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Proton magnetic resonance studies of the solvation of representative halides and of gallium (III) perchlorate in acetonitrile
JOURNAL
OF MAGNETIC
RESONANCE
7, 196-206 (1972)
Proton Magnetic ResonanceStudies of the Solvation of RepresentativeHalides and of Gallium (III) Perchloratein Acetonitile I. Y. AHMED Department of Chemistry, University of Mississippi, University, Mississippi 38677 Presented at the Fourth International Symposium on Magnetic Resonance, Israel, August, 1971 At low temperatures the proton magnetic resonance signals of coordinated acetonitrile in the solutions of some group (III) and group (V) representative element halides and of Ga+3 ion can be distinguished from those of bulk acetonitrile. From the relative intensities of these signals the solvation numbers of the metal halides and Ga+3 ion were determined. Reduction in the solvation number of some complexes from the expected value was observed and is explained in terms of contact ion-pair formation. The spectrum of Ga+3 ion solution contains two distinctive signals for coordinated acetonitrile. The relative intensities of these signals vary with temperature but are independent of concentration. This is explained in terms of an equilibrium between two kinetically distinguishable solvated Ga+3 species. From the temperature dependence of the proton relaxation rates the solvent exchange parameters k,, dH*, and dS* were calculated for various systems. Variations in the kinetic parameters among the different complexes are discussed in terms of the structure of the complex and the effect of anion participation in the coordination sphere of the complex. INTRODUCTION
The application of nuclear magnetic resonance spectroscopy to the study of solutesolvent interactions both in aqueous and nonaqueous solvents has been the subject of several recent reviews (1-4). The NMR technique can provide detailed information about solvation numbers, structure, and lability of kinetically distinguishable solute species (5-11). The mechanism by which the solvent exchange process occurs is inferred indirectly from the activation parameters which can provide information on the structure of the activated complex (5). The contribution to these parameters from second coordination sphere interactions should, however, be considered. Evidence for the formation of ion pairs, especially in solutions of low dielectric constant, has been accumulating (12-18) and its effect on the kinetic parameters of exchange has been reported (12, 13, 26). In most of the systems investigated it was only necessary to invoke a single cation-solvent complex to account for the NMR data. In a few cases, more than one complex was assumed to exist in equilibrium, especially in mixed solvents,
in order
to account
for the observed
solvation
numbers
(14-18).
No direct
observation of more than one well-defined solvated complex in solution has been reported, although Matwiyoff and Movius (11, 13, 19) have been able to characterize mixed complexes in NJ-dimethylformamide solutions of a mixture of some metal ro 1972 by Academic
Press, Inc.
196
SOLVATION
OF METAL
HALIDES
IN
CH&N
197
ions and their acetylacetonato complexes. The two mixed complexes characterized in each case have the same coordination number. The results of infrared and conductance measurements of solutions of some representative metal halides in acetonitrile (20-23) indicated that the halides interact strongly with acetonitrile and dissociate to a greater or smaller extent to give cations with an unspecified number of solvent molecules in the first coordination sphere. The possibility that gallium and aluminum cations may possess four or six coordination or that chemical equilibrium exists between four- and six-coordinate cationic species prompted this investigation. EXPERIMENTAL
Fisher reagent grade acetonitrile was purified by the method described previously (20). Gallium (III) chloride and bromide (Alfa), aluminum (III) chloride (Baker), and phosphorus (V) chloride (Baker) were sublimed under vacuum prior to use. Boron (III) chloride (Matheson) and antimony (V) chloride (Baker) were purified by fractional distillation under vacuum. Anhydrous gallium (III) perchlorate was obtained by heating the hexahydrated salt (Alfa) at 60°C under vacuum for several days. Alternatively, solutions of the anhydrous salt were prepared by the addition of slight excess of anhydrous AgCIO, solution to a solution of gallium (III) bromide in acetonitrile, followed by the separation of solid AgBr. All solutions were prepared by weight and transferred in a glove bag under an anhydrous nitrogen atmosphere to NMR tubes, which had been baked at 120”. Proton spectra were obtained at 60 MHz with a Varian A-60 spectrometer equipped with a variable-temperature probe and a variable-temperature control unit. The temperature was monitored by measuring the peak separation of methanol. The areas under the peaks were obtained by integration of the signals with the electronic integrator. RESULTS
Figure 1 shows a representative NMR spectrum of aluminum (III) chloride solution in acetonitrile at -36°C. The spectra of GaCI,, GaBr,, BC13, and SbCl, are similar in appearance. The intensity of the peak attributed to coordinated acetonitrile is directly proportional to the concentration of the halide. At -40°C the position of this peak in the spectra of the various halide systems investigated is 40 3: 5 Hz downfield from
FIG. 1. The ‘H spectrum of aluminum (III) chloride solution in acetonitrile at -36°C. A,, A,, SSB, and ‘% are designations for free acetonitrile, coordinated acetonitrile, spinning side band, and the free solvent band resulting from ‘W-splitting, respectively. 8
198
AHMED
the bulk acetonitrile peak and shows no apparent dependence on concentration. No coordinated solvent peak is observed in the spectra of PCI, solutions over a wide range of concentrations and temperatures. The separate acetonitrile peaks in solutions of aluminum and gallium halides are distinguishable even at room temperature although the coordinated solvent peak is broad due to solvent exchange. In BC13 and SbCI, solutions, the coordinated solvent peaks broaden at much lower temperatures and completely disappear at approximately 0 and -20°C respectively.
I /AA --?4
1,
-30 364 Hz j! .---.-.-...-40
-..-.-.
P . . .. /
---
___-.-
---
-’ iv
-
i AC
%
FIG. 2. The ‘H spectra of 0.324 no solution of Ga(ClO.& in acetonitrile. A, and A, represent the signals due to free acetonitrile and coordinated acetonitrile, respectively. The free acetonitrile signal is recorded at lower rf level. Temperatures in “C are indicated at the left of the curves.
