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Potentiometric study of the Ni(II)-dihydrogenviolurate system in dimethylsulphoxide (DMSO)
1786
Notes
I B
574
578
582 586 590 Wovelenqth (nm)
594
598
602 448
Fig. I. Shapes of the 405/2, 2G7/2 *-- 419/2 band in Nd 3+ complexes of (A) ODA and (B) TMODA.
452
456 460 Wovelength (nm)
464
468
Fig. 3. Shapes of the ZHII/2('-'4II5/2 band in Er3+ complexes of (A) ODA and (B) TMODA.
Acknowledgement--We thank the Bangalore NMR facility for ghe ~3CNMR spectra. One of the authors (C.P.L.) is grateful to the authorities of the Indian Institute of Science for a scholarship. C. PREMLATHA S, SOUNDARARAJAN
Department of Inorganic and Physical Chemistry Indian Institute of Science Bangalore 560012 India
I 514
518
522 526 530 Wavelength (nm)
,534
538
Fig. 2, Shapes of the 5G6 *'-s/s band in Ho a+ complexes of (A) ODA and (B) TMODA, Based on the analytical, IR, ~H and '3C NMR and electronic spectral data, it is seen that the lanthanide ion is surrounded by three TMODA ligand molecules, each bound in a tridentate "O, O, O" fashion. A coordination number of nine is assignable to the Ln3+ ions in the complexes.
REFERENCES
1. D. K. Koppikar, P. V. Sivapullaiah, L. Ramakrishnan and S. Soundararajan, Struct. Bonding 34, 135 (1978). 2, E. Stein and O. Bayer, CA 52, p. 10183b (1952). 3. C. Premlatha and S. Soundararajan, Proc. Indian Acad. Sci. 8g(A), 291 (1979). 4. R. Jagannathan and S. Soundararajan, J. Coord. Chem. 9, 31 (1979). 5. C. K. JCrgensen, Prog. lnorg. Chem. 4, 73 (1962). 6. S. P. Sinha, Spectrochim. Acta 22, 57 (1966). 7. D. G. Karraker, lnorg. Chem. 6, 1863 (1967). 8. J. Albertsson, Acta. Chem. Scand. 22, 1563 (1968).
J. inorg, nucL Chem. Vol. 42, pp. 1786-1788
0022-1902/80/1201-1786/$02.00/0
~) Pergamon Press Ltd., 1980. Printed in Great Britain
Potentiometric study of the Ni(II)-dihydrogenvioinrate system in dimethylsulphoxide (DMSO) (Received 29 August 1979; in revised form 11 February 1980) We have recently isolated and characterized several violurateNi(II) complexes. The study of their stability in aqueous solution was made difficult because the 1 : 2 (metal ion: ligand) species was insoluble and the 1:3 one was not formed. Although solid Na[Ni(H2V)3], which was isolated in ethanolic solution, was soluble in water, the complex Ni(H2V)3- soon evolved to [Ni(H:V)2(H20)2]'3H20 precipitate[l]. Since this last compound was soluble in DMSO, this medium seemed to be a more appropriate solvent to study the formation equilibria of the Ni(II)violurate complexes. Only little has been published on the determination of stability
constants of metal complexes in DMSO in spite of its increasing use as a solvent. Schriver[2] studied spectrophotometrically the Co(II)-SCN- and Co(II)-Cl- systems. Ahrland et al. [3] began a thermodynamic study of complex formation in DMSO with ligands which are coordinated through N, P, As, Sb or Bi. The stability constants are potentiometr/cally determined using metal or amalgam electrodes. Potentiometric determinations of acidity constants in DMSO of very weak acids[4-7] have been carried out in the last years using a more or less modified glass electrode. However, as far as we know, it has not been used in the study of the metal ion-ligand systems.
Notes Here we report the potentiometric determination of the stability constants of the Ni(II)-H:V- system in DMSO solution using a glass electrode as a hydrogen-ion concentration probe.
