J. Chem. SOC., Faraday Trans. 1, 1986,82, 1703-1712 Ionization Equilibria of Cobalt(I1) Chloride in N , N-Dime t h ylformamide Waclaw Grzybkowski" and Michal Pilarczyk Department of Physical Chemistry of the Institute of Inorganic Chemistry and Technology, Technical University of Gdarisk, 80-952 Gdarisk, Poland Visible absorption spectra and the molar conductance curve for CoC1, in N,N-dimethylformamide (DMF) have been determined at 25 "C. The results indicate the formation of the Co(DMF)i+ - 2CoC1,DMF- complex electro- lyte which controls the electrolytic properties of the solution. The formation constants of the individual chloro-complexes of cobalt(I1) have been calculated. A knowledge of transition-metal complexes in non-aqueous media and an understanding of the influence of solvents upon their equilibria are of major interest.However, there is a lack of information concerning the simplest transition-metal salts dissolved in non-aqueous donor solvents. It has been shown by Libui et a1.l that the transition-metal cations in strongly polar donor solvents, such as acetonitrile,,. dimethyl sulphoxide,*~ dimethylacetamide6 and dimeth~lformamide,~ exist in the absence of coordinating anions as MLi+-type solvated complexes (M = Mn2+, Co2+, Ni2+, Cu2+ or Zn2+ and L denotes the solvent molecule). Unlike perchlorates and tetrafluoroborates, the divalent transition-metal halides dissolved in dimethyl sulphoxide8 or acetonitrileg* lo exhibit a variety of electrolytic behaviour, as is shown by their molar conductance curves and absorption spectra. The properties of the solutions can be described in terms of the coordinative disproportionation reactions producing complex electrolytes consisting of a hexacoordinate cation and a tetracoord- inate anion.The formation of tetrahedral anionic species in N,N-dimethylformamide solutions of the transition metal chlorides was shown by Katzinll and ascribed to the equilibrium (original notation used) : 2°CtMC1, e OctMC1+ + tetMC1;. The proposed scheme, however, does not describe a large range of electrolytic behaviour. The dissolution of CoCl, in acetonitrilelo is accompanied by the equilibrium 3CoC1,(CH3CN), + 2CH3CN Co(CH,CN)i+ + 2CoC1,CH3CN- while in dimethyl sulphoxide solutions of NiCl, the pseudotetrahedral complexes are practically absent and the NiCl(DMSO),+ * C1- complex electrolyte is responsible for the electrolytic behaviourq8 In a previous paper1, we have shown that the dissolution of NiCl, in DMF results in the formation of the NiCl(DMF)$ NiC1,DMF- complex electrolyte being the main form of the solute in DMF.The present work was undertaken in order to establish the ionization equilibria of CoCl, in DMF. Experiment a1 N,N-Dimethylformamide (analytical grade) was dried using 4A molecular sieves and distilled under reduced pressure at 45-50 "C. The specific conductance of the purified 57 1703 FAR I1704 Ionization Equilibria in DMF solvent was in the range (3.0-8.0) x S cm-l. The density at 25 "C was 0.94402 g ~ m - ~ DMF-solvated CoCl, and C0(C104), were prepared from the corresponding hydrates by dissolving them in DMF, followed by removing any excess of the solvent under reduced pressure at 60 "C.On cooling, crystalline solids were obtained and were recrystallized twice from anhydrous DMF. Analytical-grade tetraethylammonium chloride was recrystallized twice from anhy- drous acetonitrile and dried in vacuo at 65 "C. The stock solutions of the salt were analysed by standard EDTA titrations. Solutions for measurements were prepared by weighed dilutions. The concentrations were calculated using densities determined independently. Details of the procedures for spec t rop ho t ome tric and conduct ome tric measurements were identical to those described previo~sly.