.288 0.340 0.360 dabs a t 30 "c 0.160 0.200 0.231 0.278 0.300
1 O 5 W 2 I o/f 12.01 12-01 12.01 12.01 12-01
E, = 8946 1 mol-l cm-l. K,/1 mol-1 (graphical) = 260 (at 10 "C), 192 (at 20 "C), and
137 (at 30 "C).
105Cc/M a t 10 "c 2.60 3.23 3-79 4.63 K, a t 10 "C 258 265 272 269 105c, a t 20 "C 1-98 2 4 8 2.79 3.80 K, a t 20 "C 181 187 193 199 105c, a t 30 "C 1.50 1.97 2.32 3.12 K, a t 30 "C 131 140 140 151
&(SO2) = 250 a t 320 nm or d(S0,) = 0.03 for a 12.01 ~O-~M-SO, solution.
~ ~~
11 J. A. A. KeteIaar, C. van de Stoppe, A. Goudsmit, and
12 H. A. Benesi and J. Hildebrand, J. Amsr. Chem. SOC., 1949, W. Dzcubas, Rec. Trav. chhim., 1952, 71, 1104.
71, 2703.
TABLE 4 Evaluation of K , for SO,,Cl- species in acetonitrile Solution 1 2 3 4 5
1O3[C1-]o/M 1.488 1-785 2.083 2.380 2.678
d0b at 11 "C 0-580 0.616 0-655 0.685 0.729 doba a t 20 "C 0.520 0.541 0.585 0.628 0.652 dabs a t 30 "C 0.475 0.510 0.553 0.580 0.618
105c,/n~ at 15 "C 4.88 5-68 6.32 7.04 Kc at 15 "C 86.34 86-78 85.15 86.86 105Cc/M at 25 "c 4-48 5.20 5-92 6.56 K, at 25 "C 77.8 77.6 78.2 78.1 105Cc/M at 35 "c 4.081 4.800 5.520 6.160 K, at 35 "C 69-53 70.31 71.60 71.90
(b) Calculation method. Various equations 13-15 were tried for evaluating stability constants from spectral data, but the calculated values differed by up to GU. lOOyo from our graphical results. It was found that equation (4) gave satisfactory results, where c, is the equilibrium molar con- centration of the complex species, [A]eqb is the equilibrium
(4) molar concentration of the sulphur compound or the accep- tor, and [Djeqb is the equilibrium molar concentration of the halide ion or the donor. The terms of equation (4) were
13 M. Tamers, J. Phys. Chem., 1961, 65, 654. l4 R. S. Drago, J . Amer. Chem. Soc., 1959, 81, 6138.
TABLE 12 Evaluation of K, for SO,Cl,,Br- species in acetonitrile
at 15, 25, and 35 "C S o h tion 1 2 3
1O3[Br-],/~ 2.127 2.552 2.771 1 0 4 [ ~ 0 2 c 1 ~ ] o / M 2.766 2.766 2.766
dabs at 25 "c 0.082 0.098 0.108 dobB at 35 "c 0.077 0.087 0.093
dobs at 15 O C 0-092 0.111 0.119
E, = 3376 1 mol-1 cm-1.
106Cc/M a t 15 "c 2.400 2.990 3.228 K , at 15 "C 45-17 47.82 44.47 1O6Cc/M at 25 "c 2.103 2.577 2.902 Kc at 25 "C 39.08 40.68 39.77 105c, at 35 "C 1.985 2-281 2.429 K, a t 35 "C 36.70 35-54 32.60
E[so&I,) = 43 at 322 nm or d = 0-012 for a 2. S02C1, solution.
4 3.402 2.766 0.137 0-125 0.111
3.761
3.406
2.990
46.78
41-70
35.71 766 x 10-4~-
TABLE 13 Evaluation of K , for SO,Cl,,Cl- species in acetonitrile
a t 15, 25, and 35 "C Solution 1 2 3 4
1 03[c1-]o/M 1.802 2.162 2-522 2.882 104[so2c~,]o/M 2.766 2-766 2.766 2.766 doba at 15 "c 0.085 0.095 0.105 0.117 doba at 25 " c 0.080 0.089 0.099 0.109 dabs at 35 "c 0,075 0.083 0.092 0.100
106Cc/M at 15 "c 5.639 6-628 7-617 8.804 11.28 11.27 11-44 K, at 15 "C
106Cc/M at 25 "c 5.144 6.034 7.025 8.013 Kc at 25 "C 10.54 10.35 10.35 10.38 106c,/M at 35 "c 4.551 5-342 6.368 7.025 Kc at 35 "C 9.3 1 9.13 9-36 9.06
11.58
E(SO,Cl,) = 108 at 293 nm or d = 0.03 for a 2.766 x 1 0 - 4 ~ - S 0 ,C1 solution.