EXI~AL Violuric acid is synthesized as described in Ref. [8]. The analytical results on C, H and H20 agree with the calculated values for (C4H3N304)'HzO. Sodium dihydrogenviolurate dihydrate, which is used in the spectropbotometric study, is obtained by crystallization from an aqueous solution of violuric acid neutralized with sodium hydroxide. Ni(NO~h'6H20 and all the other chemicals are Merck reagents p. a. Absorption spectra are registered with UV-V Pye-Unicam SP 100-8 spectrophotometer and data between 900-1200nm with a Beckman DU. 0.3 M NaCIO4 is used as a background electrolyte. A 0.35M tetra-n-butylammonium hydroxide solution in DMSO, recently prepared from BDH reagent, is used as a titrant. Potential measurements are performed with a Radiometer 26 pHmeter, using a combined glass electrode GK 2401 C in which we have changed the saturated aqueous KC1 solution of the reference electrode by a saturated methanolic KCI one. The electrode is immersed in DMSO solution for several days before using it. Measurements are carried out at 25.0_+0.1°C in N2 atmosphere and in 1 M NaCIO4. Nernst's equation, E = E ~'+ 0.059 log [H+], is strictly obeyed by the electrode under the conditions mentioned above, at least, over the hydrogen-ion concentration range between 10-eM and 5 x 10-TM. Apparent deviations are observed if a rough change is produced in [H+] because of the slow response of the electrode. Accurate concentration of basic titrant as well as E °' value, are determined with a 4-toluenesulfonic acid DMSO solution before each titration series. Reliable potential measurements are obtained between 5 and 10 rain after each basic solution addition. Further we have used the experimental techniques and the data treatment which are generally recommended[9].
RESULTS AND DISCUSSION
Up to now, no systematic research has been carried out on potentiometric measurements of hydrogen-ion concentration in DMSO medium. However, several modifications of the glass electrode have been suggested in order to determine acidity constants of weak acids. These changes consist of the replacement of the saturated KCI solution in the reference electrode by a methanolic one[4-6], or even more of the internal aqueous
1787
solution of glass membrane by 4-toluenesulfonic acid in DMSO or triply-destiUed mercury['/]. An electrode of the first kind has been used for this work. We have previously determined the/(as value of violuric acid in DMSO titrating a 10-z M solution with tetra-n-butylammonium hydroxide. The obtained value 3.72 × 10-~ (25°C, I M NaCIO4) is lower than the observed one in aqueous solution[10]. A titration of Ni(II)-violuric acid in DMSO solution allows to obtain the formation curve. The overall constants//,, have been computed by the extrapolation method suggested by Rossotti and Rossotti[ll]. The obtained values are ~1=9.2x103; /32= 3.1x107; //3=4.5×10 I° (25.0-+0.1°C; 1M NaCIO4). The theoretical formation curve calculated with the computed constants fits very well the experimental data ~(1 g [H2V-]). The value flz=4x I0¢, which was determined spectrophotometrically in aqueous solution at pH = 8.6112], is the only previous datum on the stability of these complexes. However, these data should not be directly compared because a deprotonation and stereochemical change in the 1:2 complex is produced in basic aqueous solution [1]. The distribution diagram a(lg [HaV-]) for the species in solution is plotted in Fig. I. The values of the stepwise stability constants, K , ( K I = 9.2 x 103; Kz = 3.4 × 103; /(3 = 1.45x 103), show the expected sequence in the successive formation of complexes when there is not any change in their coordination number. The three species are octahedral according to their absorption spectra. We have registered the absorption spectra between 25,000 and 8000cm-~ of a series of solutions where the concentration of Ni(lI) is kept constant and the molar ratio [HzV-]:[Ni(II)] is varied. The spectra obtained show a strong absorption in the UV region which decreases in the visible giving a shoulder at 19,200cm-l and a broad band at 12,500cm-L The maximum of this band is shifted to longer wavelengths when the concentration of H2V- is increased. The saturation spectrum is characterized by ~.max at 12,190cm-z and ~ =96.3 I.mof-Lcm-'. This spectrum is identical to the reflectance spectrum of the solid Na[Ni(H2V)3]. Thus, the formation of Ni(H2V); from [Ni(H2Vh(DMSO)z] in DMSO solution produces a shift of the absorption band to longer wavelengths against expectation because the anion H2V- is a strong field ligand. A similar phenomenon has previously observed and discussed in the [Ni(H:Vh(H:O)z] and Na [Ni(H2V)3] reflectance spectra[l]. The abnormal Ao value of the 1:3 complex has not any apparent effect on their stability. Therefore it is suggested that the insolubility of the 1:2 complex in aqueous solution is the main factor which determines the inexistence of the I : 3 complex in that medium.
