Weak Complexes of Sulfur Compounds with Halide Ligands

This review summarizes the study of the formation of SO2-X, SOCl2-X, SOBr2-X, SO2Cl2-X complexes (X = Cl, Br, I, and SCN) in pure and mixed solvents of acetonitrile and dimethylsulfoxide over the past 30 years. Spectrometry (290nm-380nm) was the principal technique used for the investigation, since the enthalpies of formation ∆Hf of the above complexes are low ( -6.03  -22.51 kJ/mol ). The stability of the complexes depend on the nature of the solvent, acceptor properties of the sulfur compounds, the donor properties of the halide ions and the temperature of the reaction. Also, it was found that the extent of the iodide ion replacement reactions by the other halide depends on the nature of the solvent and the halide ions. SO2-I + X → SO2-X + I The structures of SO2-X compounds are pyramidal, with X at the top of the pyramid, the S atom at the center, and the angle depends on the nature of the halide ion.

ABSTRACT: This review summarizes the study of the formation of SO 2 -X -, SOCl 2 -X -, SOBr 2 -X -, SO 2 Cl 2 -X -complexes (X -= Cl -, Br -, I -, and SCN -) in pure and mixed solvents of acetonitrile and dimethylsulfoxide over the past 30 years.Spectrometry (290nm-380nm) was the principal technique used for the investigation, since the enthalpies of formation ∆H f o of the above complexes are low ( -6.03  -22.51 kJ/mol ).The stability of the complexes depend on the nature of the solvent, acceptor properties of the sulfur compounds, the donor properties of the halide ions and the temperature of the reaction.Also, it was found that the extent of the iodide ion replacement reactions by the other halide depends on the nature of the solvent and the halide ions.
SO 2 -I -+ X - T hroughout the past 30 years there has been a growing interest in the field of sulfur compounds- halide ligands chemistry.The motivation for this interest has undoubtedly arisen from various sources.Witeckowa and Witok (1955) investigated the reaction between SO 2 and iodine in the gas phase and in solutions by spectrophotometric and kinetic techniques.They suggested that the interaction between HI and SO 2 in aqueous solutions is due to dipole-dipole interaction.Burke and Smith (1959) studied the molecular complexes between HF and SO 2 by infrared spectroscopy.Jander and Tuerk (1962;1963) studied the adduct of iodine with H 2 S in dichloroethane at -95 o C. The low enthalpy of formation (∆H f o = -31.8kJ /mol) was taken as indication of the charge transfer nature of the adduct formation.Burow (1970) studied the solvate formation between SO 2 and Cl -, Br -, and I -ligands in liquid SO 2 .Gutmann (1956) isolated a number of adducts of SOCl 2 and SO 2 Cl 2 with halide ligands.Sandhu et al (1960;1962) discussed the tendency of SO 2 Cl 2 to form adducts with Lewis acids and Lewis bases.Salama and Wasif et al (1971;1973;1994) studied the interaction of some organic sulfur compounds and SO 2 , SOCl 2 , SO 2 Cl 2 , & SOBr 2 with Cl -, Br -, I -, and SCN -ligands in acetonitrile (MeCN), dimethylsulfoxide (dmso) and water (for SO 2 species only).The collated data are summarized in four parts: 2. Detection, stoichiometry and stability of SO 2 -X -, SOCl 2 -X -, and SO 2 Cl 2 -X -complex species.
3. Effects of solvents on the stability of complex species.4. Ligand replacement reactions in complex species and factors which affect them.5. Structure of SO 2 -X -species.

Detection of Sulfur Complex Species
Figure 1 shows the absorbance peaks of mixtures of SO 2 with (A) tetramethylammonium iodide, (B) tetramethylammonium bromide and (C) tetramethylammonium chloride.Table 1 includes the absorbance peaks of sulfur compounds, SO 2 , SOCl 2 and SO 2 Cl 2 with tetramethylammonium halides in which the halide was in abundance of the sulfur compound and in acetonitrile (MeCN) solvent.Similar peaks were obtained in dmso and water confirming earlier studies (Jander et al., 1937;Seel et al., 1955).

Stoichiometry and Complex Species
Job's (1928) and Asmus's (1960) methods were used to determine the complex stoichiometry.The former gave the empirical formula while the latter gave its molecular formula.The two methods showed that all complex species were of 1:1 type irrespective of the solvent used (MeCN or dmso).