evaluated as follows: c, was calculated from E, which was found by the graphical method. For the iodide complex species where EA = 0 at Amax. of the complex species the molar concentration equals dobs/Ec. For the situation where EA # 0 at &= of the complex species, the absorbance of the complex species was corrected for a small contribu- tion from the acceptor as follows: the assumption was made that the observed absorbance is only due to the complex species and an approximate value of c, can be computed from equation (5) . From G{ an approximate value of [SO2lesb
(5)
is computed. This equals [SO,], - c,' and this is used to find da or d(S0, ) of equation (1) at & of the complex species from relation (6). For equation (6) a solution of
~S0210 - G,' + 0.09 = d(S0,) 12.5 x 10-5
12.5 x ~O-~M-SO, has an absorbance of 0.09 a t 292 nm (Amaz of SO,,Cl-). Equation (1) is then used to find dcomples and c, thus found which equals dcomp~ex/Ec. The equilibrium molar concentrations of the complexing components are found from equations (7) and (8). The K, data so calcu-
I5 F. J. C. Rossotti and H. Rossotti, ' The Determination of Stability Constants,' McGraw-Hill, New York, 1961.
lated are included in Tables 2-13 and Table 1 4 summarises
[Dleqb = LD1O - cc
prepared by weight in a known volume of solvent. All solutions were prepared freshly before each experiment. SDectral data were taken on a Unicam SP 700 automatic ('1
(8) recording spectrophotometer and the 1 cm matched cells were thermostatted to f0.05 "C. [Aleqb = CA10 - &c
all K , data evaluated by both methods. In most cases the difference does not exceed 5% and this may be taken as the limit of experimental error.
The constancy of K , [by equation (4)] is a further con- firmation that all the complex species investigated in this work are of a 1 : 1 type.l6
DISCUSSION
Table 16 includes the stability constants of the com- plex species in acetonitrile at 25 "C. The standard thermodynamic constants of these complexes point to a
TABLE 14 Summary of K , * data by graphical and calculation
methods Sulphur
compound so, so, so, SOCl, SOCl, Halide I- Br- C1- I- Br- Graphical 37.9 192 348 162 242 Calculation 38.5 190 363 150 241 Temp./"C 25 20 20 25 25
Evaluation of Standard Thermodynamic Data.-The K , data of Tables 2-13 were used to evaluate the standard thermodynamic constants for the formation of the different species. AF" was calculated from the relation AFO = -RT In K,. AH" was determined graphically, in most cases, the assumption being made that AH may be identified with AH" over the temperature range used. ASo was calculated from the relation AF" = AH" - TAS". The data are summarised in Table 15.
TABLE 15 Thermodynamic constants of the complexes in aceto-
Materials.-SO, was purified by passage through concen- trated sulphuric acid. SOCI, was purified by fractional distillation after adding triphenyl phosphite lo (16% v/v) . The middle fraction was collected at 78 "C. SO,Cl2 was purified by fractional distillation through a 80 cm glass- packed column and the middle fraction was collected at 68 "C. The solvents were purified by standard procedures. The halide solutions were prepared by weighing in a known volume of solvent. Sulphur dioxide solution was prepared by bubbling through the solvent and the concentration was checked by iodometry. Other acceptor solutions were
Q Q
TABLE 16 Stability constants of SO,,X-, SOCl,,X-, and SO,CI,,X-
in acetonitrile a t 25 "C c1- Br- I-
so, ,x- 372 160 38 3 62 241 15C SOCl,,X-
SO,Cl,,X- 10-2 41.0 76.9
weak association between the donors and the acceptors. This is shown from the standard enthalpies of formation (Table 15, column 2, 1-5 kcal mol-1). All these values correspond to weak association of a charge-transfer type.l6 Since the solvents used are polar they possess varying tendencies to solvate the species in solution (i.e. , ions, molecules, and complex species) and although we are mainly concerned with complex species in aceto- nitrile, yet a simple interpretation of such enthalpy data will be complicated by solvation and/or dipole inter- action. Comparison of the enthalpy data for different complex species cannot lead (Table 15) to linear correla- tions.
Of the components taking part in these complexes, only the acceptors have U.V. absorption peaks: SO, (280 nm), SOCl, (280 nm),17 and SO,CI, (275 nm). The appearance of new peaks owing to the formation of the complexes arise from donor-acceptor interactions. These result in spectral shifts for the acceptors, which must be a function of the donor character of each halide ion. An attempt was made to correlate such spectral shifts with the reversible potential e- + X X-. The linear plots of Figure 2 supplement this assumption for the different species.
