2075
COORDINATION DISPROPORTIONATION EQUILIBRIA IN SOLUTION
Coordination Disproportionation Equilibria in Solution. 11. Cobaltous Chloride and Zinc Chloride in Acetonitrile by W. Lib&, D. Puchalska, and T. Szuchnicka Department of Physical Chemistry of the Technical University of Gdahsk, Gdahsk, Poland Accepted and Transmitted by The Faraday Society
(October $4, 1967)
A new method of evaluating conductometric data in terms of coordination equilibria applicable to nonaqueous solvents is proposed. It is based on the assumption that the mobilities and the degree of outersphere association of complex ions of a given structure are independent of the central metal atom. An application of the method to the study of CoClzand ZnClz solutions in acetonitrile is demonstrated. Visible absorption spectra of mixed cobaltous chloride-tetraethylammonium chloride and cobaltous chloride-zinc chloride solutions have been measured and are used in the evaluation of conductometric data. The coordination disproportionation equilibrium 3[ZnClzLz] 2L [ZnL6]2+ 2[ZnCl&]tetrahedral
+ e
oatahedral
+
tetrahedral
(L = acetonitrile), analogous to that found previously for CoC12, is shown to occur in solutions of ZnClz. Disproportionation equilibrium constants both for CoClz and ZnClz are determined.
I n an earlier investigation,' the coordination disproportionation equilibrium 3[COCl2L2]
+ 2L
[COLS]2+
+ 2[CoC13L]-
(1)
(L being the solvent molecule) has been established spectrophotometrically in cobaltous chloride solutions in acetonitrile. However, it was found difficult to obtain a reliable value of the equilibrium constant of that reaction by means of the spectrophotometric measurements alone.2 I n search of a method allowing us to obtain more accurate results, we have now developed a probably new method of evaluating conductometric data in terms of coordination equilibria which, when combined with spectrophotometric measurements, allows the equilibrium constant of reaction l to be determined with considerably higher precision. Furthermore, it appears possible, by applying this method, to obtain definite conclusions regarding the nature of the equilibrium occurring in zinc chloride solutions in acetonitrile and to determine the corresponding equilibrium constant. It seems that the method developed for these special cases may prove to be of a more general applicability. The conductometric method here proposed is based on the assumption that mobilities of complex ions of a given structure as well as their degree of association (outersphere, in the sense defined by Taube3) with o p positely charged ions are independent of (or only very slightly dependent on) the nature of the central metal atom and are determined by the effective ionic composition of the solution. Two solutions are considered to have effectively identical ionic compositions if they have equal total concentrations of complex ions of a
given structure in addition to having equal concentrations of simple electrolytes which also may be present. It follows from the above assumption that the total concentrations of complex ions of a given structure in two solutions having equal .conductivities should be equal if the concentrations of any other structurally defined ions are equal. Surprisingly few experimental data which might be used to verify directly the assumptions underlying our method are to be found in the literature. From the recent work reported by Johari and T e ~ a r iit, ~follows that the following complexes in formamide solutions have (in pairs) nearly identical limiting equivalent conductances: K~[CO(CN)G]and K3[Fe(CN)a], La[Co(CN)e] and La[Fe(CN)a], [ C ~ ( e n )[Fe(CN)s] ~] and [ C ~ ( e n ) [Co(CN)6]. ~] Thus, substitution of Fea+ for Co3+has no effect on the limiting mobilities of complex ions in these cases. It is more difficult to find reliable information of this kind concerning finite concentrations. We shall refrain from any further analysis of literature data as the validity of the method proposed here may readily be verified in each particular case.