Stability Constants of Complex Species
If the complex formation is represented by the equations S compound + Halide Ligand = Complex Species then the equilibrium constants for reactions (1) -( 3), which will be defined throughout this article as stability constants, may be represented by the equation where the parentheses represent molar concentration of each species and K c is defined by the units dm 3 mol -1 .The magnitude (or value) of K c is taken as a measure of the ability of the reacting species to associate in a complex.Spectrophotometry was the principal technique used to find the concentration of all the terms in equation ( 4).Two procedures (Salama et al., 1971) were adopted to evaluate K c : (a) graphical and (b) by calculation from the molar concentrations of reactants and products.Both methods (a & b) depend on the absorbances of individual species before and after they are mixed together.Table 2 (Salama et al., 1971) includes K c values for SO 2 -X -, SOCl 2 -X -and SO 2 Cl 2 -X -in MeCN at 293° or 298°K.
In most cases the difference between K c values by the graphical and calculation methods does not exceed 5% which may be taken as the limit of the experimental error.The constancy of the K c data is further evidence to confirm that all the complex species were of a 1:1 type (Salama et al., 1971).3 includes K c data for SO 2 -X -, SOCl 2 -X -and SO 2 Cl 2 -X -over a range of temperatures and their relevant thermodynamic constants.The data in Table 3 point to a weak association between the sulfur compounds (electron acceptors) and the halide ligands (electron donors) of a charge transfer nature (Salama et al., 1971;Ketelaar et al., 1952;Benesi et al., 1949;Drago, 1959;Rossotti et al., 1969;Andrew et al., 1961).Since the solvents used are polar, they possess varying tendencies to solvate the species in solution ( ions, molecules and complex species) and although we are mainly concerned with complex species in acetonitrile, yet a simple interpretation of such enthalpy data will be complicated by solvation and/or dipole interaction.Comparison of the enthalpy data for the different complex species in Table 4  Of the components taking part in the formation of these complexes only the acceptors (sulfur compounds) have UV absorption peaks, SO 2 (280nm), SOCl 2 (280nm) and SO 2 Cl 2 (275nm) (Friedman, 1967).The appearance of new peaks due to the formation of the complex species arises from donoracceptor interactions.These result in spectral shifts for the acceptor which must be a function of the donor character of each halide ligand.An attempt was made to correlate such spectral shifts with the reversible potential for: e -+ X = X - The linear plots of Figure 2 supplement this assumption for the different species.

The Donor-Acceptor Nature of the Complex Species
In order to understand the nature of these complexes we shall try to rationalize the stability constants data of Table 4 with the nature of the halide ligands (donors) and the sulfur compounds (acceptors).Table 5 summarizes some important trends (Basolo et al., 1958 ;Gould, 1960).The iodide ion with high polarisability, low electronegativity and easy oxidation is considered to be a soft Lewis base (Pearson, 1963;Day et al., 1969).The chloride ion with low polarisability and high electronegativity is a hard Lewis base.The bromide ion is a borderline Lewis base.The acceptors include SO 2 and SOCl 2 in which the oxidation state of sulfur is four and SO 2 Cl 2 in which it is six.SO 2 is a borderline Lewis acid (Pearson, 1963;Day et al., 1969).It acts as a base toward BF 3 to form the adduct SO 2 -BF 3 and as acid towards water.Thionyl chloride SOCl 2 is similar to SO 2 in that it has a lone pair of electrons (3s 2 ) but one of the double bonded oxygen atoms is replaced by two Cl atoms.The S-Cl bond is more polarized than the S-O bond owing to the higher electronegativity of Cl -, and would be expected to act as a stronger Lewis acid than SO 2 , or a better acceptor.Sulphonyl chloride SO 2 Cl 2 , may be related to SO 3 (known as a hard Lewis acid) (Pearson, 1963;Day et al., 1969) in the same manner as SOCl 2 is related to SO 2 , and the order of acid strength is SO 2 Cl 2 > SOCl 2 > SO 2 .The formation of the present complex species is the result of acid-base interactions between the acceptors and the donors and the order of stability given in Table 4 can be discussed on this basis.