~. ' 9 l2 All the preparations and manipulations were performed in a dry box. (literature values are 0.94387 and 0.94407 g cm- 3 ).13, 1 4 Results and Discussion Fig. 1 shows the visible absorption spectra of a series of solutions of CoCl, in DMF within the concentration range 0.002-0.02 mol dm-3 at 25 "C. As is seen, the spectrum consists of a broad band with maxima located at 610 and 680 nm. The band position, contour, and high intensity are typical of cobalt(I1) in a pseudotetrahedral environment8* 9; l5 Further inspection of fig. 1 shows that the band position is independent of salt concentration, while a variation of the intensity with the increase of CoCl, concentration can be observed. The effect of concentration becomes more distinct when the molar absorption coefficients of cobalt(I1) at the maxima are plotted against the square root of the concentration of CoC1, (fig.2). The most characteristic feature of this dependence is the relatively sharp decrease in the intensity with the decrease of CoCl, concentration below 0.0004 mol dm-3. This fact may be interpreted in terms of replacing the tetrahedral I I I 5 7 300 - ,-I 'E 200 "E - 0 - E a IUJ 1 550 600 650 700 750 wavelength/ nm Fig. 1. Visible absorption spectra of CoCl, solutions in DMF at 25 "C. The molar concentrations of CoCl, are: (1) 0.00 1935; (2) 0.002429; (3) 0.004837; (4) 0.01083; and (5) 0.016 18.W. Grzybkowski and M . Pilarczyk - 7 --. I E 300 E 200- ; m -a IIU . 100 1705 - r-=yGT'' n n ,. V " , I , ] I I I I I , , , cobalt(I1) complex with an octahedral one. The hexacoordinated species do not exhibit absorption in this spectral range.l The independence of the band position and contour of concentration of CoCl, suggest that only one tetracoordinated complex of cobalt(r1) exists in solution.Moreover, an increasing concentration of CoC1, in the solution brings about an increase in the relative content of the complex. Fig. 3 shows the molar conductance curve of CoCl, in DMF at 25 "C. The experimental values are listed in table 1. In the same figure is presented the molar conductance curve of Co(ClO,), reported previous1y.l It can be seen that the molar conductance curve of CoCl, runs well below the curve of Co(ClO,),, known to exist in the form of a Co(DMF)i+ - 2C104-type complex electrolyte, being only slightly associated. As is seen the molar conductance curve of CoC1, exhibits a slight decrease in conductivity with increasing concentration at the lowest Concentration range, while the experimental points run almost horizontally at higher CoCl, concentrations.The relatively low value of the molar conductance suggests a high degree of complex formation. The limiting molar conductance calculated for the Co(DMF)i+ * 2C1- complex electrolyte from the known ionic conductancesly l6 amounts to 188.4 S cm2 mol-l, and the corresponding conductometric curve is expected to run close to the curve for Co(ClO,),. As can be seen, the variation of the molar conductances is roughly reflected in the concentration changes of the spectrum of cobalt(I1). Such behaviour can be due to the formation of ionic species in the solution rather than neutral ones. The effect of increasing the conceqtration of the ionic complexes may compensate the effect of decreasing ionic mobilities due to increasing ionic strength.Thus, we infer that the pseudotetrahedral complex of cobalt(I1) is the CoC1,DMF- anion. A qualitative confirmation of this conclusion is provided by the effect which addition of a non-coordinating diluent of low polarity exerts on the spectrum of CoCl, dissolved in DMF. The spectra of cobalt(I1) observed at high toluene contents are shown in fig. 4 along with the spectrum of CoCl, in DMF. Inspection of fig. 4 shows that addition of toluene results in drastic changes in the spectrum of cobalt(I1). The effect consists of the development of a new spectrum with absorption maxima at 582,640 and 670 nm. Similar spectral changes were induced by addition of chlorobenzene.Moreover, the effects are accompanied by the essential decrease in conductivity. The molar conductance of a 0.00289 mol dm-, solution of CoC1, in mixed solvent (87 mol % of chlorobenzene) amounts to 1.1 S cm2 mol-1 only, while the value for the corresponding DMF solution is 26.5 S cm2 mol-l. It may be expected that the decrease in dielectric constant of the medium favours formation of