Experimental Section Materials. I n preparing anhydrous cobaltous chloride, the procedure described by Janz, et al.,5 has been (1) W. Lib& Rocz. Chem., 36, 999 (1962); Proceedings of the 7th ICCC,Stockholm and Uppsala, Sweden, June 25-29, 1962. (2) W. Lib& and M. Walczak, J . Inorg. Nucl. Chem., in press. (3) H. Taube and F. Possey, J . Am. Chem. Soc., 7 5 , 1463 (1953). (4) G. P. Johari and P. H. Tewari, J . Phys. Chem., 69, 2862 (1965). (5) G. J. Janz, A. E. Marcinkowski, and H. V. Venkatasetty,
Electrochim. Acta, 8 , 867 (1963). Volume 7.2, Number 6 June 1068
W. LIBUS,D. PUCHALSKA, AND T. SZUCHNICKA
2076 closely followed. The product obtained in this way dissolved readily in acetonitrile to give neutral and perfectly clear solutions. These were analyzed for cobalt by EDTA titrations. Anhydrous solutions of zinc chloride in acetonitrile were prepared from the solid complex ZnC12L2 (L = acetonitrile). The latter crystallized readily when a concentrated solution of commercial anhydrous zinc chloride in acetonitrile was allowed to stand for several hours at 0". It was recrystallized twice from and finally dissolved in anhydrous acetonitrile to give clear stock solution. The solid complex and the stock solution were analyzed for zinc by EDTA titration. Anhydrous solutions of cobaltous perchlorate in acetonitrile were obtained from a solid complex of the salt with acetonitrile. This complex, shown by analysis to be C O ( C ~ O ~ ) ~was . ~ Lpre, pared by the method recently developed by Hathaway.* It consisted of the reaction of the metal with a suspension of nitrosyl perchlorate in acetonitrile. The product was recrystallized twice and analyzed for cobalt by EDTA titration. Purification of acetonitrile was as described before.' The physical constants of the final material were: bp 81.6" (760 mm), specific conductance 3 X lo-' ohm-' cm-', at 25". Water content of the solvent, determined by Karl Fischer titration, was less than O.Ol$& Procedures. Preparations of solutions and further manipulations were carried out in a drybox. Conductivities were measured at a constant temperature of 25 & 0.01" using the Jones and Josephs type of conductance bridge, as modified by Luder.* Spectrophotometric measurements were carried out at 25.0 f 0.1" by means of a Unicam SP500 spectrophotometer.
Results and Discussion The visible absorption spectrum of a dilute solution of cobaltous chloride in acetonitrile consists of a broad, intense band which, on account of its intensity and spectral range (550-720 mp), is ascribed to the transition 4Aa(F) + 4T1(P) in tetrahedral cobalt(I1) complexes. It has been shown earlier' that of the four absorption maxima observed within this band the ones at 571 and 614 mp belong to theneutralcomplex [CoCl2LZ],while the maximum at 590 mp is characteristic of [CoC13L]-. The highest maximum, observed in the spectrum under consideration at approximately 680 mH, is less useful in discussing coordination equilibria, as it results from an overlap of bands occurring in the spectra of both [ C O C ~ ~and L ~ ] [CoC13L]- complexes. A section of the spectrum important in further discussion is represented by curve 1 in Figure 1. It should be noted that in the corresponding solutions, in addition to the above-mentioned tetrahedral chloro complexes, octahedral cations, [CoL,jI2+,are also present, but their absorption is negligibly small in the spectral region here considered. An interesting peculiarity of cobalt(I1) in acetonitrile solution lies in the fact that The Journal of Physical Chemistry
360
300
250 lui
200
150
100 550
560
570
580
590
600
610
620
Wavelength, mp.
+
Figure 1. Absorption spectra of CoClz (0.000980 M ) EtaNCl solutions in acetonitrile a t 25'. The curves correspond to the following molar concentrations of EtdNC1: 1, 0.0; 2, 0.000459; 3, 0.000656; 4, 0.000858.
it does not form detectable quantities of the monochloro complex, either octahedral or tetrahedral. We refrain here from the detailed discussion which leads to these conclusions, as it has been given elsewhere.lS2 When chloride ions (e.q., in the form of tetraethylammonium chloride) are added gradually to a dilute solution of cobaltous chloride in acetonitrile, a remarkable increase in intensity of the band at 590 mp belonging to the complex [CoC13L]- is observed. At the same time the bands at 571 and 614 mp, characteristic of [CoCl2LZ],decrease in intensity. Within a certain concentration range of EtdYC1 three isosbestic points appear (at 576, 600, and 633 mp) in the set of absorption curves. It is obvious that the only forms of cobalt(I1) present in this concentration range are the [CoC12L2] and [CoC13L]- tetrahedral complexes, while [CoLBl2+is no longer present. Two of these isosbestic points denoted by P'23and p"23, of which use will be made further, are seen in Figure 1. The molar extinction coefficients, 623, at these points found in the pres(6) B. J. Hathaway and A. E. Underhill, J . Chem. SOC.,3091 (1961). (7) W. Libus' and D. Puchalska, J . Phys. Chem., 71, 3549 (1967). (8) W. F. Luder, J . Am. Chem. SOC.,62, 89 (1940).