Correlation of the Order of Stability Constants of Different Complexes with Donor-Acceptor Properties
Table 4 shows that the stability of SO 2 -X -species falls in the order SO 2 -Cl -> SO 2 -Br -> SO 2 -I -.Sulfur (IV) forms coordination compounds owing to the electrofilic and nucleofilic nature of the sulfur atom.The former is due to the availability of the empty 3d electron orbitals and the latter to the presence of a lone pair of 3s 2 electrons on the sulfur atom.Thus, in such compounds as SO 2 , sulfur acts as a σ-donor only or a Π-acceptor.However, the donor-acceptor properties of the sulfur atom are exhibited almost synonymously.If the donor (or ligand) contains d-orbitals of the appropriate symmetry (i.e.not diffuse) back-donation from the sulfur atom to the donor may occur, giving rise to the d-d multiple bonding which will strengthen the ligand acceptor bond.The Cl -and Br -ions may accept back-donation but this seems doubtful for the I -ion because the d-orbitals become progressively diffuse and less available for backdonation as we go down the halide group.The order of complex stability can be explained on this basis.
This interpretation of the stability constants order is supplemented by the classification of the halide ions as hard (Cl -) borderline (Br -) and soft (I -) bases and of SO 2 as a borderline Lewis acid.The order of stability of the SO 2 -X -complex species would follow the strength of the base and SO 2 -Cl -species would be the strongest and SO 2 -I -the weakest, as actually found.We may now consider the SOCl 2 -X -species.Table 6 includes the ratios of stability constants of SO 2 -X -and SOCl 2 -X -species.Table 6.Stability Constants Ratios for SO 2 -X -and SOCl 2 -X -in MeCN Table 4 shows that the order of stability of SOCl 2 -X -species is similar to that of SO 2 -X -where Cl -> Br -> I -.Table 6 shows that the stability constants of SO 2 -Cl -and SOCl 2 -I -are nearly of the same order of magnitude but K c (SOCl 2 -I -) is merely 4 times greater than K c (SO 2 -I -) and K c (SOCl 2 -Br -) and K c (SOCl 2 -Cl -) are only 1.6 and 2.4 times greater than K c (SOCl 2 -I -) which calls attention to new factors responsible for the observed change in ratios.
The order of stability constants ratio of SOCl 2 -X -species shows that as in the case of SO 2 -X -the chloride species is the most stable and the iodide is the least stable.This order suggests that the nature of association between the Cl -and SO 2 is much the same as with SOCl 2 .Back-donation may be considered to be the factor contributing to the stability of SOCl 2 -X -species.In SOCl 2 the electrophilic nature is enhanced over SO 2 by the replacement of one oxygen atom by two chlorine atoms and the d-orbitals of sulfur are more exposed for coordination because the electron cloud is removed by the electronegative chlorine atoms, and this makes SOCl 2 a better Lewis acid than SO 2 .
Although the halide ions were classified by Pearson (1963) as Lewis bases of varying strengths and such classification could account for the order of stabilities of SO 2 -X -species, the situation may be different with SOCl 2 .The increased acceptor character does not appear to have changed or to have affected dramatically the nature of association with the Cl -ligand.For the I -ligand this increased acid character appears to have increased the basicities of the I -and Br -ligands relative to that of the Cl -, so SOCl 2 appears to be leveling up the basic character of the I -and Br -ligands towards that of the Cl -ligand.This leveling of relative basicities of the halide ligands appears to be another factor which determines the ratios of Table 6.
A third factor relevant to Table 6 is the increased ionic radii and polarisabilities of the donors.Table 5 shows that the I -ligand is the most polarisable of the halide ions.Other factors remaining equal, an increase in the polarisability of the donor would make the donor-acceptor interaction stronger.The dipole moments of SO 2 and SOCl 2 are 1.61 and 1.60D respectively.If polarisability was 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 ligands with SOCl 2 .Sulphonyl chloride, SO 2 Cl 2 , is the strongest acceptor of this group of sulfur compounds; it has the highest dipole moment (1.86 D).The d-orbitals of sulfur here are the most exposed for coordination than in the other acceptors.The order of stability of its halide ligand complexes is: SO 2 Cl 2 -I -> SO 2 Cl 2 -Br -> SO 2 Cl 2 -Cl -which is the reverse of SO 2 -X -and SOCl 2 -X -species (Table 4) suggesting that back-donation cannot be strong in the formation of SO 2 Cl 2 -X -complexes.
The increased acidity of SO 2 Cl 2 seems now to be very important.In the presence of such a relatively strong Lewis acid the three Lewis bases appear to lose their identity and are of merely equal strength.Thus the leveling effect observed for SOCl 2 is probably more strongly displayed.In protonic systems this leveling explains why benzoic acid and sulfuric acid are equally strong in liquid ammonia while water, alcohol, ketones….etc are equally strong bases in pure sulfuric acid (Bell, 1965;Waddington, 1965).
It appears that increased polarisability towards I -, the increased polarity towards SO 2 Cl 2 and the increased leveling effect can account for the order of stability constants observed for SOCl 2 -X -complex species.The effect of polarisability of the halide ligands on the order of stability of some metal complexes has been reported (Gould, 1960).
We may conclude that as the acceptor is changed from SO 2 to SO 2 Cl 2 the nature of association also changes.With SO 2 d Π -d Π multiple bonding from back-donation makes its association with halide ligands quite strong but with SO 2 Cl 2 the dipole-dipole interaction seems to be a weaker force of association, as shown from ∆H f o values in Table 3 and the K c values of Table 4.