Figure 2 shows the NMR spectra of anhydrous gallium (III) perchlorate solution in acetonitrile. The peaks attributed to coordinated acetonitrile are reproduced at different temperatures. The positions of these peaks are about 365 and 380 Hz downfield from the bulk solvent peak and vary slightly with temperature and concentration without any apparent trend. The relative intensities of the coordinated solvent peaks are in the ratio of 2: 5 and vary slightly with temperature. The separation between the two coordinated solvent peaks is constant (15 Hz) at temperatures below zero and decreases to 10 Hz at 16°C when the two peaks are discernable. At temperatures higher
SOLVATION OF METAL HALIDES IN CH$N
i99
than 16” the two signals start to coalesce. The results of chemical shifts, relative intensities and solvation number of Ga(ClO,), solutions in acetonitrile at various temperatures and concentrations are summarized in Table 1. When very small amounts of water or DzO were added to these solutions, the coordinated solvent peaks completely disappeared. TABLE PROTON
CHEMICAL
SHIFTS,
RELATIVE NUMBER
INTENSITIES OF Ga (III)
1
OF COORDINATED ACETONITRILE ION IN ACETONITRILE
AND
SOLVATION
Chemical shift” (Hz) Molality Cm) 0.108
Temperature CC) -44 ~-20
0.114 0.221
-40
6Cl
6 c*
Relative intensity Ac,IAcz
Solvation number (n)
365 367
380 381
2.5 2.3
5.2 4.9
374
389
3.0
2.2
376
391
2.5
2.5
0.228
-40 -25
377 371
392 386
2.2 2.2
2.4 2.3
0.324
-40 -24
364 366
379 380
2.3 2.1
2.9
’ c, and c2 refer to the high-field and low-field signals, respectively, of coordinated the shift is downfield from bulk acetonitrile.
-
acetonitrile, and
The solvation number was determined from the known solution composition and the relative areas under the signals of coordinated and bulk acetonitrile in the spectrum. Because the amplitude of the signals differs appreciably for some solutions, it was not always possible to record both signals at the same recorder input level and obtain an accurate area for both. Instead, the area of one of the peaks resulting from 13C-splitting, measured at the same signal amplitude as that of coordinated acetonitrile, was used to calculate the relative area of bulk acetonitrile. The calculated solvation numbers at the respective temperature and solute concentration are summarized in Table 2 for the metal halides, and in Table 1 for gallium (III) perchlorate. The temperature dependence of the line widths of coordinated acetonitrile signal in the solutions of gallium (III) chloride and gallium (III) perchlorate are summarized in Fig. 3 in which log (rrdv) is plotted as a function of reciprocal temperature. Since the chemical shifts are independent of concentration and temperature in the range studied, the temperature dependence of the line width should conform to the equation (24,25)
PI l/T, = (l/T;) + (l/T), which relates the observed transverse relaxation rate l/T, to the mean lifetime of acetonitrile in the complex T and the transverse relaxation rate in absence of exchange
200
AHMED
TABLE SALVATION
NUMBERS
Molality Metal
halide
AK&
(4 0.6343
number 02) ~~ .---~
Average (n) 1.50 + 0.03
-40 -40 -40 -40 -40
0.96 1.00 1.08 1.11 I .07
0.5355 0.6022
-40 -40 -27 -42 -35 -28 -40 -44 -40 -25
0.61 0.55 0.61 0.51 0.51 0.54 0.51 0.58 0.65 0.59
0.2521 0.2802 0.3184 0.3295 0.4220 0.5438 0.8665 0.9847
-40 -40 -31 -40 -46 -34 --40 -40 -17
0.94 1.00 1.04 1.06 0.96 1.04 1.02 1.03 1.10
1.02 It 0.01
0.3342 1.062 1.163 1.438 1.772
-40 -40 -40 -40 -40
0.79 0.82 0.86 0.79 0.83
0.82 + 0.01
0.7130 1.072 1.079
SbCI,
Solvation
0.6115 1.029 1.064 1.288 1.843
0.6170
GaBr,
IN ACETONITRILE
1.40 1 so 1.60 1.64 1.49 1.45 I .43
0.956
GaCI,
Temperature (T)
HALIDES
-40 -17 -50 -40 -25 -40 26
0.8464
BC13
OF METAL
2
1.04 j: 0.02
0.57 i 0.01
I/T!. The observed transverse relaxation rate is related to the full width of the NMR line AV by l/T2 = rrdv. PI Equation [l] becomes nAv = nAv” + k,, [31
SOLVATION
OF METAL
HALIDES
IN
CH,CN
201
100
40
30 -7 8 i!.! -P I
10 9 8 7 6 5 4
3
3.0
I
I
I
3.2
3.4
3-6
I
I
loo0
T
-'
I 4.2
1 4'*
K
FIG. 3. The temperature dependence of the line width of coordinated acetonitrile in the solutions of GaC& and Ga(ClO&. The lower curve is for GaC&. The upper curves are for Ga(ClO&, with the open circles indicating the high-field signal and the solid circles representing the low-field signal.
where ki is the pseudo-first-order rate constant for the exchange of acetonitrile between the free solvent and the compiex. The temperature dependence of l/r and l/T; can be represented by the equations l/~ = k, = (kT/h)exp [(-dHt/RT) 1/T;
a exp (-Eu/RT),
+ (LlSt/R)],
[41 [51
where d/D and dSt represent the enthalpy and entropy of activation for the solvent exchange reaction, and E, can be viewed as an apparent Arrhenius activation energy associated with the transverse relaxation of a proton in the complex (11). The line width data for gallium (III) chloride and gallium (III) perchlorate in Fig. 3 can be fitted to two straight lines: (a) the ones at low temperatures, with small positive
202
AHMED
slopes, can be associated with the temperature dependence of the relaxation rate l/T: of the proton in the first coordination sphere of the complex; (b) the ones at high temperatures, with large negative slopes, are associated with line broadening due to solvent exchange. In the case of boron (TII) chloride and antimony (V) chloride solutions, solvent exchange is significant even in the low temperature region and it was not possible to go to temperatures low enough to determine the temperature dependence of l/T; for these systems since the solutions freeze below the temperatures recorded here. Only straight lines with very large negative slopes, which describe the variation of l/~ with temperature, were obtained. However, there is a curvature in the line fitting the data of BC13 solutions below -40” but not enough to construct a line describing the variation of l/T!: with temperature. Chemical exchange parameters calculated from the data are summarized in Table 3. In solutions of gallium (III) bromide and aluminum (III) chloride, insufficient line broadening, in the concentration range TABLE KINETIC
PARAMETERS
FOR ACETONITRRILE
EXCHANGE
SbCl,
k.25. (see-‘) AH * (kcal/mole) AS% (eu) E. (kcal) n From Ref. * First entry
3 IN SOLUTIONS
BCI,
3.5 x lo3 16.3
5.6 x 10’ 13.9
13
1
AICI,”
OF METAL
high-field
signal
and the second
AND Ga (III)
GaCI,
8.1
9.6
57
9.4
12.0
9.7
-22
is for low-field
signal
ION
Ga(ClO&
19.1
-0.2 (18). is for
HALIDES
81 11.5
-10
-11
-1.3
of coordinated
*
-0.3
acetonitrile.