The Dortor-Acceptor Nature of the Comj?dex S$ecies.--In order to understand the present complexes we will try to rationalise the stability constants of Table 14 with
TABLE 17 c1 Br I
Ionisation potentialslev 13.0 11.8 10.4
Hydration energy of X- * 85.0 74.0 61.0 i Polarisability /As 2.3 3.3 5.1
Electron affinity/eV 4.02 3.78 3.44
Electronegativity 2.83 2.74 2.2 1 * Kcal g-ion-'. t F. Bas010 and R. G. Pearson, ' Mech-
All anisms of Inorganic Reactions,' Wiley, New York, 1958. other values are from ref. 22.
16 L. J. Andrew and R. M. Keefer, Adv. Inorg. Chem. Radio-
1 7 L. Friedman and W. P. Wetters, J . Chem. SOC. ( A ) , 1967, 36. chew., 1961, 3, 91.
1116 J. Chem. SOC. (A), 1971 the nature of the donors and acceptors. Table 17 summarises some important trends. The iodide ion with high polarisability, low electronegativity, and easy oxidation is considered to be a soft Lewis base.l8J9 The chloride ion with low polarisability and high electro- negativity is a hard Lewis base. The bromide ion is a borderline base.
The acceptors include SO, and SOC1, in which the oxidation state of sulphur is four, and SO,Cl, in which it is six. Sulphur dioxide is a borderline Lewis acid.18J9 It acts as a base towards BF, to form an adduct BF,,SO, and as an acid towards water. Thionyl chloride is similar to SO, in that it has a lone pair of electrons (3s2) but one of the doubly bonded oxygen atoms is
Spectral shift / n m FIGURE 2 Correlation of E" for the reaction e- + X + X-
with acceptor spectral shifts: A, SO,-X-; B, SO,CI,-X-; c, soc1,-x-
replaced by chlorine atoms. The S-Cl bond is more polarised than the S-0 bond owing to the higher electro- negativity of chlorine. It would be expected that thionyl chloride can act as a better Lewis acid or better acceptor than SO,. Sulphuryl chloride may be related to sulphur trioxide (known to be a hard Lewis acid) l9 in the same manner as SOCl, is related to SO,. Sulphuryl chloride would be expected to act as the strongest acceptor and the order of acid strength to be SO,Cl, > SOCl, > SO,. The formation of the present complex species is the result of acid-base interactions between the acceptors and the donors and the orders of stability given in Table 16 can be discussed on this basis.
Correlation of the Order of Stability Constants of D;f- ferent Complexes with Donor-Acceptor Properties.-(a) The sulphw dioxide-halide ion complex species. Table 16 shows that the stability of SO,,X- complex species fall in the order SO,,Cl- > SO,,Br- > SO,,I-. Sulphur (IV) forms co-ordination compounds owing to the electro- philic and nucleophilic nature of the sulphur atom.
The former is due to the availability of empty 3d- orbitals and the latter to the presence of a lone pair of 3s2-electrons on the sulphur atom. Thus in such com- pounds as sulphur dioxide, sulphur acts as a G-donor only or a x-acceptor. However, the donor-acceptor properties of the sulphur atom are almost exhibited synonomously. If the donor (or ligand) contains d- orbitals of the appropriate symmetry (i.e., not diffuse) back-donation from the sulphur atom to the donor may occur giving rise to d-d multiple bonding which will strengthen the ligand-acceptor bond. The chloride ion and also the bromide ion may accept back-donation but this seems doubtful for iodide because the d-orbitals of the halide ions become progressively diffuse and less suitable for back-donation as we go down the group. The order of stability can be explained on this basis.
This interpretation of the stability constant order is supplemented by the classification of the halide ions as hard (Cl-), borderline (Br-), and soft (I-) bases and of SO, as a borderline Lewis acid. The order of stability of the SO,,X- complex species would thus follow the strength of the base and the SO,,Cl- complex would be the strongest and the SO,,I- the weakest, as found.
The Thionyl Chloride-Halide Ion Complex Species.- Table 18 includes the ratios of stability constants for SO,,X- and SOCl,,X- species. Tables 16 and 18 show
that the order of stability for SOCl,,X- species is similar to that of SO,,X-, i.e., C1- > Br- > I-. Table 16 shows that the stability constants of SO,,Cl- and SOCl,,Cl- are nearly of the same order of magnitude, but K,(SOCl,,I-) is nearly 4 times greater than K,(SO,,I-) and K,(SOCl,,Br-) nearly 1.6 times greater than K,(SO,,- Br-). The large increase in K, for SOCl,,I- and the smaller one for SOCl,,Br- calls attention to new factors responsible for this rise.