a neutral species. Thus, it is clear that the pseudotetrahedral complex of cobalt(@ existing in the presence of the inert diluents is CoCl,(DMF),. Fig. 5 shows the visible absorption spectra of a series of solutions containing Co(ClO,), 57-21706 200 150 r( - I 0 E *s 100 e LA \ 50 0 Ionization Equilibria in DMF \ \ \ \ \ 1 I I I I I I 1 I 0.02 0.04 0.06 0.08 (clmol dm-3)3 Fig.3. Plot of the molar conductivity against the square root of concentration for CoCl, in DMF solution at 25 "C (0). The broken line represents the conductometric curve obtained previously for Co(ClO,),. at an approximately constant concentration of 0.012 mol dm-3 and Et,NCl at a number of different concentrations not exceeding the 2:l mole ratio of Et,NCl to Co(ClO,),. As is seen, an addition of Et,NCl brings about a development of an intensive absorption band with maxima at 610 and 680 nm. The same maxima were observed for solutions of CoCl, in DMF (fig. 1). With increasing concentration of Et,NCl the band changes in intensity only, indicating the presence of one tetrahedral complex of cobalt(I1) in the solutions. Table 1.Molar conductivities of CoCl, in DMF at 25 "C c Am C Am / 1 O-* mol dm-3 / S cm2 mol-l / 1 0-4 mol dm-3 / S cm2 mol-1 1.9450 2.8773 4.0909 5.3970 8.4335 12.134 14.377 21.890 33.52 26.103 27.33 33.04 32.671 26.90 3 1.44 43.936 26.46 30.57 52.140 26.21 29.30 63.528 25.84 28.68 81.639 25.39 28.26 99.587 25.10 27.52 - -400 300 r( I E "E - 2 200 a IUJ 1 100 0 W. Grzybkowski and M. Pilarczyk I 1 I I 1707 550 600 650 700 7 50 wavelength/nm Fig. 4. Absorption spectra of CoCl, in DMF-toluene mixtures at 25 "C. The concentrations (mol dm-3) of CoCl, and the mole fractions of toluene in the mixed solvent are, respectively: (1) 0.003 025,O.OO; (2) 0.003 180,0.156; (3) 0.003077,0.428; (4) 0.003009,0.592; (5) 0.003 158,0.755; and (6) 0.001 675, 0.925. Further changes in the spectrum of cobalt@) induced by an increasing concentration of Et4NCl added to a 0.0012 mol dm-3 solution of C0(C104), in DMF are illustrated in fig.6. At Et4NCl to Co(ClO,), ratios exceeding 2.8 an increase of Et4NC1 concentration results in the gradual disappearance of the maximum at 610 nm and the simultaneous development of a new spectrum consisting of three bands with maxima at 637.5,673 and 700 nm. Subsequently, three well defined isosbestic points can be observed on the spectra at 622,648 and 661 nm. When the Et,NCl to Co(ClO,), ratio exceeds 50, further increase of chloride concentrations does not affect the spectrum. The limiting spectrum shows the characteristics of the tetrachloro-complexes. The facts indicate a two-species equilibrium : CoC1,DMF- + C1- + CoC1;- + DMF (1) established within the 30-50 range of the C1- to Co2+ ratio. The above results permit calculation of the equilibrium concentrations of CoC1,DMF- and CoC1;- complexes.Thus, ignoring absorption due to the octahedral complexes, the mole fraction of the tetrahedral complexes may be calculated as1708 300 2 00 r( I 8 - 0 E E m a Iu, 1 100 0 Ionization Equilibria in DMF I I I 550 600 650 700 750 w aveleng t h/nm Fig. 5. Absorption spectra of Co(ClO,),-Et,NCl solutions in DMF at 25 "C. The concentrations (mol dm-3) of Co(ClO,), and Et,NCl are, respectively: (1) 0.01 1896, 0.004528; (2) 0.01 1947, 0.006713; (3) 0.012187, 0.008912; (4) 0.012475, 0.01272; (5) 0.012 119, 0.01561; (6) 0.01253, 0.01887; and (7) 0.012 195, 0.021 97. where E denotes the measured mean molar absorption coefficient of cobalt(I1) at the wavelength of the isosbestic point where the molar absorption coefficient of the two complexes have the common values of e3,.For any solution in which the tetrachloro- complex is absent eqn (2) has the form: On the other hand, for the solutions in which the octahedral complexes are absent, as indicated by an existence of the abovementioned isosbestic points, we have c4 - &,--e c E 3 - E 4 -- - (4) where E relates to any selected wavelength at which the molar absorption coefficients, e3 and E,, of CoC1,DMF- and CoCli-, respectively, are markedly different. The necessary values of