Conductometric study of some divalent transition metal perchlorates in acetone

Ektrochimica

km,

Printed 10 Great

Vol.

35. No.

2. ,sp

351-354.

1990.

Britain.

c

CONDUCTOMETRIC TRANSITION METAL

00134686i90 S3Ml+O.W 1990 Pergaman Press pk

STUDY OF SOME DIVALENT PERCHLORATES IN ACETONE

WACLAW GRZYBKOWSKI and MICHAL RLARCZYK

Department of Chemistry, Technical University of Gdahsk, S&952 Gdatisk, Majakowskiego 1l/12, Poland (Received 16 January 1959)

Abstract-Molar conductances have been determined for Mn(CIO,),, Co(CIO,),, Ni(CIO,),, Cu(ClO,), and Zn(CIO,), in acetone solutions at 25°C. Visible absorption spectra of Co(CIO,),, Ni(CIO,), and Cu(ClO& indicate the existence of octahedral species in these solutions. All the systems show a high degree of association. The first and second step association constants are estimated.

INTRODUCTION In previous papers we investigated systematically the behaviour of divalent first-row transition metal perchlorates in strongly polar coordinating solvents, such as dimethyl sulphoxide[ l] and N,N-dimethylformamide[2]. It has been shown that the ML:+ type solvocations display rather low ability for outersphere association, being practically independent of the nature of the central metal ion. This results in the fact that equimolar solutions of the respective salts may be considered as effective constant ionic media, and may be used to keep ionic strength constant without affecting chemical equilibria in the solutions when complex formation is studied[3]. The present work extends these investigations to acetone. It may be expected that the lowering of the dielectric constant markedly enhances association, thus making detection of possible differences in the association constants easier. All the salts examined show a considerable degree of ion pairing in acetone solution. The association constants obtained for alkali metals vary from 132 for LiClO, to ca 40 000 M-’ for LiNO,[4]. Thus, a considerably higher degree of association may be expected for the divalent metal perchlorates.

EXPERIMENTAL Acetone was purified by means of KMnO,. After dehydration with a molecular sieve, acetone was distilled. The product was kept in a dry-air atmosphere. The specific conductance of the purified solvent was in the range of 8.2 x 10m9-1.8 x lo-* Scm-‘. The transition metal perchlorates, recrystallized from water, were dried over phosphorus pentoxide in vacua and dissolved in dehydrated acetone. The resulting solutions were dried using molecular sieves. After removing any excess of solvent the crystalline solids were isolated and recrystallized from anhydrous acetone. The crystalline products were dissolved in fresh solvent prior to taking measurements. The individual stock solutions were standardized by titration with

EDTA. Solutions for measurement were prepared by weighed dilutions and their concentrations were calculated using densities determined independently. Details of the procedures for measurements were identical to those described previously[ 1,2].

RESULTS AND DISCUSSION Figure 1 shows the molar conductances of Mn(ClO,),, CO(CIO,)~, Ni(ClO,),, Cu(ClO& and Zn(ClO,), in acetone at 25°C. The experimental values are listed in Table 1. For the sake of clarity the data for two salts only are presented within the whole range of concentrations studied. The solid line shows the molar conductance of Cu(ClO,),, whilst the behaviour of Ni(CIO,), is described by the broken line. Inspection of the diagram shows that the lines for Cu(ClO,), and Ni(ClO,), display similarity. Moreover, the two salts exhibit the boundary values of molar conductances, ie the values obtained for Mn(ClO,),, Co(ClO,), and Zn(ClO,), fall between the lines. The points obtained for all the metal perchlorates are shown for the most dilute solutions only, for concentrations below 0.003 mol dmm3. As is seen, the points for Co(ClO,), and Ni(ClO,), form a single common curve. The points for Zn(ClO,), and Mn(ClO,), also exhibit a close similarity, and they are observed between the line for Cu(ClO,), and the common line for Co(ClO,), and Ni(CIO,),. The resemblance between the molar conductances suggests that the mobilities and abilities for association of the solvated cations existing in the acetone solutions are also very close. Of the five divalent metal perchlorates studied in this work, only Co(ClO,),, Ni(ClO,), and Cu(ClO,), exhibit visible absorption spectra. The visible absorption spectrum of Co(ClO,), in acetone consists of an absorption band characteristic of Co2+ in an octahedral environment. Both the position and intensity of the band are concentration-independent indicating that the perchlorate anion does not enter the coordination sphere of cobalt (II). The same remarks are valid for Ni(ClO,), and Cu(ClO,),. However, the visible absorption spectrum of Cu(C10J2 may be ascribed as

WACLAW GRZYBKOWSKI and MICHAL PILARCZYK

352

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Fig. 1. Plots of molar conductance against the square root of concentration for divalent transition metal perchlorates in acetone solution at 25°C. (0) Mn*+; (+) Co2+; (0) Ni’+; (0) Cu’+; (a) Zn2+. The solid line represents the conductometric curve for Cu(ClO,), and the broken line shows the molar conductance of Ni(ClO,&.