ID'
05
-5
-4
i
log [H2V-]
Fig. I. Distribution diagram a (log[H2V-]) for Ni(II)-dihydrogenviolurate system (1) ao, (2) al, (3) a2, (4) a3.
1788
Notes
Acknowledgement--We thank the Comisiofi Asesora de Investigaci6n Cientffica y T~cnia de la Presidencia del Gobierno (Spain) for financial support of this work. Departamento de Qufmica Inorg6nica Facultad de Ciencias Qufmicas de la Universidad de Valencia Spain J. FAUS J. MORATAL M. JULVE RIgFIgRENCES !. J. Moratal and J. Faus, Rev. Chim. Min. 16, 203 (1979). 2. A. Schriver, Bull. Soc. Chim. Fr. ll (1973).
3. S. Ahrland, T. Berg and P. Trinderup, Acta Chem. Scand. A31,775 (1977); A32, 933 (1978). 4. K. K. Barnes and C. K. Mann, Anal. Chem. 36, 2502 (1964). 5. D. L. Wetzel and C. E. Meloan, Anal. Chem. 36, 2474 (1964). 6. R. Morales, Anal. Chem. 40, 1148 (1968). 7. C. D. Ritchie and R. E. Uschold, L Am. Chem. Soc. 89, 1721 (1967). 8. (a) M. Ceresole, Chem. Ber 16, 1133 (1888); (b) C. Guinchard, Chem. Ber. 32, 1723 (1899). 9. H. S. Rossotti, Talanta 21, 809 (1974). 10. (a) P. A. Leermakers and W. A. Hoffman, J. Am. Chem. Soc. 80, 5663 (1958); (b) S. Sueur, C. Bremard and G. Nowogrocld, J. lnorg. Nucl. Chem. 38, 2037 (1976). 1I. F. J. C. Rossotti and H. S. Rossotti, Determination of Stability Constants, p. 110. McGraw-Hill (1961). 12. L. Ershova and V. V. Noskov, Zh. Anal. Khim. 26, 2406 (1971).
Notes
I B
574
578
582 586 590 Wovelenqth (nm)
594
598
602 448
Fig. I. Shapes of the 405/2, 2G7/2 *-- 419/2 band in Nd 3+ complexes of (A) ODA and (B) TMODA.
452
456 460 Wovelength (nm)
464
468
Fig. 3. Shapes of the ZHII/2('-'4II5/2 band in Er3+ complexes of (A) ODA and (B) TMODA.
Acknowledgement--We thank the Bangalore NMR facility for ghe ~3CNMR spectra. One of the authors (C.P.L.) is grateful to the authorities of the Indian Institute of Science for a scholarship. C. PREMLATHA S, SOUNDARARAJAN
Department of Inorganic and Physical Chemistry Indian Institute of Science Bangalore 560012 India
I 514
518
522 526 530 Wavelength (nm)
,534
538
Fig. 2, Shapes of the 5G6 *'-s/s band in Ho a+ complexes of (A) ODA and (B) TMODA, Based on the analytical, IR, ~H and '3C NMR and electronic spectral data, it is seen that the lanthanide ion is surrounded by three TMODA ligand molecules, each bound in a tridentate "O, O, O" fashion. A coordination number of nine is assignable to the Ln3+ ions in the complexes.
REFERENCES
1. D. K. Koppikar, P. V. Sivapullaiah, L. Ramakrishnan and S. Soundararajan, Struct. Bonding 34, 135 (1978). 2, E. Stein and O. Bayer, CA 52, p. 10183b (1952). 3. C. Premlatha and S. Soundararajan, Proc. Indian Acad. Sci. 8g(A), 291 (1979). 4. R. Jagannathan and S. Soundararajan, J. Coord. Chem. 9, 31 (1979). 5. C. K. JCrgensen, Prog. lnorg. Chem. 4, 73 (1962). 6. S. P. Sinha, Spectrochim. Acta 22, 57 (1966). 7. D. G. Karraker, lnorg. Chem. 6, 1863 (1967). 8. J. Albertsson, Acta. Chem. Scand. 22, 1563 (1968).