E, were taken from the limiting spectrum, and those of E , were calculated as E, = (G)/C, from the absorption curves of the Co(ClO,),-Et,NCl solutions in which the tetrachloro-complexes were absent.The concentrations of CoC1,DMF-, C,, were calculated using eqn (3), valid for the wavelength of the isosbestic points. The mole fraction of the CoC1,DMF- complex in the CoC1, solutions amounts to 60% for the more concentrated solutions. Thus, the octahedral species must be the Co(DMF),2- solvo-complex. The same is indicated by the results obtained for the Co(ClO,),-Et,NC1 solutions; the chloride ions for the most part are consumed for CoC1,DMF- complex formation. An illustrative example is the mole fraction of the CoC1,DMF- complex calculated for the solution at the mole ratio of Et4NC1 to C0(C104), equal to 2.0. In this solution 65% of cobalt(r1) exists as the trichloro-complex.800 70 0 600 500 - I E c( 400 m E a IUJ \ 300 200 I 100 W.Grzybkowski and M. Pilarczyk 1709 1o.11 I A I 550 6 00 6 50 700 750 wavelength/nm Fig. 6. Absorption spectra of Co(ClO,),-Et,NCl solutions in DMF at 25 "C. The concentrations (mol dm-3) of Co(ClO,), and Et,NCl are, respectively: (1) 0.001 244, 0.002642; (2) 0.001 248, 0.003043; (3) 0.001 254,0.003366; (4) 0.001 261, 0.003627; ( 5 ) 0.001 240; 0.003957; (6) 0.001 247, 0.005626; (7) 0.001 236, 0.01 103; (8) 0.001 225, 0.021 51 ; (9) 0.001 226, 0.03271 ; (10) 0.001 268, 0.08858; and (11) 0.001241, 0.1808.1710 Ionization Equilibria in DMF 100 h E E .- % 50 5 E P) 0 c-. 0 0.1 1 10 100 Et 4 Ncl~~Co(clo4 )2 Fig. 7. The ranges of existence of the chloro-complexes of cobalt@) in DMF solutions of Co(ClO,), in the presence of Et,NCl at 25 "C.The fact that its abundance is somewhat higher than the corresponding value for the CoCl, solution is due to a presence of Et,N+ and C10, ions enhancing the ionic strength of the solution. The results presented suggest that the solute undergoes a coordinative disproportion- ation producing a complex electrolyte of the Co(DMF)i+ - 2CoC1,DMF- type, being the main form of existence of CoCl, in DMF solution. However, formation of the corresponding mono- and di-chloro-complexes cannot be ignored. This is indicated by the 60% abundance of the trichloro-complex in the CoCl, solution and some non- linearity of the plot of the abundance vs. mole ratio for the Co(ClO,),-Et,NCl solution. However, the lack of spectral evidence suggests their octahedral structure.The above results provide the possibility of calculating stability constants of all the complexes assumed to be formed in the solutions under consideration, provided that proper allowance can be made for the variation in the activity coefficients. In the calculation actually performed we used the data obtained for the Co(ClO,),-Et,NCl-DMF system. The results obtained for the three-component solutions were much more useful than the data derived for the CoC1, solution. For the latter system the differences in the intensity of the spectral bands are rather small at the higher concentration range, while the data obtained for the very dilute solutions are less accurate. In the computer analysis performed we assumed formation of the CoCl+, CoC1, and CoCl; formal complexes, the corresponding formation constants being defined as KO, = cn Yn (5) Cn-1 LC1-1 where n = 1, 2 or 3 and Yn is quotient of the respective activity coefficient.Variations in the activity coefficients were assumed to follow the Debye-Huckel equation involving the ion-size parameter, BH, the latter being estimated from conductivity data7 as 3.29. Taking into account the equations arising from the material balance for the cation and anion, we attempted to find the best set of KO, values describing the spectral properties of the three-component solutions in which the CoCli- complexes were absent. The resulting values of the logarithms of the formation constants of CoCF, CoC1, and CoC1; were 3.5k0.5, -2.Ok0.5 and 1l.Ok 1. The high uncertainty of the derived values is related to the very high stability of the CoC1,DMF- complex.The data suggest