studied, made it difficult to obtain accurate kinetic data. By comparison with the gallium (III) chloride data, the pseudo-first-order constant for solvent exchange in gallium (III) bromide solutions at 25°C is estimated to be less than 5 set-‘. A value of 8.1 set-’ had been reported for k, of AIC13 solution in acetonitrile at 25°C (18). DISCUSSION
Metal Halide Solutions. An apparent solvation number of 1 for boron (III)
and gallium (III) halides is consistent with the presence of four-coordinate species in solution either as molecular adducts CH,CN*MXj or ionic species (CH,CN),MXz, MX,. Although the boron trichloride-acetonitrile adduct is molecular in the solid state (20, 22), its solutions in acetonitrile have been shown to contain appreciable concentrations of BCl; (20). In view of the low conductance of these solutions, the ions must be present predominantly as ion pairs. There is no direct evidence for the existence of the molecular adduct in equilibrium with the ionic species in solution such as the appearance of two bands for coordinated acetonitrile in the spectra due to CH3CN*BC13 and (CH,CN),BClf. However, the presence of two solvated boron species in solution would produce a single resonance band if either the actual chemical
SOLVATION
OF METAL
HALIDES
IN
CH,CN
203
shifts of the two species are experimentally indistinguishable or rapid ligand exchange between the species produces an average chemical shift. On the other hand, the gallium trihalide-acetonitrile complexes have the ionic structure (CH,CN),GaX:, GaX; in the solid state (23). Infrared and conductance measurements established the stability of GaX; ion in solution (25). The solvation number of 1 for the strongly conducting gallium (III) bromide can be accounted for only if the species present are (CH,CN),GaBr: and GaBr;, and the results firmly establish the preference of gallium for four coordination. The low solvation number of 0.57 observed for gallium (III) chloride can occur if significant concentration of four-coordinate, chlorine-bridged gallium species, such as Ga,Cl, or (CH,CN)C&GaClGaCl,, are present. These species are produced if contact ion pairing occurs, whereby acetonitrile molecules in the first coordination sphere of the cation are completely or partially replaced by the anion. Similar results were obtained for gallium (III) chloride solutions in aqueous-acetone mixtures (1.5, 17). It was observed (17) that ion-pairing ability of the anions in gallium (III) halide solutions decreases in the order Cl- > Br- z I -. The greater strength of the Ga-Cl bond in the dimeric gallium halides (26) must be responsible for this variation in solvation numbers of the two halides. The faster rate of exchange of coordinated acetonitrile in gallium (III) chloride solutions, compared to gallium (III) bromide, must be a consequence of ion-pair formation. The rate of solvent exchange is enhanced by the presence of the anion in the close vicinity of the solvated cation. Similar observations have been reported for solutions of AIXj in N,N-dimethylformamide, where the rates of DMF exchange increase in the order Cl > Br > I (27). It has also been noted that the *‘Al line width in DMF solutions of Al(ClO& increases with the addition of halide ions to these solutions (Z2, 13). These observations are significant in view of the SN’ mechanism proposed for solvent exchange in solutions of metal ions (28). In addition, ultrasonic studies (29, 30) of the kinetics of ion association in water have been interpreted in terms of multistep mechanisms in which the rate-determining last step is the formation of a close ion pair. The rate of this process was found to be independent of the incoming anions and is determined solely by the rate of elimination of a solvent molecule from the first coordination sphere. Although such data are not available for acetonitrile solutions, it is evident that variations in the anion have appreciable effect on the rates of solvent exchange. The effect of ion pairing on the rate of solvent exchange is more apparent in solutions of boron (III) chloride. The rate of exchange of acetonitrile coordinated to the boron solute species exceeds that of gallium by about two orders of magnitude. Tn this case, however, the ion pair must be solvent-separated because a constant solvation number of 1 was obtained over a wide range of concentration and temperature, and the solution exhibits very low conductance (20). Further evidence for ion pairs in the solutions of boron and gallium chlorides is suggested by the small or negative values of dS$, which can result if the rate-determining step involves exchange of highly oriented solvent molecules between the first and second coordination spheres of the cation. Steric crowding of four ligands around boron could also contribute to the lability of the solvated cation. Reduction in solvation number and faster rate of solvent exchange are also observed for the antimony (V) chloride solutions. The apparent solvation number of 0.82 in this case indicates that two acetonitrile molecules are present in the first coordination
204
AHMED
sphere of the cation and supports the suggestion (20, 31) that the cation is present as trans-(CH3CN)$bCl’. The slight reduction in the solvation number from the expected value of 1 indicates contact ion-pair formation to produce species of decreased solvation numbers such as Sb2Cl,,, or (CH3CN)Cl,Sb-ClSbCl,. The rate of solvent exchange in solutions of antimony (V) chloride exceeds those of boron (III) and gallium (III) chlorides, and its activation enthalpy and entropy are also larger. Thus in the three chlorides the activation parameters decrease in the order SbCl, > BCl, > GaCl,. This order is parallel to the order of decreasing acidity of these Lewis acids and cannot be attributed to the stability of their acetonitrile complexes. Arguments based on cation charge-to-size ratio cannot be used to account for this order because of lack of information on these cations. Rather, this trend must have its origin, as discussed earlier, in second coordination sphere interactions. Comparison with the aluminum (III) chloride parameters is not meaningful because of the different structure that the solvated cation has in this system. The apparent solvation number of 1.5 can be explained on the basis of the equation 4AlClj + 6CHjCN
+ Al(CH&N);+
+ 3AlCl;,
[61