J. inorg, nucL Chem. Vol. 42, pp. 1786-1788
0022-1902/80/1201-1786/$02.00/0
~) Pergamon Press Ltd., 1980. Printed in Great Britain
Potentiometric study of the Ni(II)-dihydrogenvioinrate system in dimethylsulphoxide (DMSO) (Received 29 August 1979; in revised form 11 February 1980) We have recently isolated and characterized several violurateNi(II) complexes. The study of their stability in aqueous solution was made difficult because the 1 : 2 (metal ion: ligand) species was insoluble and the 1:3 one was not formed. Although solid Na[Ni(H2V)3], which was isolated in ethanolic solution, was soluble in water, the complex Ni(H2V)3- soon evolved to [Ni(H:V)2(H20)2]'3H20 precipitate[l]. Since this last compound was soluble in DMSO, this medium seemed to be a more appropriate solvent to study the formation equilibria of the Ni(II)violurate complexes. Only little has been published on the determination of stability
constants of metal complexes in DMSO in spite of its increasing use as a solvent. Schriver[2] studied spectrophotometrically the Co(II)-SCN- and Co(II)-Cl- systems. Ahrland et al. [3] began a thermodynamic study of complex formation in DMSO with ligands which are coordinated through N, P, As, Sb or Bi. The stability constants are potentiometr/cally determined using metal or amalgam electrodes. Potentiometric determinations of acidity constants in DMSO of very weak acids[4-7] have been carried out in the last years using a more or less modified glass electrode. However, as far as we know, it has not been used in the study of the metal ion-ligand systems.
Notes Here we report the potentiometric determination of the stability constants of the Ni(II)-H:V- system in DMSO solution using a glass electrode as a hydrogen-ion concentration probe.
EXI~AL Violuric acid is synthesized as described in Ref. [8]. The analytical results on C, H and H20 agree with the calculated values for (C4H3N304)'HzO. Sodium dihydrogenviolurate dihydrate, which is used in the spectropbotometric study, is obtained by crystallization from an aqueous solution of violuric acid neutralized with sodium hydroxide. Ni(NO~h'6H20 and all the other chemicals are Merck reagents p. a. Absorption spectra are registered with UV-V Pye-Unicam SP 100-8 spectrophotometer and data between 900-1200nm with a Beckman DU. 0.3 M NaCIO4 is used as a background electrolyte. A 0.35M tetra-n-butylammonium hydroxide solution in DMSO, recently prepared from BDH reagent, is used as a titrant. Potential measurements are performed with a Radiometer 26 pHmeter, using a combined glass electrode GK 2401 C in which we have changed the saturated aqueous KC1 solution of the reference electrode by a saturated methanolic KCI one. The electrode is immersed in DMSO solution for several days before using it. Measurements are carried out at 25.0_+0.1°C in N2 atmosphere and in 1 M NaCIO4. Nernst's equation, E = E ~'+ 0.059 log [H+], is strictly obeyed by the electrode under the conditions mentioned above, at least, over the hydrogen-ion concentration range between 10-eM and 5 x 10-TM. Apparent deviations are observed if a rough change is produced in [H+] because of the slow response of the electrode. Accurate concentration of basic titrant as well as E °' value, are determined with a 4-toluenesulfonic acid DMSO solution before each titration series. Reliable potential measurements are obtained between 5 and 10 rain after each basic solution addition. Further we have used the experimental techniques and the data treatment which are generally recommended[9].