Pure Solvents
Solvent molecules are not impartial in chemical processes and the extent to which they participate sometimes overshadows that of the other species in the reaction media.This is because the solvent represents the environment in which a chemical reaction takes place and in most cases plays the role of a donor or acceptor.The role of environment and solvent effects on chemical reactions has been discussed by a number of workers (Bell, 1965;Waddington, 1965;Frost et al., 1961;Benson, 1960;Amis, 1965;Gutmann, 1967Gutmann, , 1971;;Grunwald, 1949) .The K c values for SO 2 -X -, SOCl 2 -X -and SO 2 Cl 2 -X -in MeCN, dmso and water recorded in Table 7 illustrate the solvent effects.Table 7. Stability Constants for SO 2 -X -, SOCl 2 -X -and SO 2 Cl 2 -X -in MeCN, dmso and water at 298° K.
(a = MeCN, b = dmso, and c = water) Comparison of K c data is limited to the values in MeCN and dmso.With the exception of SO 2 Cl 2 -X - species, the stability constants of different complexes decreased by a factor of nearly 20 in dmso as compared to MeCN which is shown from the data in Table 8.The data in Tables 7 and 8 express significant changes in complex stabilities between MeCN and dmso, as the K c values are lower in dmso than in MeCN.There are at least two possible roles that can be played by dmso (or water) in affecting the stabilities of the complex species.It may solvate the halide ligands, which prevents them from interacting with the acceptors, i.e. sulfur compounds, or it may act as a competing acceptor, i.e. competes against SO 2 , SOCl 2 , and SO 2 Cl 2 and thus makes a complex species with the ligands and perhaps it may play the two roles depending on the environmental conditions.Table 8 shows some horizontal and vertical trends.
The data in column (1) show maximum decrease in K c for SO 2 -Cl -and also for SOCl 2 -Cl -in dmso.This can be attributed to solvation of the Cl -ligand and possibly ion-dipole interaction in view of the high dipole moment of dmso (4.3D).In these species, stabilization results from back-donation (Mines et al., 1972;Chadwick, 1973) from sulfur 3d-orbitals to the donor ligand.Minimum effect is shown for SO 2 Cl 2 -Cl - where K c (MeCN)/K c (dmso) equals 0.3, which shows the importance of polarization and not back-donation in stabilizing SO 2 Cl 2 -Cl -species.
In column (2) ratios for SO 2 -Br -and SOCl 2 -Br -are nearly half those reported for the Cl -species, which is regarded as reflecting the lower tendency to solvation of the Br -ligand as compared to Cl -by dmso.In column (3) the ratios of SO 2 -I -and SOCl 2 -I -are nearly ¼ of the values for the Cl -species.This shows the lower tendency of I -ligands to solvate and this is not unexpected due to the larger ionic size of I -.
The horizontal trends in SO 2 -X -and SOCl 2 -X -appear to agree with the conclusion that solvation of the donor ligands by dmso is important in decreasing the stability constants compared with those in MeCN and that this lowering is maximal with Cl -ligands and minimal with I -ligands.The K c values in water appear to support this view.
For SO 2 Cl 2 -X -the horizontal tend is reversed compared to that shown by SO 2 -X -and SOCl 2 -X -.This supports the view that the nature of association in SO 2 Cl 2 -X -is different from that in SO 2 -X -and SOCl 2 -X - (Salama et al., 1971), the former being mainly ion-dipole interaction and the latter back-donation as mentioned earlier.The lowering in stability constants of SO 2 Cl 2 -I -and SO 2 Cl 2 -Br -is evidence that iondipole interaction is particularly strong between dmso and iodide ion (Salama et al., 1991), which is understandable in view of the higher dipole moment of dmso compared with SO 2 Cl 2 .The data in Table 8, column (3) for the iodide species show that ion-dipole interaction outweighs solvation, while those in column (1) show that solvation has the greater effect.

Mixed Solvents: (MeCN-dmso)
Few workers have reported on chemical processes in mixed solvents and also specified the role of solvent.The work to be outlined reports on the stability of SO 2 -X -, SOCl 2 -X -and SO 2 Cl 2 -X -in MeCNdmso mixed solvent, and Table 9 (Salama et al., 1978) includes the K c data of these complex species in MeCN, dmso and mixtures of the two solvents at 298º K.
The data in columns 2 and 6 were quoted from Table 2.One feature appears throughout Table 9.The stability constants of all complex species, at 298ºK, vary with solvent composition.In order to rationalize the K c values we shall discuss the horizontal and vertical trends in Table 9.