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Plots of equivalent conductance against the square root of equivalent concentration for Co(ClO,), (0), LiClOd @) and LiNO, (0) at 25°C. The data for lithium salts are taken from[4].

the Cuz+ ion in a tetragonally distorted octahedra1 copper (II) complex. The spectra1 data for the metal perchlorates in acetone are summarized in Table 2. Thus, the conclusion that all five perchlorates occur as complex electrolytes involving the hexa-solvated cations of the M(acetone)g* type seems to be rather obvious. Inspection of the data presented in Fig. 2 shows the equivalent conductances of Co(CIO,),, LiClO., and LiNO,. In order to compare the data for the electro-

lytes differing in charge of the cation, Fig. 2 shows the conductance plotted against the square root of the equivalent concentration. As is seen, the points for Co(ClO& run well below the points for LiCLO.+ This drastic differences suggests a high degree of ion pairing in acetone solutions of Co(ClO,), as well as in solutions of other metal perchlorates. Their behaviour seems to be more akin to the electrolytic properties of LiNO, than to the behaviour of LiClO,. T,he former salt may be considered as a typical weak electrolyte.

Conductances

of divalent

Table 1. Molar conductances of divalent transition perchlorates in acetone solution at 25°C c x 1041 moldme

MW0.A

1.0491 1.4834 2.3380 3.2623 4.1973 5.3034 6.4434 7.7332 9.2892 12.598 16.441 20.790 26.992 Co(C10& 1.0750 1.2662 1.4866 1.8163 2.3501 3.0209 3.7192 4.9463 6.3285 1.8622 10.241 12.938 15.896 Ni(CIO,), 1.0774 1.3465 1.5945 1.9780 2.2357 2.7200 3.1489 3.6121 4.0770 5.7978 8.1540 11.975 15.695 Cu(C103, 0.7766 1.1579 1.9217 2.9250 3.6411 4.6209 5.7593 7.0493 8.2558 10.855 12.797 15.149 17.692 Zn(CIO,), 1.2072 1.4346 1.I643 2.0445 2.2211 2.6742 3.0703 3.5971 4.5445 7.1105 10.148 13.699 18.015 22.645

Al Scm2mol-’

c x 104/ mol dm- 3

metal

metal perchlorates

37.688 50.676 66.928 83.138 103.207 148.24 194.86 252.67 321.22 396.32 475.90 625.50 802.65

129.39 121.64 114.11 109.34 103.74 95.69 90.12 85.13 81.01 77.69 74.74 70.75 67.80

283.15 272.63 263.57 253.87 240.30 227.92 218.06 205.18 194.72 185.95 176.19 167.44 160.73

20.192

151.60 143.29 132.64 123.53 110.39 100.47 92.26 85.94 81.05 76.59 72.87 69.66 65.68

283.23 269.89 261.99 248.37 243.48 232.27 225.82 219.10 213.44 198.28 184.17 169.91 160.95

20.161 26.730 32.088 44.509 61.589 87.142 127.35 175.49 228.17 298.65 374.50 484.65 655.01

151.88 142.85 137.20 126.68 117.37 108.83 98.45 90.96 85.39 80.14 75.48 70.86 66.11

291.54 260.60 233.23 213.64 206.14 194.38 185.90 178.48 112.65 164.15 157.94 152.71 148.24

22.847 3 1.462 43.592 63.620 100.16 144.40 196.56 258.25 321.99 398.47 525.45 741.80

140.80 131.10 122.48 112.94 102.87 94.48 88.73 83.92 80.31 76.95 72.88 68.08

264.90 256.77 245.15 238.04 233.69 223.47 218.90 209.73 199.74 182.35 169.91 159.82 151.23 144.69

24.940 28.525 36.330 50,870 66.370 107.28 151.26 201.57 262.00 338.51 416.35 576.52 812.87

142.10 138.00 130.20 121.11 113.73 102.01 94.31 88.46 83.57 79.32 75.68 70.99 65.80

353

Table 2. Spectral bands for Cu(ClO,), in acetone solution

Al Scm2mol-’