where ion aggregates (e.g., (CH3CN),Al, AlCl:+ etc.) exist in solution. Raman spectral measurements (32) of concentrated solutions of AlC13 in acetonitrile showed that AlCl; ion is present in high concentrations. The low conductance and irregular rl t’s. C112 plots of AlCl, solutions in acetonitrile (20) require that a variety of ion aggregates be present in equilibrium. The solvation number results are in excellent agreement with recent proton and 27Al NMR studies (I& 33, 34). Aluminum (III) chloride is unique among the group (III) halides studied in that no halide ion is associated with the cation and a coordination number of six is thermodynamically preferred. This is also the result observed in other nonaqueous solvents (12, 27, 35). The fact that similar chemical shifts are observed in solutions of different halides despite variations in the stoichiometries of solvated cations and the effects of second coordination sphere interactions is quite surprising. If the interaction of acetonitrile with various metal halides is simple Lewis acid-base interaction resulting in polarization of electron density towards the acid and the consequent deshielding of the protons, one would expect variation of the chemical shift as a function of the acid strength of the metal halides, Even if the electrostatic interaction is the dominant factor in complex ion formation, variations of the shift as a function of ion charge-to-size ratio are expected. The electron density redistribution in CHJN upon complexation is not easily predicted from simple considerations, as evidenced by infrared (36, 37) and NMR (10, 38) measurements of acetonitrile complexes. There is probably more than one factor contributing to the shifts observed by both techniques. In the metal halide solutions, the chemical shift of coordinated acetonitrile is influenced not only by the electric field of the cation, but also by the effect of the cation on the anisotropy of the C-N n system of acetonitrile. These effects are difficult to separate and are not necessarily related to the strength of the coordinate bond. Gallium (HZ) Perch/orate Solutions. Examination of the NMR spectral results (Table 1) for gallium (III) perchlorate solutions in acetonitrile reveals three observations : (1) The presence of two separate signals for coordinated solvent; (2) Reduction in the solvation number from the expected value of 6 for Ga (III) ion found in other
SOLVATION
OF METAL
HALIDES
IN
CHJN
205
solvents (5, 6, 11); (3) The abnormally large chemical shift of coordinated acetonitrile downfield from the bulk solvent resonance. The appearance of two separate signals for coordinated acetonitrile in the spectrum of anhydrous gallium (III) perchlorate solution seemed surprising. Solutions of Ga(ClO,), prepared by the addition of slight excess of anhydrous AgClO, to gallium (III) bromide solutions in acetonitrile also show the two signals although broadened and not as well defined, probably due to the presence of Ag+ and/or Rrions in solution. (When insufficient amount of AgClO, is added, a signal for acetonitrile coordinated to gallium (III) bromide is observed around 40 Hz in addition to two broad signals around 370 Hz downfield from the bulk solvent signal for acetonitrile coordinated to Ga3’). When a very small amount of water (or D,O) is added to the solution of anhydrous Ga(ClO,), in acetonitrile, the coordinated acetonitrile signals completely disappear. These observations rule out the possibility that the signals may be due to water impurities in the solutions. The presence of two signals for coordinated acetonitrile must be due to two kinds of solvated Ga (III) species in equilibrium. It has been reported (14, 18) that spectra of aluminum (III) perchlorate solution in acetonitrile contain several signals for coordinated solvent; these were attributed to different kinds of coordinated acetonitrile. Solvation numbers of 2.8 (24) and 2.9 (18) were calculated for Al (III) in these solutions. The solvation numbers for Ga (III) in acetonitrile found in this study are, on the average, close to these values. Only at the lowest concentration studied a solvation number of about 5 was obtained. The low solvation number can be attributed to the participation of the perchlorate ion in the first coordination sphere of the complexed species. The presence of only two well-defined signals with their positions invariant to temperature, and the variation of their relative intensities with temperature indicate an equilibrium between two solvated Ga (III) species. Because Ga (III) shows the tetrahedral-octahedral configuration transformation (39) the two solvated Ga (III) species in acetonitrile can be tetrahedral Ga(CH3CN)j+ and octahedral Ga(CH,CN)i’ or their derivatives Ga(CH,CN),-,(ClO,),“-” and Ga(CH,where ?I and m have constant values. It has been shown that thermoCN)6-,,(C10,)3;-’ dynamic parameters for such an equilibrium can be calculated from NMR data (I I)In this case, however, the inexact solvation numbers precluded a meaningful calculation. The chemical shift of coordinated acetonitrile in the gallium (III) perchlorate solution is exceptionally large compared to the shifts observed in gallium (III) halide solutions. The only plausible explanation for this difference is the presence of acetonitrile molecules in the field of a tripositive ion in the case of Ga(C10,)3 solutions as compared to the monopositive cations in case of the halides. The large negative entropies of activation calculated for the solvent exchange from both species can be due to the effect of ion pairing on the orderly orientation of the exchanging solvent molecules. However, these results are not inconsistent with the proposition that activated complex has a higher coordination number than the solvated cation and the solvent exchange proceeds via an SN2 mechanism (5).
ACKNOWLEDGMENT
Support of this work by the University of Mississippi through a Faculty Research Grant is gratefully acknowledged.
206
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KUSTIN AND J. SWINEHART, Prog. Inorg. Chem. 13, 107 (1970). BURGESS AND M. C. R. SYIUONS, Quart. Revs. 22,276 (1968). F. HINTON AND E. S. AMIS, Chem. Revs. 67,367 (1967). R. STENGLE AND C. H. LANGFORD, Coord. Chem. Rev. 2,349 (1967). FIAT AND R. E. CONNICK, J. Amer. Chem. Sot. 90,608 (1968). N. FIAT AND R. E. CONNICK, J. Amer. Chem. Sot. S&4754 (1966). J. SWIFT AND W. G. SAYRE, J. Chem. Phys. 44,3567 (1966). NAKAMURA AND S. MEIBOOM, J. Amer. Chem. Sot. 89,1765 (1967). A. MATWIYOFF, Znorg. Chem. 5,788 (1966). A. MATWIYOFF AND S. V. HOOKER, Znorg. Chem. 6,1127 (1967). G. MOVIUS AND N. A. MATWIYOFF, Znorg. Chem. 8,925 (1969). G. MOVIUS AND N. A. MATWIYOFF, J. Phys. Chem. 72,3063 (1968). A. MATWIYOFF AND W. G. MOVIUS, J. Amer. Chem. Sot. 89,6077 (1967). SUPRAN AND N. SHEPPARD, Chem. Commun. 832 (1967). FRATIELLO, R. E. LEE, AND R. E. SCHUSTER, Chem. Commun. 37 (1969). FRATIELLO, R. E. LEE, V. M. NISHIDA, AND R. E. SCHUSTER, J. Chem. Phys. 48, 3705 (1968); 50,3624 (1969). A. FRATIELLO, R. E. LEE, AND R. E. SCHUSTER, Znorg. Chem. 9,82 (1970). J. F. O’BRIEN AND M. ALEI, JR., J. Phys. Chem. 74,743 (1970). W. G. MOVIUS AND N. A. MATWIYOFF, J. Phys. Chem. 90,5452 (1968). C. D. SCHMULBACH AND I. Y. AHMED, J. Chem. Sot. A 3008 (1968). I. Y. AHMED AND C. D. SCHMULBACH, Znorg. Chem. 8,141l (1969). C. D. SCHMULBACH AND I. Y. AHMED, Znorg. Chem. 8,1414 (1969). C. D. SCHMULBACH AND I. Y. AHMED, Znorg. Chem. 10, 1902 (1971). L. H. P~E-~TE AND W. A. ANDERSON, J. Chem. Phys. 30,899 (1959). H. M. MCCONNELL, J. Chem. Phys. 28,430 (1958). S. C. WALLWORK AND I. J. WORALL, J. Chem. Sot. 1816 (1965) and references therein. A. FRATIELLO, D. P. MILLER, AND R. E. SCHUSTER, J. Phys. Chem. 71,1948 (1967). M. EIGEN AND R. G. WILKINS, “Mechanisms of Inorganic Reactions,” pp. 55-56, American Chemical Society, Washington, DC, 1965. M. EIGEN AND L. DEMAEYER, “Techniques of Organic Chemistry” (A. Weissberger, Ed.), Vol. III, Part II, Chapter 18, Interscience, New York, 1961. G. ATKINSON AND S. K. KORR, J. Phys. Chem. 60,128 (1965). I. R. BEATTIE AND M. WEBSTER, J. Chem. Sot. 38 (1963). C. D. SCHMULBACH, J. Znorg. Nucl. Chem. 26,745 (1964). J. F. HON, Mol. Phys. 15,57 (1968). H. HARAGUCHI AND S. FUJIWARA, J. Phys. Chem. 73,3467 (1969). S. THOMAS AND W. L. REYNOLDS, J. Chem. Phys. 44,3148 (1966). I. Y. AHMED, Ph.D. dissertation, Southern Illinois University, 1968. K. F. PURCELL AND R. S. DRAGO, J. Amer. Chem. Sot. 88,919 (1966). C. C. HINCKLEY, Znorg. Chem. 7,396 (1968). L. I. KATZIN, “Transition Metal Chemistry” (R. L. Carlin, Ed.), Vol. 3, p. 57, Marcel Dekker, New York, 1966.