2077
COORDINATION DISPORPORTIONATION EQUILIBRIA IN SOLUTION
+
ZnClz solutions in acetonitrile at 25'. The curves correspond to the Figure 2. Absorption spectra of CoClz following molar concentrations of CoCh and ZnClz, respectively: 1, 0.000977, 0.0; 2, 0.00248, 0.000621; 3, 0.00186, 0.00124; 4, 0.00124, 0.00186; 5, 0.00103, 0.00207.
ent work are 294 and 265 mp, respectively. These values are somewhat different from those found previouslyl (291 and 267, respectively). For solutions in which [COC~ZLZ] and [CoC13L]- are the only light-absorbing species, the following equation must hold €23(C2
+
c3)
=
(2)
where a is the average molar extinction coefficient of cobalt(I1) a t the wavelengths of that isosbestic point to which 623 refers, Cz and C3 are the equilibrium concentrations of the [CoCI2L2]and [CoCl3L]- complexes, respectively, and C is the total concentration of cobaltous chloride in the solution. Since C = Co C2 C3,where Cois the Concentration of [CoLaI2+,it follows from eq 2 that
+ + (3)
Application of this relation to dilute solutions of cobaltous chloride in acetonitrile does not lead to reliable values of Co/C, as then the measured difference €23 i is very small. It may, however, give considerably
better results in the case of solutions in which C$C is higher as, for example, in the case of the three-component solutions cobaltous chloride-zinc chloride-acetonitrile discussed below. When zinc chloride is added to an acetonitrile solution of cobaltous chloride, the absorption spectrum changes in the way illustrated by Figure 2. As is seen, the bands characteristic of the tetrahedral complexes decrease gradually as the concentration of zinc chloride increases. It is noteworthy, however, that the band at 590 mp characteristic of the complex [CoClaL]- decreases in intensity more rapidly than do the two bands a t 571 and 614 mp characteristic of [CoC12L2]. It disappears completely when the ratio of the concentration of ZnClz to that of CoClz becomes 2, while the other two bands indicating the presence of [CuClzL2]become insignificant only when the concentration of zinc chloride becomes as high as 0.1 M . I n the latter case the solution becomes pink, indicating the presence of octahedral cobalt(I1) complexes. The corresponding spectrum, together with that of anhydrous cobalt(I1) perchlorate in acetonitrile, is shown in Figure 3. The two spectra are seen to be similar in the range of the 450-550-mp Volume 72,Number 6 June 1968
2078
W. LIB&, D. PUCHALSKA, AND. T. SZUCHNICKA the corresponding disproportionation equilibrium constant for zinc chloride, as compared with cobaltous chloride in the same solvent. This expectation is supported qualitatively by the small electrical conductance of zinc chloride solutions in acetonitrile (see further). It should be noted that other types of coordination disproportionation, e.g., those involving [ZnClL6]+ and/or [ZnClrI2- ions, cannot, a priori, be excluded. It also seems possible that, in addition to coordination disproportionation, polymerization equilibria might occur in these solutions. As far as we know, there have been no investigations in the past on coordination equilibria of zinc chloride in acetonitrile. I n these circumstances'we assume, as a working hypothesis to be verified in further experiments, that the equilibrium
16
10 lui
6
500
450
550
600
Wavelength, mp.
Figure 3. Absorption spectra of: 1,0.0135 M Co(ClO&, and 2, 0.0163 M CoClz ZnC& (0.156 M ) solutions in acetonitrile at 25".