269.06 250.93 221.90 212.51 202.58 193.07 185.60 179.27 173.33 164.38 153.81 146.97 137.81

37.115 49.885 80.456 119.76 168.25 224.8 1 286.47 356.78 439.84 528.96 675.73

transition

l’mJ nm WCIO,), Ni(CIO,), Cu(CQ),

523 407 740 822

Co(CIO,),, at 25°C

s InPII dm3mol-‘cm-’

Ni(CIO,),

and

Assignment

12.8 11.6 3.7 29.4

It is obvious that determinations of A” from extrapolated Onsager plots are not permissible. Our attempts to derive the limiting equivalent conductances made use of the Fuoss and Edelson method as described in previous papers[l, 33. The analysis was performed for the most dilute solutions only and the resulting values may be considered as very rough approximations because the Fuoss-Edelson treatment is useful for electrolytes associated to a much smaller extent. Thus, we report the average value of 208 S cm’ mol- I for the limiting equivalent conductance of the metal perchlorates in acetone at 25°C. Taking into account known values of the limiting ionic conductance of the ClO, anion varying from ca 121 to 122 S cm2mol-’ [4], the limiting equivalent conductance of the hexa-solvated cations may be estimated as 86 S cm2 equiv.- I. This value agrees well with the values reported for alkali metal cations and varies from 72.5 S cm2 mol- 1 for Li+ to 79.9 S cm2 mol- 1 for Cs+[4]. It is obvious that equivalent conductance of the divalent cation is higher than that of the monovalent cation. Moreover, our previous observations suggest that the respective ratio amounts to 1.25[1,5], while the Stokes equation results in the value of 2 on the assumption of constant radii of the solvated cations. The Fuoss-Edelson plots were used to estimate the value of the first-step association constant. The value of (8 f 3) x lo3 dm3 mol- ’ seems to reflect the ability of the hexasolvated cation towards association with the perchlorate anion. Further attempts to derive more detailed characteristics of the transition metal perchlorates in acetone made use of the procedure described in detail in a previous paper concerning more associated electrolytes, such as transition metal nitrates in DMF. In these systems ion-pair formation is accompanied by second-step association[6]. The procedure results in the estimations of the first- and second-step association constants as well as in the value of the molar conductance. Unfortunately, due to high degrees of association the procedure cannot be fully useful for the acetone solutions of the perchlorates. The analysis actually performed yields values which can be considered as approximations only. They are: A” [iM(acetone)g +] = 83 S cm* mol- ‘, log K I = 4.04 and log K, = 2.7. The latter value indicates that the second-step association is one of the important factors controlling the electrolytic properties of the transition metal perchlorates in acetone. High ability for association is in part the result of a low value of the dielectric constant of acetone, while interactions of the ClOi anion with the solvent mole-

354

WACLAW GRZYBKOWSKIand MICHAL PILARCZYK

cules from the first coordination sphere are a consequence of a specific nature of the solvent modified by complex formation. Perchlorate salts have often been used to keep ionic

strength constant in studies of complex formation. It is obvious that interferences from association can cause systematic errors in the chemical quantities derived from the measurements. Sawada er al. have studied the equilibria of CO(C~O,)~ with LiCl and LiBr in 0.1 mol drne3 LiClO,-acetone solution[7, 83. Their statement that the ionic species form uncharged ionpairs with CIO; seems to be insufficient.

Acknowledgement-This CPBP-01.15.

work was supported by Program

REFERENCES 1. W. Libus, B. Chachulski, W. Grzybkowski, M Pilarczyk and D. Puchalska, J. Solution Gem. 10, 631 (1981). 2. W. Grzybkowski and M. Pilarczyk, J. &em. Sot., Faraday Trans. 1 79, 2319 (1983). 3. W. Grzybkowski and M. Pilarczyk, J. them. Sot., Faraday Trans. 1 82, 1745 (1986). 4. N. Schmelzer, J. Einfeldt and M. Grigo, J. them. Sot., Faraday Trans. 1 84, 931 (1988). 5. W. Grzybkowski and M. Pilarczyk, J. them. Sot., Faraday Trans. 1 83, 281 (1987). 6. M. Pilarczyk and W. Grzybkowski, J. them. Sot., Faraday Trans. 1 (submitted). 7. K. Sawada, T. Onoda and T. Suzuki, J. inorg. nucl. Chem. 43, 3263 (1981). 8. K. Sawada, T. Onoda and T. Suzuki, J. them. Sot., Dalton Trans. 1565 (1983).