The SO 2 -X -Species
The K c values of SO 2 -I -change gradually between the limits set for MeCN and dmso as the solvent composition changes.In MeCN-dmso solvent (3:1) the K c value is reduced by 2.7% compared with that in 25% dmso solvent.For SO 2 -Br -and SO 2 -Cl -the K c values are reduced in the same solvent by 75% and 85% respectively.(The K c value in MeCN is taken as a reference in each case).Solvation of Br -and Cl - ligands by dmso is clearly greater than that for the I -ligand.A similar conclusion was reached when K c for the same species were determined in MeCN and dmso (Salama et al., 1975).To correlate the variations in K c values with solvent composition it should be remembered that the molecules of different solvents can act as donors and/or acceptors.Even if both solvent molecules have donor or acceptor character a slight difference in the donor or acceptor properties between different solvent molecules will invite donoracceptor interaction between them.The nature of such interactions in non-protic solvent mixtures such as MeCN-dmso has not been studied before (Waddington, 1965).
Over a wide range of molecular ratios of MeCN and dmso one expects such interactions to exhibit different patterns which depend on the structural and geometrical characters of the molecules.We shall call this solvent-solvent interaction and may define it in terms of donor-acceptor interaction or perhaps association which vary with solvent composition.
Such solvent-solvent interactions may occur at the expense of other interactions in solution.For example in MeCN-dmso containing SO 2 molecules and Cl -ligands and SO 2 -Cl -complex species the following interactions are likely to occur (a) Cl --dmso, (b) Cl --MeCN, (c) SO 2 -dmso, (d) SO 2 -MeCN and (e) MeCN-dmso.The stability constant of the SO 2 -Cl -species is determined by the relative magnitudes of such interactions, a strong Cl --solvent interaction (solvation) would reduce the stability of SO 2 -Cl -species, since this steric factor may prevent, to some extent, SO 2 and Cl -from approaching each other for coordination.On the other hand a strong MeCN-dmso interaction would allow more SO 2 to coordinate with Cl -ligands and K c values become greater than the limits set for each solvent.
The data for SO 2 -X -show that MeCN-dmso interaction accounts partly for the change in K c with solvent changes.Ligand solvation is also important in determining K c values.The K c data show that while the I --solvent interaction is relatively weak and reduces K c by 2.7% (in 25% dmso relative to its value in MeCN) those of Cl --solvent interactions and Br --solvent interactions are much stronger and reduce K c by 85% and 75% for SO 2 -Cl --and SO 2 -Br -respectively.

The SOCl 2 -X -and SO 2 Cl 2 -X -Series
The K c values for SOCl 2 -Cl -and SOCl 2 -Br -fall between the limits set for MeCN and dmso but that for SOCl 2 -I -species exceeds the upper limit in MeCN by 30% (in 25% dmso solvent).A change in solvent composition from 50 to 25% dmso is coupled with a sudden change in K c for SOCl 2 -I -.It appears that over this critical range of solvent composition the solvent-solvent interaction reaches its maximum.The nature of this interaction is not yet clear but is detectable from vapor pressure measurements (Salama et al., 1985).Such interactions have freed sufficient I -ligands and SOCl 2 molecules for coordination which makes K c 30% greater than the value in MeCN.With SOCl 2 -Cl -and SOCl 2 Br -(in 25% dmso solvent) the K c values are reduced by 69% and 71% from the value in MeCN.Thus, despite strong solvent-solvent interaction the Br --solvent and Cl --solvent interactions have outweighed solvent-solvent interactions.A similar argument applies to SO 2 Cl 2 -I -and SO 2 Cl 2 -Br -where K c increases by 61% for the iodide species and decreases by 15% for the bromide species.

The Iodide Complex Series
Although K c values for SOCl 2 -I -and for SO 2 Cl 2 -I -in 25% dmso are 30% and 64% greater than the value in MeCN we find that K c for SO 2 -I -is 2.7% less than its value in MeCN.Such differences in behavior are probably due to differences in the manner in which the I -ligand is coordinated to the three acceptors.The I -ligand is polarisable with diffuse d-orbitals suitable for ion-dipole interaction.The dipole moment of the three acceptors are in the order SO 2 (1.61D) SOCl 2 (1.60D) and SO 2 Cl 2 (1.86D).In 25% dmso solvent strong solvent-solvent interaction favors SO 2 Cl 2 -I -coordination so that the K c value exceeds that in MeCN by 64%.The acceptor character of SOCl 2 is enhanced by the replacement of one O atom in SO 2 by 2 Cl atoms and coordination by ion-dipole interaction is relatively stronger with I -than with SO 2 .Solvent-solvent interaction helps coordination of SOCl 2 to I -and results in a K c value which exceeds that in MeCN by 30%.For SO 2 -I -the ion-dipole interaction is probably so weak that is nearly balanced by solventsolvent interaction and K c is slightly reduced by 2.7%.