also that the existence of the dichloro-complex can be ignored, while the monochlorideW. Grzybkowski and M. Pilarczyk 171 1 complex is present in the solution containing the relatively low amount of the chloride ion donor. In order to complete the characteristics of the chloro-complexes of cobalt(I1) in DMF solution we calculated the formation constant of the CoCli- complex. The calculation was based on the C,/C values determined for the Co(C104),-Et,NC1 solutions at the range of the coexistence of the two tetrahedral complexes only. The corresponding logarithm is equal to 1.9kO.l. The equilibrium concentrations of the single complexes found at the same time for the different Et,NCl to Co(ClO,), ratios have been used for preparing the distribution diagram shown in fig.7. Commenting on the results obtained, we note that the derived formation constant of the CoC1,DMF- complex is much higher than that previously found for the trichloro- complexes of cobalt(I1) in DMSOs and DMA17 solutions. This striking difference cannot be related to the donor properties of the solvents, since their donor numbers are rather similar and close to the donor number of C1- ion.ls Conclusions The observed behaviour of CoC1, in DMF solutions is a consequence of the high stability of the CoC1,DMF- complex resulting in the formation of the Co(DMF);+ - 2CoC1,DMF- complex electrolyte. We note that >60% of cobalt(I1) exists in the form of the pseudotetrahedral trichloro-complex.In the previous paper1, from this laboratory it has been shown that the dominating form of an existence of NiCl, in DMF solution is the NiCl(DMF)'; NiC1,DMF- complex electrolyte and almost 50% of nickel@) exists as the trichloro-complex. It is interesting to compare the abovementioned coordination forms of CoCl, and NiCl, with those being formed in DMSO solutions. It has been shown by Libui et aL8 that the corresponding coordination forms are the CoCl(DMS0)'; * CoC1,DMSO- and NiCl(DMS0): - Cl- complex electrolytes. As is seen, CoCl, exhibits a significant tendency towards tetrahedral complex formation, while NiCl, prefers the octahedral structure. Similar abilities were found for CoBr, and NiBr, dissolved in a~etonitrile.~? lo However, it should be noted that the abovementioned complex electrolytes are formed at a higher concentration range.In very dilute solutions the simple ionization and association equilibria are responsible for the properties of the systems. The conclusion follows that the main factor governing the coordination states of transition metal salts in solution is the relative stability of tetrahedral and octahedral complexes. References 1 W. Libus, J . Solution Chem., 1981, 10, 631. 2 W. LibuS and H. Strzelecki, Electrochim. Acta, 1970, 15, 703; 1971, 16, 1749. 3 W. LibuS, B. Chachulski and L. Frqczyk, J. Solution Chem., 1980, 11, 355. 4 W. LibuS and M. Pilarczyk, Bull. Acad. Pol. Sci., Ser. Sci. Chim., 1972, 20, 359; 1974, 22, 717. 5 W. LibuS, W. Grzybkowski and R. Pastewski, J. Chem. Soc., Faraday Trans. 1, 1981,77, 147. 6 E. Kamienska and I. Uruska, Electrochim. Acta, 1977, 22, 181. 7 W. Grzybkowski and M. Pilarczyk, J . Chem. Soc., Faraday Trans. 1, 1983, 79, 2319. 8 W. LibuS, Electrochim. Acta, 1975, 20, 831; 1982, 27, 573. 9 W. LibuS, W. Grzybkowski and M. Walczak, Bull. Acad. Pol. Sci., Ser. Sci. Chim., 1970, 18, 141. 10 W. LibuS and W. Grzybkowski, Electrochim. Acta, 1978, 23, 791. 11 L. Katzin, J. Chem. Phys., 1962, 36, 3034. 12 M. Pilarczyk and L. Klinszporn, Electrochim. Acta, in press. 13 M. R. J. Dack, K. J. Bird and A. J. Parker, Aust. J . Chem., 1975, 28, 955. 14 D. Hamilton and R. H. Stokes, J. Solution Chem., 1972, 1, 213. 15 L. Sestili, C. Furlani and G. Festuccia, Inorg. Chim. Acta, 1970, 4, 542.1712 Ionization Equilibria in DMF 16 A. C. Covington and T. Dickinson, Physical Chemistry of Organic Solvent Systems (Plenum Press, 17 E. Kamienska and I. Uruska, Bull. Acad. Pol. Sci., Ser. Sci. Chim., 1976, 24, 576. 18 V. Gutman and U. Mayer, Monatsh., 1968,99, 1383. London, 1973), p. 677. Paper 5/759; Received 2nd July, 1985