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OF MAGNETIC
RESONANCE
7, 196-206 (1972)
Proton Magnetic ResonanceStudies of the Solvation of RepresentativeHalides and of Gallium (III) Perchloratein Acetonitile I. Y. AHMED Department of Chemistry, University of Mississippi, University, Mississippi 38677 Presented at the Fourth International Symposium on Magnetic Resonance, Israel, August, 1971 At low temperatures the proton magnetic resonance signals of coordinated acetonitrile in the solutions of some group (III) and group (V) representative element halides and of Ga+3 ion can be distinguished from those of bulk acetonitrile. From the relative intensities of these signals the solvation numbers of the metal halides and Ga+3 ion were determined. Reduction in the solvation number of some complexes from the expected value was observed and is explained in terms of contact ion-pair formation. The spectrum of Ga+3 ion solution contains two distinctive signals for coordinated acetonitrile. The relative intensities of these signals vary with temperature but are independent of concentration. This is explained in terms of an equilibrium between two kinetically distinguishable solvated Ga+3 species. From the temperature dependence of the proton relaxation rates the solvent exchange parameters k,, dH*, and dS* were calculated for various systems. Variations in the kinetic parameters among the different complexes are discussed in terms of the structure of the complex and the effect of anion participation in the coordination sphere of the complex. INTRODUCTION
The application of nuclear magnetic resonance spectroscopy to the study of solutesolvent interactions both in aqueous and nonaqueous solvents has been the subject of several recent reviews (1-4). The NMR technique can provide detailed information about solvation numbers, structure, and lability of kinetically distinguishable solute species (5-11). The mechanism by which the solvent exchange process occurs is inferred indirectly from the activation parameters which can provide information on the structure of the activated complex (5). The contribution to these parameters from second coordination sphere interactions should, however, be considered. Evidence for the formation of ion pairs, especially in solutions of low dielectric constant, has been accumulating (12-18) and its effect on the kinetic parameters of exchange has been reported (12, 13, 26). In most of the systems investigated it was only necessary to invoke a single cation-solvent complex to account for the NMR data. In a few cases, more than one complex was assumed to exist in equilibrium, especially in mixed solvents,
in order
to account
for the observed
solvation
numbers
(14-18).
No direct
observation of more than one well-defined solvated complex in solution has been reported, although Matwiyoff and Movius (11, 13, 19) have been able to characterize mixed complexes in NJ-dimethylformamide solutions of a mixture of some metal ro 1972 by Academic
Press, Inc.
196
SOLVATION
OF METAL
HALIDES
IN
CH&N
197
ions and their acetylacetonato complexes. The two mixed complexes characterized in each case have the same coordination number. The results of infrared and conductance measurements of solutions of some representative metal halides in acetonitrile (20-23) indicated that the halides interact strongly with acetonitrile and dissociate to a greater or smaller extent to give cations with an unspecified number of solvent molecules in the first coordination sphere. The possibility that gallium and aluminum cations may possess four or six coordination or that chemical equilibrium exists between four- and six-coordinate cationic species prompted this investigation. EXPERIMENTAL
Fisher reagent grade acetonitrile was purified by the method described previously (20). Gallium (III) chloride and bromide (Alfa), aluminum (III) chloride (Baker), and phosphorus (V) chloride (Baker) were sublimed under vacuum prior to use. Boron (III) chloride (Matheson) and antimony (V) chloride (Baker) were purified by fractional distillation under vacuum. Anhydrous gallium (III) perchlorate was obtained by heating the hexahydrated salt (Alfa) at 60°C under vacuum for several days. Alternatively, solutions of the anhydrous salt were prepared by the addition of slight excess of anhydrous AgCIO, solution to a solution of gallium (III) bromide in acetonitrile, followed by the separation of solid AgBr. All solutions were prepared by weight and transferred in a glove bag under an anhydrous nitrogen atmosphere to NMR tubes, which had been baked at 120”. Proton spectra were obtained at 60 MHz with a Varian A-60 spectrometer equipped with a variable-temperature probe and a variable-temperature control unit. The temperature was monitored by measuring the peak separation of methanol. The areas under the peaks were obtained by integration of the signals with the electronic integrator. RESULTS
Figure 1 shows a representative NMR spectrum of aluminum (III) chloride solution in acetonitrile at -36°C. The spectra of GaCI,, GaBr,, BC13, and SbCl, are similar in appearance. The intensity of the peak attributed to coordinated acetonitrile is directly proportional to the concentration of the halide. At -40°C the position of this peak in the spectra of the various halide systems investigated is 40 3: 5 Hz downfield from
FIG. 1. The ‘H spectrum of aluminum (III) chloride solution in acetonitrile at -36°C. A,, A,, SSB, and ‘% are designations for free acetonitrile, coordinated acetonitrile, spinning side band, and the free solvent band resulting from ‘W-splitting, respectively. 8
198
AHMED
the bulk acetonitrile peak and shows no apparent dependence on concentration. No coordinated solvent peak is observed in the spectra of PCI, solutions over a wide range of concentrations and temperatures. The separate acetonitrile peaks in solutions of aluminum and gallium halides are distinguishable even at room temperature although the coordinated solvent peak is broad due to solvent exchange. In BC13 and SbCI, solutions, the coordinated solvent peaks broaden at much lower temperatures and completely disappear at approximately 0 and -20°C respectively.