The Bromide and Chloride Complex Series
The decrease in the K c values for SO 2 -Br -, SOCl 2 -Br -and SO 2 Cl 2 -Br -(in 25% dmso solvent) by 76, 71 and 15% respectively resulted from Br --solvent interaction.The difference arises from differences in the modes of coordination of the three acceptors.For SO 2 and SOCl 2 back-donation is the principal mechanism by which coordination takes place to S(IV) .For SO 2 -Br -and SOCl 2 -Br -solvent-solvent interaction is outweighed by Br --solvent interaction and K c is reduced accordingly.For SO 2 Cl 2 -Br -coordination occurs by an ion-dipole mechanism involving S(VI).The Br -ligand, being a borderline Lewis base, responds favorably to this mechanism and by its solvation by dmso slightly outweighs solvent-solvent interaction and K c is reduced to a smaller extent than in the other bromides.
For SO 2 -Cl -and SOCl 2 -Cl -back-donation is the principal mechanism for coordination.In both species the Cl --solvent interaction outweighs solvent-solvent interaction and K c values are reduced by 85 and 69% respectively.This differing effect on K c might be due to selective solvation in the presence of different acceptors which we may describe as chemical environmental factors.

Spectroscopic
Using the IR (infra red) techniques, it is found that the S=O vibration band of dmso appears at 1080 cm -1 and on addition of MeCN the band is shifted to 1070cm -1 .The C ≡ N vibration band of MeCN appears at 2250cm -1 and adding dmso at x 1 = 0.5 (x 1 = mol fraction) the band is shifted to 2240cm -1 (Salama et al., 1985).
Using Raman spectra it is found that the S = O vibration band appears at 1044cm -1 and on adding MeCN a peak appears at 1062cm -1 while that at 1044cm -1 disappears.For MeCN the C ≡ N vibration band appears at 2255cm -1 and on addition of dmso the band disappears gradually.The observed vibrational shifts may be taken as evidence for solvent-solvent interaction through adduct formation of the type represented by Figure 3.Further confirmation for solvent-solvent interaction was obtained from measurements of vapor pressure and viscosity of MeCN-dmso mixtures.Table 10 includes vapor pressure, viscosity, ∆H vap , ∆S vap , and ∆H vis , of the mixtures over the whole composition range.Figure 4 shows a plot of ∆H vap and ∆H vis against solvent composition.The plots are not linear and deviate over the composition range 50-70% (maximum deviation which may be taken to indicate solventsolvent interaction).The ∆S vap data show a minimum value at 75% dmso suggesting maximum order for the system at this composition with probable formation of MeCN-dmso adduct.The excess functions from refractive index n E , dielectric constant ε E and densities V E of MeCN-dmso mixtures are given in Table 11 (Salama et al., 1985).The data in Table 11 show that the magnitude of any excess function reaches a maximum at approximately 1:1 composition.This is again taken to indicate solvent-solvent interaction.Table 12 includes the standard free energies of formation ∆G f o of SO 2 -X -and SOCl 2 -X -in MeCN, dmso, and their mixtures.The data in Table 12 show that for every complex species there are several free energy minima, each corresponding to a different solvent composition.This situation is only possible if a change in the solvent composition affects continuously the coordinating ability of the halide ligand (X -) with the sulfur acceptor (SO 2 or SOCl 2 ) and partially hinders them from complex formation and which was described as solvation.Preliminary studies (Wasif, unpublished work) show that halide ion solvation in MeCN and dmso falls in the order Cl -> Br > I -,which agrees with their ionic radii and charge densities.
Solvation of the sulfur acceptors was studied in the present work by UV spectroscopy.Figure 5 shows the absorbance of SOBr 2 in (a) MeCN, (b) dmso and (c) 1:1 mixed solvent of MeCN-dmso.It shows three distinct species which obey Beer's law.The intermediate absorbance of the 1:1 mixed solvent shows that SOBr 2 forms an absorbing species of intermediate character between the species in dmso and MeCN.There are two possibilities in which this could happen: (1) The formation of a constant ratio of the adduct species SOBr 2 -MeCN and SOBr 2 -dmso, (2) that SOBr 2 makes a species with a mixed solvent adduct e.g.(SOBr 2 -MeCN:dmso).Using CCl 4 as solvent the species SOBr 2 -MeCN and SOBr 2 -dmso were detectable and their stability constants are given in Table 13.
The data in Table 13 show that solvents MeCN and dmso play a competing role against halide ligands in their coordination with the sulfur acceptors.The data also show dmso to have a greater destabilizing role for the complex species than does MeCN, by its strong ability to solvate the halide ligands and the sulfur compounds.A second thermodynamic aspect would be to consider the significance of ∆H f o and ∆S f o for complex species in mixed solvents.Table 14 includes the standard thermodynamic constants for the formation of the complex species SOBr 2 -Cl -and SOBr 2 -Br -.
Table 14 shows that the standard enthalpy of formation varies as the solvent composition changes from MeCN to dmso.For SOBr 2 -Br -∆H f o is nearly 10 times greater in MeCN than in dmso but for SOBr 2 -Cl -it is nearly 5 times greater than the value in dmso.Such differences in ∆H f o values suggest that the measured enthalpy of formation is a rather complex function.It does not probably represent the heat of formation of the complex species but other heat terms are possibly embodied in this term such as heats of solvation of the halide ligands and sulfur compounds in MeCN and dmso.If ∆H f o be taken as a rough measure for the complex stability, then data in Table 14 would show that the complex species SOBr 2 -X -(X - = Cl, Br) are more stable in MeCN than they are in dmso.A thorough discussion of the significance of ∆H f o data requires a knowledge of the heats of solvation of the different species in both solvents which are not at present available.This situation permits a qualitative discussion of ∆H f o data.Since solute-solvent interactions are a dynamically changing process we may expect the magnitude of ∆H solvation values to change over the solvent concentration range, which is confirmed directly from Table 14.With this situation in mixed polar solvents a discussion of ∆S f o values would be difficult to interpret.