+
absorption band which, according to its position and small intensity, should be ascribed to the transition 4T~,(F)-+ 4T1,(P) of cobalt(I1) in octahedral environment. The interpretation of the spectral effects described above in terms of cobalt(I1) coordination changes is as follows. With increasing concentration of zinc chloride in the three-component solution the tetrahedral complexes [CoC13L]- and [CoCI2L2]are decomposed gradually, while the octahedral complex [CoL6I2+is formed in their place. The formation of the latter, which might be anticipated based on the knowledge of equilibrium 1, is confirmed by the above-mentioned identity of the spectra of CoC1, in the presence of an excess of ZnClz on one hand and of Co(ClO& in acetonitrile solution on the other. Since [cOL6l2+is the only existing cobalt(I1) species in dilute solutions of Co(C10J2 in acetonitrile, the band a t 480 mp must belong to this complex. It follows from the above discussion that the reaction taking place between zinc chloride and cobaltous chloride in acetonitrile solution consists in the transfer of chloride ions from cobalt(I1) to zinc(I1). The products of this process involving zinc(I1) are not immediately apparent, and to substantiate our expectations a brief comment is necessary. Coordination properties of cobalt(I1) and zinc(I1) are known to be rather similar. However, an important quantitative difference between them consists in a marked difference in the relative stabilities of tetrahedral and octahedral structures, the tendency of zinc to form the tetrahedral structure being higher.9 According to the discussion given in the preceding paper,' this difference should result in a much lower value of The Journal of Physical Chemistry
+
~ [ Z ~ C I ~ 2L L~] tetrahedral
+ 2[ZnC13Ll-
[ZnL612+ octahedral
(4)
tetrahedral
analogous to that occurring in solutions of CoC12, is responsible for the electrical conductance of zinc chloride in acetonitrile. Of course, this assumption does not exclude the possible occurrence of polymerization equilibria involving neutral complexes. Taking into consideration the above argument as well as the conclusions drawn from the spectral effects shown in Figure 2, it seems very probable that the reaction taking place between cobaltous chloride and zinc chloride in acetonitrile involves the formation of [ZnC13LI- tetrahedral anions. At the same time, the concentration of [ZnL6I2+initially present in small quantities in the two-component solution zinc chloride-acetonitrile must decrease very considerably if equilibrium 4 is assumed to be valid. Ultimately, the only ionic species expected to be present in significant quantities in the three-component solutions zinc chloride-cobaltous chloride-acetonitrile are [coL6]2+, [CoC13L]-, and [ZnC13L]-, the relative proportions of the latter two changing with changing composition of the solution. These expectations may be verified in the following way. Since the structure of the anions [CoC13L]- and [ZnCLLJ- should be the same and their dimensions practically identical, the specific conductance of the solutions should be determined solely by the total concentration of the complex electrolyte (expressed as [i\!tL6]2+'2[i\IIC13L]-), and be independent of the relative contents of the [CoC13L]- and [ZnCl3L]- ions. This independence has, in fact, been observed experimentally, as may be seen from Figure 4. The specific electrical conductance of the three-component solutions cobaltous chloride-zinc chloride-acetonitrile has, in this figure, been plotted against the concentration of the complex cation [MLs]", assuming it to be equal to Co. Values of COwere determined spectrophotometrically making use of eq 3 and (9) W. Libus' and I. Uruska, Inorg. Chem., 5, 266 (1966).
2079
COORDINATION DISPROPORTIONATION EQUILIBRIA IN SOLUTION
determine the associated equilibrium constant more precisely. Contrary to this, in the case of zinc chloride the more fundamental question must be answered whether or not coordination disproportionation equilibrium 4 is the real one, and if so, whether it is the only one of importance in this system. The thermodynamic equilibrium constant of reaction 1 is given by the expression
where f* is the mean ionic activity coefficient of the complex electrolyte [CoL6]*+. 2 [CoC13L]-, fo is the activity coefficient of the neutral complex [CoC12L2], and Co, C2, and C3 are the equilibrium concentrations of the [CoL6I2+, [CoClzL2], and [CoC13L]- complexes, respectively. Taking into account that C8 = 2C0 and C = Co Cz C3,and assumingfo = 1,we obtain
+ +
Figure 4. 1, The dependence of the specific conductance of CoC12 ZnClz solutions in acetonitrile on the concentration of [MLs]2+.2[MClaL]- complex electrolyte. The points correspond to the following molar proportions of CoClz to ZnClz: 0, 4:l; 0, 3:2; A, 2:3; 0, 1:2; A, 1 : 4 ; 2 and 3, the dependences of the specific conductance of CoCll (curve 2) and of ZnClz (curve 3) solutions in acetonitrile on concentration; temperature 25’.