I /AA --?4
1,
-30 364 Hz j! .---.-.-...-40
-..-.-.
P . . .. /
---
___-.-
---
-’ iv
-
i AC
%
FIG. 2. The ‘H spectra of 0.324 no solution of Ga(ClO.& in acetonitrile. A, and A, represent the signals due to free acetonitrile and coordinated acetonitrile, respectively. The free acetonitrile signal is recorded at lower rf level. Temperatures in “C are indicated at the left of the curves.
Figure 2 shows the NMR spectra of anhydrous gallium (III) perchlorate solution in acetonitrile. The peaks attributed to coordinated acetonitrile are reproduced at different temperatures. The positions of these peaks are about 365 and 380 Hz downfield from the bulk solvent peak and vary slightly with temperature and concentration without any apparent trend. The relative intensities of the coordinated solvent peaks are in the ratio of 2: 5 and vary slightly with temperature. The separation between the two coordinated solvent peaks is constant (15 Hz) at temperatures below zero and decreases to 10 Hz at 16°C when the two peaks are discernable. At temperatures higher
SOLVATION OF METAL HALIDES IN CH$N
i99
than 16” the two signals start to coalesce. The results of chemical shifts, relative intensities and solvation number of Ga(ClO,), solutions in acetonitrile at various temperatures and concentrations are summarized in Table 1. When very small amounts of water or DzO were added to these solutions, the coordinated solvent peaks completely disappeared. TABLE PROTON
CHEMICAL
SHIFTS,
RELATIVE NUMBER
INTENSITIES OF Ga (III)
1
OF COORDINATED ACETONITRILE ION IN ACETONITRILE
AND
SOLVATION
Chemical shift” (Hz) Molality Cm) 0.108
Temperature CC) -44 ~-20
0.114 0.221
-40
6Cl
6 c*
Relative intensity Ac,IAcz
Solvation number (n)
365 367
380 381
2.5 2.3
5.2 4.9
374
389
3.0
2.2
376
391
2.5
2.5
0.228
-40 -25
377 371
392 386
2.2 2.2
2.4 2.3
0.324
-40 -24
364 366
379 380
2.3 2.1
2.9
’ c, and c2 refer to the high-field and low-field signals, respectively, of coordinated the shift is downfield from bulk acetonitrile.
-
acetonitrile, and
The solvation number was determined from the known solution composition and the relative areas under the signals of coordinated and bulk acetonitrile in the spectrum. Because the amplitude of the signals differs appreciably for some solutions, it was not always possible to record both signals at the same recorder input level and obtain an accurate area for both. Instead, the area of one of the peaks resulting from 13C-splitting, measured at the same signal amplitude as that of coordinated acetonitrile, was used to calculate the relative area of bulk acetonitrile. The calculated solvation numbers at the respective temperature and solute concentration are summarized in Table 2 for the metal halides, and in Table 1 for gallium (III) perchlorate. The temperature dependence of the line widths of coordinated acetonitrile signal in the solutions of gallium (III) chloride and gallium (III) perchlorate are summarized in Fig. 3 in which log (rrdv) is plotted as a function of reciprocal temperature. Since the chemical shifts are independent of concentration and temperature in the range studied, the temperature dependence of the line width should conform to the equation (24,25)
PI l/T, = (l/T;) + (l/T), which relates the observed transverse relaxation rate l/T, to the mean lifetime of acetonitrile in the complex T and the transverse relaxation rate in absence of exchange
200
AHMED
TABLE SALVATION
NUMBERS
Molality Metal
halide
AK&
(4 0.6343
number 02) ~~ .---~
Average (n) 1.50 + 0.03
-40 -40 -40 -40 -40
0.96 1.00 1.08 1.11 I .07
0.5355 0.6022
-40 -40 -27 -42 -35 -28 -40 -44 -40 -25
0.61 0.55 0.61 0.51 0.51 0.54 0.51 0.58 0.65 0.59
0.2521 0.2802 0.3184 0.3295 0.4220 0.5438 0.8665 0.9847
-40 -40 -31 -40 -46 -34 --40 -40 -17
0.94 1.00 1.04 1.06 0.96 1.04 1.02 1.03 1.10
1.02 It 0.01
0.3342 1.062 1.163 1.438 1.772
-40 -40 -40 -40 -40
0.79 0.82 0.86 0.79 0.83
0.82 + 0.01
0.7130 1.072 1.079
SbCI,
Solvation
0.6115 1.029 1.064 1.288 1.843
0.6170
GaBr,
IN ACETONITRILE
1.40 1 so 1.60 1.64 1.49 1.45 I .43
0.956
GaCI,
Temperature (T)
HALIDES
-40 -17 -50 -40 -25 -40 26
0.8464
BC13
OF METAL
2
1.04 j: 0.02
0.57 i 0.01
I/T!. The observed transverse relaxation rate is related to the full width of the NMR line AV by l/T2 = rrdv. PI Equation [l] becomes nAv = nAv” + k,, [31
SOLVATION
OF METAL
HALIDES
IN
CH,CN
201
100
40
30 -7 8 i!.! -P I
10 9 8 7 6 5 4
3
3.0
I
I
I
3.2
3.4
3-6
I
I
loo0
T
-'
I 4.2
1 4'*
K
FIG. 3. The temperature dependence of the line width of coordinated acetonitrile in the solutions of GaC& and Ga(ClO&. The lower curve is for GaC&. The upper curves are for Ga(ClO&, with the open circles indicating the high-field signal and the solid circles representing the low-field signal.