Ligand Replacement Reactions
The coordination of different ligands (Cl -, Br -I -or SCN -) with the same sulfur acceptors giving varying stability constants suggested that they are differently coordinated and could accordingly be able to replace each other with the same acceptor.Equation (5) shows a general replacement reaction between SO 2 -I -and X -(X -=Cl, Br, SCN) and Figure 6 illustrates a spectrophotometric scan when SCN -is added to SO 2 -I -in MeCN (Salama et al., 1978).2), 1.0(3), 2.5(4), 4.0(6) 0.20(-), 0.50(1.8),1.0(2.9),2.5(4), 4.0(5.9)(b) In dmso Reaction 9) Cl -+ SO 2 -I - 10) Cl -+ SOCl 2 -I - 11) Cl -+ SO 2 Cl 2 -I - [Cl -] : [I -] 0.05(3), 1.0(5), 1.5 (7), 2.0(10), 3.0(13), 4.0(17) 0.50(3), 1.0(4), 1.5(6), 2.0(9), 3.0(12), 4.0(15) 2.0(8), 4.0(16) 12) Br -+ SO 2 -I - 13) Br -+ SOCl 2 -I - [Br -] : [I -] 0.50(4), 1.0(6), 2.0(11), 4.0(13) 0.50(4), 1.0(5), 1.5 (7) In all the above reactions the extent (or magnitude) of replacement depends on the stability constant for the reactant and product complex species.Table 16 includes the K c data for reactants and products in the replacement reactions.Another factor which determines the magnitude of the replacement is the Lewis basic character of the ligands and a third factor appears to be related to the donor and acceptor number of the solvent used (Salama et al., 1971;Pearson, 1963;Day et al., 1969).The data in Table 15 show that in the presence of two halide ligands, Cl -and I -and acceptors such as SO 2 , SOCl 2 and SO 2 Cl 2 in MeCN solvent the thermodynamics would be more favorable for SO 2 -Cl -and SOCl 2 -Cl -than for SO 2 -I -and SOCl 2 -I -.We may add that K c (SO 2 -Cl -) is nearly equal to K c (SOCl 2 -Cl -) and both are much higher than K c (SO 2 Cl 2 -Cl -)in the ratio 37: 36: 1.Such a large difference in stability constants makes replacement of I -by Cl -much easier for SO 2 -I -and SOCl 2 -I -than it is for SO 2 Cl 2 -I -.The data in Table 15 and the plot of Figure 7 illustrate this observation and we note that the percentage replacement in SO 2 -I -and SOCl 2 -I -fall at a low [Cl -]/[I -] on the same line and are much higher for SO 2 Cl 2 -I -.The replacement reaction, equation ( 6), seems anomalous in view of the K c (SO 2 Cl 2 -I -) / K c (SO 2 Cl 2 -Cl -) ratio = 8.That this reaction, equation ( 6), can take place despite the reversed order of stability constants for reactant and product can only be due to the relative abundance of the Cl -ligand which seems to outweigh the difference in stability constants.Table 15 shows that a 20% disappearance of SO 2 -I -requires a ligand ratio of 0.35:1 while in the case of SO 2 Cl 2 -I -ligand ratio [Cl -] / [I -] = 1 : 1 was necessary for the same percentage replacement.By changing the ligand ratio we are merely increasing the chances of effective collisions leading to a replacement by the more abundant ligand.The magnitudes of replacement for the ligands Br -and SCN -are not widely different (Table 15).This is not unexpected since the stability constants for SO 2 -Br -, SOCl 2 -Br -and SO 2 Cl 2 -Br -are in the ratio 4:6:1 and for SO 2 -SCN -and SOCl 2 -SCN -and SO 2 Cl 2 -SCN -are in the ratio 1: 1.2 : 2.4. Figure 8 shows a comparison of the replacing ability of the SCN -ligand with that of Cl -and Br -ligands.The replacing ability of ligands fall in order Cl -> Br -> SCN -, which parallels their Lewis base character as hard, borderline and soft respectively (Pearson, 1963;Day et al., 1969).