+
of molar extinction coefficients measured at the wavelengths of two isosbestic points (at 576 and 598 mp). Differences between two values of COobtained in this way for each of the solutions did not exceed 5% of their absolute values; the average values of these two were used in constructing Figure 4. It is obvious that on passing from solutions containing CoClz only to those with an approximately fivefold excess of ZnClz, the anion [CoC13L]- must be replaced gradually by an anionic complex involving zinc(I1). The fact that despite the changing composition of the solution the dependence of its specific conductance on the concentration of [coL6]2+remains the same within the experimental error is most simply explained by the assumption that the anionic complex of zinc(I1) formed is, in accord with expectations, [ZnC13L]--. It is clear that the curve shown in Figure 4 does represent the dependence of the specific conductance on the concentration of [MLB]2+.2[P\/IC13L]- complex electrolyte in acetonitrile solution. Once determined, this curve may be used as a reference curve in conductometric determinations of the concentration of complex electrolytes of the same structure in other solutions. This procedure is applied below to the investigation of two-component cobaltous chloride-acetonitrile and zinc chloride-acetonitrile systems. The occurrence of coordination disproportionation equilibrium 1 in the former has been established previously, and the purpose of the present investigation was only to
The requisite values of Co have been determined conductometrically making use of the curve %( [h’IL6]) in the way described above. The dependence of the specific conductance on the concentration of solutions of CoClz in acetonitrile is shown in Figure 4 together with the reference curve, x ( [MLo] ), and the analogous dependence for ZnClz. It will be noted that the three curves in Figure 4 have been drawn in different concentration scales, so that the concentration range covered in the case of ZnC12 is approximately 10 times greater than that corresponding to the curve for CoC12. The values of Cowere found graphically in the way shown in the figure. The assumption that [&&I = [COLG] becomes invalid a t very low concentrations of the corresponding solutions. Consequently, only solutions M were taken into having [?L/IL6]greater than account in further calculations. I n order to determine the coordination disproportionation constant, D, values of log D, = log 4Co3/(c - 3 c 0 ) ~ have been plotted against the square root of the concentration of [h!tL6]2+ (ionic strength of the solution = 3[ML6]= 3C0), as shown in Figure 5b. The plot appears to be linear, if the region of lowest concentrations, [MLG] below M , is disregarded. Extrapolation of the linear part of the curve to zero ionic strength leads to the value of the coordination disproportionation constant, D, for CoClz in acetonitrile of (1.55 3t 0.05) X at 25”. The dependence of log D,on the concentration of the complex electrolyte [hlL6]”+.2[MC13L]- calculated from the limiting Debye-Huckel equation, assuming 36.7 for the dielectric constant of acetonitrile a t 25”, is represented by the full line in Figure 5b. It is seen to be very close to the experimental points within the whole concentration range investigated. This observation is important, as (a) it confirms the correctness of the Volume 70. Number 6 June 1968
W. LIBUB,D. PUCHALSKA, AND. T. SZUCHNICKA
10*5-
2080
-4.5
-
0
10
20
108
x
30
40
[MLs1''2.
Figure 5. The dependences of the concentration equilibrium quotients of reactions 1 (curve b) and 4 (curve a) on the square root of the concentration of the complex electrolyte [MLe]z+ .2[MCI,L] -; temperature 25'.
method applied, and (b) it indicates the validity of the limiting Debye-Huckel equation in the case of the solutions under discussion in an unexpectedly wide concentration range, corresponding to the ionic strength up to approximately 0.01. The coordination disproportionation constant of zinc chloride in acetonitrile may be determined in precisely the same way as used above, assuming that reaction 4 is valid. It should be noted, however, that eq 6 involves the assumption that C = CO CZ Ca,
+ +
The Journal of Phuatkal Chemistry
where C is (in this case) the total concentration of ZnClz and Co, Cz, and CB are equilibrium concentrations of [ZnL6I2+,[ZnC12Lz],and [ZnC13L]- complexes, respectively. Consequently, occurrence of equilibria other than that given by (4)would invalidate eq 6, although it would not necessarily preclude other possible ways of analyzing the data (but only if not more than one additional equilibrium involving neutral complexes were to be taken into account). Fortunately, complications arising from additional equilibria do not seem to occur because, as may be seen from Figure 5a, application of eq 6 to the calculation of Do for ZnClz in acetonitrile by the conductometric method leads to results perfectly consistent with the Debye-Huckel limiting law. The value of D for this system, found by extrapolation, is (9.78 f 0.05) X lo-' a t 25". In this extrapolation experimental points corresponding to lowest concentrations have been disregarded. The theoretical Debye-Huckel dependence of log D, on the ionic concentration is shown by the full line and also in this case is seen to be very close to experimental points, except at lowest concentrations. We believe that, as shown above, the occurrence of equilibrium 4 and the absence of any other equilibria in solutions of ZnClz in acetonitrile have been definitely established.
Acknowledgment. The skilful assistance of Mrs. G. Czerwinska in performing some of the experiments is gratefully acknowledged.