RESULTS AND DISCUSSION
Up to now, no systematic research has been carried out on potentiometric measurements of hydrogen-ion concentration in DMSO medium. However, several modifications of the glass electrode have been suggested in order to determine acidity constants of weak acids. These changes consist of the replacement of the saturated KCI solution in the reference electrode by a methanolic one[4-6], or even more of the internal aqueous
1787
solution of glass membrane by 4-toluenesulfonic acid in DMSO or triply-destiUed mercury['/]. An electrode of the first kind has been used for this work. We have previously determined the/(as value of violuric acid in DMSO titrating a 10-z M solution with tetra-n-butylammonium hydroxide. The obtained value 3.72 × 10-~ (25°C, I M NaCIO4) is lower than the observed one in aqueous solution[10]. A titration of Ni(II)-violuric acid in DMSO solution allows to obtain the formation curve. The overall constants//,, have been computed by the extrapolation method suggested by Rossotti and Rossotti[ll]. The obtained values are ~1=9.2x103; /32= 3.1x107; //3=4.5×10 I° (25.0-+0.1°C; 1M NaCIO4). The theoretical formation curve calculated with the computed constants fits very well the experimental data ~(1 g [H2V-]). The value flz=4x I0¢, which was determined spectrophotometrically in aqueous solution at pH = 8.6112], is the only previous datum on the stability of these complexes. However, these data should not be directly compared because a deprotonation and stereochemical change in the 1:2 complex is produced in basic aqueous solution [1]. The distribution diagram a(lg [HaV-]) for the species in solution is plotted in Fig. I. The values of the stepwise stability constants, K , ( K I = 9.2 x 103; Kz = 3.4 × 103; /(3 = 1.45x 103), show the expected sequence in the successive formation of complexes when there is not any change in their coordination number. The three species are octahedral according to their absorption spectra. We have registered the absorption spectra between 25,000 and 8000cm-~ of a series of solutions where the concentration of Ni(lI) is kept constant and the molar ratio [HzV-]:[Ni(II)] is varied. The spectra obtained show a strong absorption in the UV region which decreases in the visible giving a shoulder at 19,200cm-l and a broad band at 12,500cm-L The maximum of this band is shifted to longer wavelengths when the concentration of H2V- is increased. The saturation spectrum is characterized by ~.max at 12,190cm-z and ~ =96.3 I.mof-Lcm-'. This spectrum is identical to the reflectance spectrum of the solid Na[Ni(H2V)3]. Thus, the formation of Ni(H2V); from [Ni(H2Vh(DMSO)z] in DMSO solution produces a shift of the absorption band to longer wavelengths against expectation because the anion H2V- is a strong field ligand. A similar phenomenon has previously observed and discussed in the [Ni(H:Vh(H:O)z] and Na [Ni(H2V)3] reflectance spectra[l]. The abnormal Ao value of the 1:3 complex has not any apparent effect on their stability. Therefore it is suggested that the insolubility of the 1:2 complex in aqueous solution is the main factor which determines the inexistence of the I : 3 complex in that medium.
ID'
05
-5
-4
i
log [H2V-]
Fig. I. Distribution diagram a (log[H2V-]) for Ni(II)-dihydrogenviolurate system (1) ao, (2) al, (3) a2, (4) a3.
1788
Notes
Acknowledgement--We thank the Comisiofi Asesora de Investigaci6n Cientffica y T~cnia de la Presidencia del Gobierno (Spain) for financial support of this work. Departamento de Qufmica Inorg6nica Facultad de Ciencias Qufmicas de la Universidad de Valencia Spain J. FAUS J. MORATAL M. JULVE RIgFIgRENCES !. J. Moratal and J. Faus, Rev. Chim. Min. 16, 203 (1979). 2. A. Schriver, Bull. Soc. Chim. Fr. ll (1973).
3. S. Ahrland, T. Berg and P. Trinderup, Acta Chem. Scand. A31,775 (1977); A32, 933 (1978). 4. K. K. Barnes and C. K. Mann, Anal. Chem. 36, 2502 (1964). 5. D. L. Wetzel and C. E. Meloan, Anal. Chem. 36, 2474 (1964). 6. R. Morales, Anal. Chem. 40, 1148 (1968). 7. C. D. Ritchie and R. E. Uschold, L Am. Chem. Soc. 89, 1721 (1967). 8. (a) M. Ceresole, Chem. Ber 16, 1133 (1888); (b) C. Guinchard, Chem. Ber. 32, 1723 (1899). 9. H. S. Rossotti, Talanta 21, 809 (1974). 10. (a) P. A. Leermakers and W. A. Hoffman, J. Am. Chem. Soc. 80, 5663 (1958); (b) S. Sueur, C. Bremard and G. Nowogrocld, J. lnorg. Nucl. Chem. 38, 2037 (1976). 1I. F. J. C. Rossotti and H. S. Rossotti, Determination of Stability Constants, p. 110. McGraw-Hill (1961). 12. L. Ershova and V. V. Noskov, Zh. Anal. Khim. 26, 2406 (1971).