where ki is the pseudo-first-order rate constant for the exchange of acetonitrile between the free solvent and the compiex. The temperature dependence of l/r and l/T; can be represented by the equations l/~ = k, = (kT/h)exp [(-dHt/RT) 1/T;
a exp (-Eu/RT),
+ (LlSt/R)],
[41 [51
where d/D and dSt represent the enthalpy and entropy of activation for the solvent exchange reaction, and E, can be viewed as an apparent Arrhenius activation energy associated with the transverse relaxation of a proton in the complex (11). The line width data for gallium (III) chloride and gallium (III) perchlorate in Fig. 3 can be fitted to two straight lines: (a) the ones at low temperatures, with small positive
202
AHMED
slopes, can be associated with the temperature dependence of the relaxation rate l/T: of the proton in the first coordination sphere of the complex; (b) the ones at high temperatures, with large negative slopes, are associated with line broadening due to solvent exchange. In the case of boron (TII) chloride and antimony (V) chloride solutions, solvent exchange is significant even in the low temperature region and it was not possible to go to temperatures low enough to determine the temperature dependence of l/T; for these systems since the solutions freeze below the temperatures recorded here. Only straight lines with very large negative slopes, which describe the variation of l/~ with temperature, were obtained. However, there is a curvature in the line fitting the data of BC13 solutions below -40” but not enough to construct a line describing the variation of l/T!: with temperature. Chemical exchange parameters calculated from the data are summarized in Table 3. In solutions of gallium (III) bromide and aluminum (III) chloride, insufficient line broadening, in the concentration range TABLE KINETIC
PARAMETERS
FOR ACETONITRRILE
EXCHANGE
SbCl,
k.25. (see-‘) AH * (kcal/mole) AS% (eu) E. (kcal) n From Ref. * First entry
3 IN SOLUTIONS
BCI,
3.5 x lo3 16.3
5.6 x 10’ 13.9
13
1
AICI,”
OF METAL
high-field
signal
and the second
AND Ga (III)
GaCI,
8.1
9.6
57
9.4
12.0
9.7
-22
is for low-field
signal
ION
Ga(ClO&
19.1
-0.2 (18). is for
HALIDES
81 11.5
-10
-11
-1.3
of coordinated
*
-0.3
acetonitrile.
studied, made it difficult to obtain accurate kinetic data. By comparison with the gallium (III) chloride data, the pseudo-first-order constant for solvent exchange in gallium (III) bromide solutions at 25°C is estimated to be less than 5 set-‘. A value of 8.1 set-’ had been reported for k, of AIC13 solution in acetonitrile at 25°C (18). DISCUSSION
Metal Halide Solutions. An apparent solvation number of 1 for boron (III)
and gallium (III) halides is consistent with the presence of four-coordinate species in solution either as molecular adducts CH,CN*MXj or ionic species (CH,CN),MXz, MX,. Although the boron trichloride-acetonitrile adduct is molecular in the solid state (20, 22), its solutions in acetonitrile have been shown to contain appreciable concentrations of BCl; (20). In view of the low conductance of these solutions, the ions must be present predominantly as ion pairs. There is no direct evidence for the existence of the molecular adduct in equilibrium with the ionic species in solution such as the appearance of two bands for coordinated acetonitrile in the spectra due to CH3CN*BC13 and (CH,CN),BClf. However, the presence of two solvated boron species in solution would produce a single resonance band if either the actual chemical
SOLVATION
OF METAL
HALIDES
IN
CH,CN
203
shifts of the two species are experimentally indistinguishable or rapid ligand exchange between the species produces an average chemical shift. On the other hand, the gallium trihalide-acetonitrile complexes have the ionic structure (CH,CN),GaX:, GaX; in the solid state (23). Infrared and conductance measurements established the stability of GaX; ion in solution (25). The solvation number of 1 for the strongly conducting gallium (III) bromide can be accounted for only if the species present are (CH,CN),GaBr: and GaBr;, and the results firmly establish the preference of gallium for four coordination. The low solvation number of 0.57 observed for gallium (III) chloride can occur if significant concentration of four-coordinate, chlorine-bridged gallium species, such as Ga,Cl, or (CH,CN)C&GaClGaCl,, are present. These species are produced if contact ion pairing occurs, whereby acetonitrile molecules in the first coordination sphere of the cation are completely or partially replaced by the anion. Similar results were obtained for gallium (III) chloride solutions in aqueous-acetone mixtures (1.5, 17). It was observed (17) that ion-pairing ability of the anions in gallium (III) halide solutions decreases in the order Cl- > Br- z I -. The greater strength of the Ga-Cl bond in the dimeric gallium halides (26) must be responsible for this variation in solvation numbers of the two halides. The faster rate of exchange of coordinated acetonitrile in gallium (III) chloride solutions, compared to gallium (III) bromide, must be a consequence of ion-pair formation. The rate of solvent exchange is enhanced by the presence of the anion in the close vicinity of the solvated cation. Similar observations have been reported for solutions of AIXj in N,N-dimethylformamide, where the rates of DMF exchange increase in the order Cl > Br > I (27). It has also been noted that the *‘Al line width in DMF solutions of Al(ClO& increases with the addition of halide ions to these solutions (Z2, 13). These observations are significant in view of the SN’ mechanism proposed for solvent exchange in solutions of metal ions (28). In addition, ultrasonic studies (29, 30) of the kinetics of ion association in water have been interpreted in terms of multistep mechanisms in which the rate-determining last step is the formation of a close ion pair. The rate of this process was found to be independent of the incoming anions and is determined solely by the rate of elimination of a solvent molecule from the first coordination sphere. Although such data are not available for acetonitrile solutions, it is evident that variations in the anion have appreciable effect on the rates of solvent exchange. The effect of ion pairing on the rate of solvent exchange is more apparent in solutions of boron (III) chloride. The rate of exchange of acetonitrile coordinated to the boron solute species exceeds that of gallium by about two orders of magnitude. Tn this case, however, the ion pair must be solvent-separated because a constant solvation number of 1 was obtained over a wide range of concentration and temperature, and the solution exhibits very low conductance (20). Further evidence for ion pairs in the solutions of boron and gallium chlorides is suggested by the small or negative values of dS$, which can result if the rate-determining step involves exchange of highly oriented solvent molecules between the first and second coordination spheres of the cation. Steric crowding of four ligands around boron could also contribute to the lability of the solvated cation. Reduction in solvation number and faster rate of solvent exchange are also observed for the antimony (V) chloride solutions. The apparent solvation number of 0.82 in this case indicates that two acetonitrile molecules are present in the first coordination