The order of stability constants of the SOCl,,X- species shows that, as in SO,,X-, the chloride species is the most stable and the iodide the least. This order suggests that the nature of association between the chloride ion and SO, is much the same as with SOC1,. Back-dona- tion may be considered to be one factor contributing to the stability of SOCl,,X- species.
In thionyl chloride one oxygen atom is replaced (in SO,) by two chlorine atoms and the electrophilic nature of sulphur is enhanced as its d-orbitals are more exposed for co-ordination because the electron cloud is removed by the electronegative chlorine atoms. Thus SOCl, is a better Lewis acid than SO,.
Although the halide ions were classified by Pearson l* R. G. Pearson, J . Amer. ?hem. SOC., 1963, 85,.3533. 19 M. C. Day and J. Selbin, Theoretical Inorganic Chemistry,'
as Lewis bases of varying strengths and such classifica- tion could account for the order of stabilities of the SO,,X- species, the situation may be different with SOCl,. The increased acceptor character does not appear to have changed or affected dramatically the nature of association with the C1- ligand. For the I- and Br- ligands this increased acid character appears to have increased the basicities of the I- and Br- ligands relative to that of C1-, so SOCl, appears to be levelling- up the basic character of the Br- and I- towards that of the C1- ligand. This levelling of relative basicities of the halide ions appears to be another factor which de- cides the ratios of Table 18.
h third factor relevant to Table 18 is the increased ionic radii and polarisabilities of the donors. Table 17 shows that the I- ligand is the most polarisable of the halide ions. Other factors remaining equal, an increase in polarisability of the donor would make the donor- acceptor interaction stronger. The dipole moments of SO, and SOCl, are 1.61 and 1 . 6 0 ~ respectively. If polarisability were the only factor, one would expect the iodide complex to be the most stable. This was not so, indicating that back-donation is a still more important factor in deciding the nature of association of the halide ions with thionyl chloride.
The Sulphuryl Chloride-Halide Ion Cotq5le.x S+ecies.- Sulphuryl chloride, the strongest acceptor of this group of sulphur compounds, has the highest dipole moment (1.860). The d-orbitals of sulphur in it are more ex- posed for co-ordination than in the other acceptors. The order of stabilities of its halide complexes (Table 16; I- > Br- > Cl-) is the reverse of that of SO,,X- and SOCI,,X- species, suggesting that back-donation cannot be strong in the formation of these complexes.
The increased acidity of sulphuryl chloride seems now to be very important. In the presence of such a rela- tively strong Lewis acid the three bases appear to lose their identity and are of nearly equal strength. Thus the levelling effect observed for thionyl chloride is probably more displayed.
In protonic systems this levelling explains why benzoic and sulphuric acids are equally strong in liquid
2o R. P. Bell, ' The Proton in Chemistry,' Methuen, London,
21 T. C. Waddington, Non-aqueous Solvent Systems,' 1965.
Academic Press, London, 1965.
ammonia, and water, alcohols, ketones, etc., are equally strong bases in sulphuric acid solutions.20?21
It appears that the increased polarisability towards I-, the increased polarity towards SO,Cl,, and the in- creased levelling effect can account for the order of stability constants observed for the SO,Cl,,X- complex species. The effect of polarisability of the halide ions on the order of stability of some metal complexes has been reported.22
We conclude that as the acceptor is changed from SO, to SO,CI, the nature of association also changes. With SO,, d,-d, multiple bonding from back-donation makes its association with the halide ions quite strong but with SO,Cl, the dipole interaction seem to be a weaker force of association, as shown by AH" values in Table 15 and the values of K, in Table 14.
Solvent Efect and Levelling of Basic Character of Halide Iosts.-The effect of solvent was only studied on the SO,,X- species. Table 19 contains the ratio of K, in dimethyl sulphoxide and water to Kc in acetonitrile. These ratios show that water lowers the basicity of C1-
much more than I-.l* This can be explained in the light of the hydration energies of Table 17. The in- creased tendency to solvation (hydration) of C1- pre- vented the formation of its complex species. Dimethyl sulphoxide produces a smaller differentiation between the chloride and the iodide ions, confirming Pearson's observations. It may be that dimethyl sulphoxide and water level up the basicities of the halide ions to different extents and this might account for the results.
We thank Professor T. H. Norris, Oregon State University, U.S.A., P. Vokral, University of Prague, and A. Zarroug of this Department for help.
[9/2172 Received, December 18th, 19691
2a E. S. Gould, ' Inorganic Reactions and Structure,' Holt, Reinhart, and Winston, New York, 1960.