The Role of Solvents in Replacement Reactions
Table 15 and Figure 9 show the effect of solvents on the reaction SO 2 -I -+ X -where X -= Cl -.For a ligand ratio [Cl -] /[I -] = 4 the extent of replacement is 70% in MeCN but only 16% in dmso and less than 2% in water.Clearly a change of solvent greatly affects the degree of replacement.
In dmso two factors are operative: a) dmso may solvate the Cl -ligand and b) dmso may act as a potential acceptor at its S atom and compete with SO 2 in solution.The conclusion that dmso acts as a potential acceptor was reached using the observation of the spectrum of a solution containing Cl -and I - ligands.This solution shows slow growth of two peaks at 292 nm and 365 nm and the rate of peaks growth depends on the ligand concentration and a rate constant was found to be 2 x 10 -2 min -1 at 298°K.
The replacement reaction SO 2 -I -+ Cl -= SO 2 -Cl -+ I -in dmso is ionic with a rate constant of 10 10 sec -1 and therefore the addition of Cl -to SO 2 -I -in dmso is accompanied by the disappearance of the peak at 378nm (SO 2 -I -) and the emergence of the new peak at 292 nm (SO 2 -Cl -).If this solution is left for some time the peaks at 292nm and 365nm will slowly appear for the complex species of dmso with Cl -and I - ligands respectively.The sulfur atom in dmso will be the acceptor center with the following structures: OMe 2 S-Cl -and OMe 2 S-I -respectively.
Recent work on photoelectron spectroscopy of sulfur compounds favors this view (Guest et al., 1972;Buncel et al., 1975).Water appears to be a more drastic solvating agent towards the Cl -ligand which results in a low degree of replacement with high ligand ratio of [Cl -] / [I -] = 12.

Replacement Reactions in Mixed Solvents
Table 17 includes the replacement reaction data for the SO 2 -I -+ Cl -reaction in MeCN-dmso mixed solvent.For all runs the ligand ratio was [Cl -]/[I -] = 4.The replacement percentage of iodide is given in parentheses.
As the percentage of dmso in the solvent increases the ability of Cl -to replace I -diminishes.Here it is possible that the solvating power of dmso towards Cl -makes it sterically difficult to replace the I -in SO 2 -I -.

Structure of SO 2 -X -
The structures of SO 2 -X -(X -= F, Cl, Br & I) (Latajka et al., 1995) were investigated using ab initio method at the electron correlation level with effective core potential double zeta valence basis set with polarization functions.It has been found that the minimum on the potential energy surface corresponds to the C S structure of the complexes whereas the planar C 2V structure is the transition state for the inversion process.The stability of SO 2 -X -complexes fall in the order SO 2 -F -> SO 2 -Cl -> SO 2 -Br -> SO 2 -I -.
Careful studies of the potential energy surfaces of the complexes clearly indicates that only one structure corresponds to the minimum.The X -is in close contact with the S atom having a positive total charge.The intermolecular distance R(S …..X) increases in this series with the increase of the atomic number of X -.The shortest intermolecular distance is noted for SO 2 -F -complex.Replacement of F -by Cl - increases the distance by 2.61Å and successive replacements by Br -and I -ligands cause an increase in R(S….O) value by about 0.3Å.
The complex formation slightly distorts the SO 2 subunit.The SO bond is stretched while the OSO bond angle slightly decreases in comparison with the values for the isolated SO 2 subunit.Perturbation of the geometrical structure of the SO 2 subunit is essentially pronounced with the F -ligands.

Figure 5 .
Figure 5. Dependence of Absorbance of SOBr 2 On Solvent Mole Fraction

Figure 6 .
Figure 6.Replacement of I -by SCN -in MeCN a) No SCN -, b) [SCN -] / [I -] = 0.4, c) [SCN -] / [I -] = 1, d) [SCN -] / [I -] = 2In Figure6the gradual addition of SCN -solution causes a gradual disappearance of the SO 2 -I -peak at 378 nm and appearance of a new peak at 322 nm for the SO 2 -SCN -species.Table15includes the results of the above replacement reactions.Table 15.Replacement Reactions in MeCN, dmso and Water at 298 o K* was observed by [SCN] -in dmso.Percentage replacements of the iodide species are given in parentheses.

Figure 8 .
Figure 8.Comparison of Amounts of Replacements of I -in SOCl 2 -I -

Table 5 .
Physical Constants of Halide Ions.

Table 8 .
Comparison of Stability Constants in MeCN and dmso K c (MeCN)/K c (dmso) at 298 o K.

Table 10 .
Vapour pressures and viscosities of MeCN-dmso mixtures

Table 11 .
Excess Functions of MeCN-dmso mixtures at 298 o K 11(a).Refractive Index (n E )

Table 13 .
Stability Constants of Adducts of MeCN and dmso with Sulfur Compounds at 298°K.

Table 16 .
Stability Constants for Reactants and Products in Replacement Reactions at 298 o K