204
AHMED
sphere of the cation and supports the suggestion (20, 31) that the cation is present as trans-(CH3CN)$bCl’. The slight reduction in the solvation number from the expected value of 1 indicates contact ion-pair formation to produce species of decreased solvation numbers such as Sb2Cl,,, or (CH3CN)Cl,Sb-ClSbCl,. The rate of solvent exchange in solutions of antimony (V) chloride exceeds those of boron (III) and gallium (III) chlorides, and its activation enthalpy and entropy are also larger. Thus in the three chlorides the activation parameters decrease in the order SbCl, > BCl, > GaCl,. This order is parallel to the order of decreasing acidity of these Lewis acids and cannot be attributed to the stability of their acetonitrile complexes. Arguments based on cation charge-to-size ratio cannot be used to account for this order because of lack of information on these cations. Rather, this trend must have its origin, as discussed earlier, in second coordination sphere interactions. Comparison with the aluminum (III) chloride parameters is not meaningful because of the different structure that the solvated cation has in this system. The apparent solvation number of 1.5 can be explained on the basis of the equation 4AlClj + 6CHjCN
+ Al(CH&N);+
+ 3AlCl;,
[61
where ion aggregates (e.g., (CH3CN),Al, AlCl:+ etc.) exist in solution. Raman spectral measurements (32) of concentrated solutions of AlC13 in acetonitrile showed that AlCl; ion is present in high concentrations. The low conductance and irregular rl t’s. C112 plots of AlCl, solutions in acetonitrile (20) require that a variety of ion aggregates be present in equilibrium. The solvation number results are in excellent agreement with recent proton and 27Al NMR studies (I& 33, 34). Aluminum (III) chloride is unique among the group (III) halides studied in that no halide ion is associated with the cation and a coordination number of six is thermodynamically preferred. This is also the result observed in other nonaqueous solvents (12, 27, 35). The fact that similar chemical shifts are observed in solutions of different halides despite variations in the stoichiometries of solvated cations and the effects of second coordination sphere interactions is quite surprising. If the interaction of acetonitrile with various metal halides is simple Lewis acid-base interaction resulting in polarization of electron density towards the acid and the consequent deshielding of the protons, one would expect variation of the chemical shift as a function of the acid strength of the metal halides, Even if the electrostatic interaction is the dominant factor in complex ion formation, variations of the shift as a function of ion charge-to-size ratio are expected. The electron density redistribution in CHJN upon complexation is not easily predicted from simple considerations, as evidenced by infrared (36, 37) and NMR (10, 38) measurements of acetonitrile complexes. There is probably more than one factor contributing to the shifts observed by both techniques. In the metal halide solutions, the chemical shift of coordinated acetonitrile is influenced not only by the electric field of the cation, but also by the effect of the cation on the anisotropy of the C-N n system of acetonitrile. These effects are difficult to separate and are not necessarily related to the strength of the coordinate bond. Gallium (HZ) Perch/orate Solutions. Examination of the NMR spectral results (Table 1) for gallium (III) perchlorate solutions in acetonitrile reveals three observations : (1) The presence of two separate signals for coordinated solvent; (2) Reduction in the solvation number from the expected value of 6 for Ga (III) ion found in other
SOLVATION
OF METAL
HALIDES
IN
CHJN
205
solvents (5, 6, 11); (3) The abnormally large chemical shift of coordinated acetonitrile downfield from the bulk solvent resonance. The appearance of two separate signals for coordinated acetonitrile in the spectrum of anhydrous gallium (III) perchlorate solution seemed surprising. Solutions of Ga(ClO,), prepared by the addition of slight excess of anhydrous AgClO, to gallium (III) bromide solutions in acetonitrile also show the two signals although broadened and not as well defined, probably due to the presence of Ag+ and/or Rrions in solution. (When insufficient amount of AgClO, is added, a signal for acetonitrile coordinated to gallium (III) bromide is observed around 40 Hz in addition to two broad signals around 370 Hz downfield from the bulk solvent signal for acetonitrile coordinated to Ga3’). When a very small amount of water (or D,O) is added to the solution of anhydrous Ga(ClO,), in acetonitrile, the coordinated acetonitrile signals completely disappear. These observations rule out the possibility that the signals may be due to water impurities in the solutions. The presence of two signals for coordinated acetonitrile must be due to two kinds of solvated Ga (III) species in equilibrium. It has been reported (14, 18) that spectra of aluminum (III) perchlorate solution in acetonitrile contain several signals for coordinated solvent; these were attributed to different kinds of coordinated acetonitrile. Solvation numbers of 2.8 (24) and 2.9 (18) were calculated for Al (III) in these solutions. The solvation numbers for Ga (III) in acetonitrile found in this study are, on the average, close to these values. Only at the lowest concentration studied a solvation number of about 5 was obtained. The low solvation number can be attributed to the participation of the perchlorate ion in the first coordination sphere of the complexed species. The presence of only two well-defined signals with their positions invariant to temperature, and the variation of their relative intensities with temperature indicate an equilibrium between two solvated Ga (III) species. Because Ga (III) shows the tetrahedral-octahedral configuration transformation (39) the two solvated Ga (III) species in acetonitrile can be tetrahedral Ga(CH3CN)j+ and octahedral Ga(CH,CN)i’ or their derivatives Ga(CH,CN),-,(ClO,),“-” and Ga(CH,where ?I and m have constant values. It has been shown that thermoCN)6-,,(C10,)3;-’ dynamic parameters for such an equilibrium can be calculated from NMR data (I I)In this case, however, the inexact solvation numbers precluded a meaningful calculation. The chemical shift of coordinated acetonitrile in the gallium (III) perchlorate solution is exceptionally large compared to the shifts observed in gallium (III) halide solutions. The only plausible explanation for this difference is the presence of acetonitrile molecules in the field of a tripositive ion in the case of Ga(C10,)3 solutions as compared to the monopositive cations in case of the halides. The large negative entropies of activation calculated for the solvent exchange from both species can be due to the effect of ion pairing on the orderly orientation of the exchanging solvent molecules. However, these results are not inconsistent with the proposition that activated complex has a higher coordination number than the solvated cation and the solvent exchange proceeds via an SN2 mechanism (5).
ACKNOWLEDGMENT
Support of this work by the University of Mississippi through a Faculty Research Grant is gratefully acknowledged.
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