Radiochim. Acta 94, 859–864 (2006) / DOI 10.1524/ract.2006.94.12.859 © by Oldenbourg Wissenschaftsverlag, München
Studies on protonation and Th(IV) complexation behaviour of dihydroxybenzenes in aqueous 1 M NaClO4 medium By U. K. Thakur1 , D. Shah1 , R. S. Sharma2 , R. M. Sawant1 and K. L. Ramakumar1 , ∗ 1 2
Radioanalytical Chemistry Section, Radiochemistry & Isotope Group, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India Research Reactor Services Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India
(Received November 15, 2005; accepted in revised form May 15, 2006)
Thorium / Dihydroxybenzene / Catechol / Resorcinol / Hydroquinone / Stability constant / Protonation constant / Thermodynamic database
Summary. Complexing behaviour of three dihydroxybenzenes (DHB) namely, hydroquinone, resorcinol and catechol with Th(IV) along with their protonation constants were studied in 1 M NaClO4 medium at 23 ± 0.5 ◦ C, using pH titration technique. Both protonation and metal-ligand equilibrium constants of dihydroxybenzenes were computed using advance software suite of program HYPERQUAD. Logarithmic values of overall protonation constants (log β1H and log β2H ) were found to be 11.47 ± 0.05 and 21.45 ± 0.06, for hydroquinone, 11.76 ± 0.04 and 20.98 ± 0.04 for resorcinol and 12.58 ± 0.03 and 21.87 ± 0.08 for catechol respectively. Complex formation has been investigated in the pH range 2 to 4. The logarithmic values of metal-ligand equilibrium constants obtained in the present work were β1 3 −1 = 48.51 ± 0.67 and β1 4 −1 = 64.86 ± 1.25 for hydroquinone, β1 1 0 = 16.98 ± 0.16, β1 3 −1 = 46.46 ± 0.18 and β1 4 −2 = 59.65 ± 0.20 for resorcinol and β1 1 −1 = 14.06 ± 0.10 for catechol. The literature values were reviewed.
1. Introduction Over the last 30 years there has been interest in utilizing thorium as a nuclear fuel since it is three times as abundant in the earth’s crust as uranium. Also, all of the mined thorium is potentially useable in a reactor, compared with the 0.7% of natural uranium, so some 40 times the amount of energy per unit mass might be available. Thorium may therefore, dominate the future nuclear industry as a fuel in future advanced reactors. With about six times more thorium than uranium, India has made utilization of thorium for large-scale energy production a major goal in its nuclear power program, utilizing a three-stage concept: (i) Pressurized Heavy Water Reactors (PHWRs,) fuelled by natural uranium generate plutonium. (ii) Fast Breeder Reactors (FBRs) use this plutonium-based fuel to breed U-233 from thorium, and then (iii) Advanced Heavy Water Reactors burn the U-233 with thorium, getting about 75% of their power from the thorium. Invariably environmental and biological considerations may eventually play an important role in assessing the large*Author for correspondence (E-mail:
[email protected]).
scale utilization of thorium on biological and environmental systems. These issues are also being addressed contemporarily. For example, various treatment and procedures are being developed against radiothorium poisoning. Internal contamination occurs when radiothorium is ingested, inhaled, or absorbed from a contaminated wound. The work has already been undertaken by many research groups to explore different sequestering agents for decorporating the radiothorium from biological systems. Emphasis is now on synthesizing relatively specific and very strong complexing sequestering agents. One important consideration for metal decorporation therapy is the relative affinity of the ligand for the target metal as compared to its affinity for essential biological metal ions which are required for the body. The main problem associated with currently used diethylenetriaminepentaacetic acid (DTPA) for actinide decorporation in humans is the lack of specificity. The derivatives of hydroquinone, resorcinol and catechol are promising decorporating agents [1] for radiothorium through urine and feces from biological systems. Dihydroxybenzenes (DHBs) are rapidly absorbed through the digestive tract. Their absorption through skin is low in aqueous solution. After absorption they distribute rapidly and widely among tissues but bioaccumulation is low. The metabolism of hydroquinone has been studied and a complete materials balance has been reported [2]. The radiolytic effect on hydroquinone in methanol solution at different doses of gamma irradiation has also been studied [3] and radiolytic effects were found to be independent of irradiation doses. Catechol is a well known bidentate chelating agent for its particular affinity towards metal ions that exhibits high oxidation states or high charge to metal ions radius ratios. Though the catechol series is a proven group of sequestering agents, significant efforts are however, being directed towards hydroquinone based liogands due to their low toxicity [2, 4]. The lack of sufficient information on physicochemical parameters of their complexes, protonation and stability constants in particular, has hampered the further progress in this field. The work was, therefore, undertaken to characterize Th(IV)-DHBs complexes by model selection criteria built into HYPERQUAD [5]. Th(IV)-DHB system also serves as a convenient model for Pu(IV) binding in biological systems. This work may therefore help in predicting Pu(IV)DHBs complexation behaviour.
860
2. Experimental 2.1 Reagents Perchlorate solution of thorium was prepared by repeatedly evaporating a portion of nuclear grade nitrates of the metal ions with HClO4 and finally taking up in 4 M HClO4 . Metal ion concentration was determined by EDTA titration. Free acid concentration of the thorium stock solution was determined by Gran’s method [6] by titrating a small portion of metal stock solution against standard alkali.
2.2 Dihydroxybenzene solutions Analytical grade, vacuum dried, dihydroxybenzenes (hydroquinone, resorcinol and catechol, C6 H6 O2 , M. Wt. 110.11) were dissolved in water to prepare 0.01 M solutions. Appropriate amount of NaClO4 was added to adjust the ionic strength to 1 M and the solution was covered with black paper. Only freshly prepared solutions were used.
2.3 NaClO4 solution A standardized solution of A.R. grade Sodium Perchlorate was used for maintaining ionic strength.
2.4 NaOH solution Analytical grade NaOH was dissolved in distilled water containing NaClO4 so as to adjust the final ionic strength at 1.0 M. It was standardized against dried potassium hydrogen phthalate.
2.5 Apparatus The potentiometric titrations were performed using an ORION 940 pH-meter equipped with a ORION 960 autotitrator capable of dispensing as low as 0.05 ml of titrant with a 0.1% precision. The autotitrator was coupled to a personal computer through RS-232 serial port for data acquisition. The software interface to computer can acquire pH, millivolts and the volume of titrant, simultaneously. A preliminary data analysis including Gran analysis can also be performed to find the end point of the titrations. A combination glass electrode ORION-910600 was used for potentiometric measurements. Electrode response can be read to the third place of decimal in terms of pH units with a precision ±0.001 and the potential with a precision of ±0.1 mV.
2.6 Titration procedure Each set of experiments essentially constituted the following steps, i) calibration of pH electrode: titration of HClO4 against standard NaOH, ii) determination of pK a : titration of DHB against standard NaOH and iii) determination of equilibrium constants: titration of thorium(IV) containing DHB solution with standard NaOH, all under the similar experimental conditions. For pK a determination, 25 ml of 0.01 M DHB solution in 1.0 M NaClO4 ionic medium was taken in the titration vessel covered with a black paper and suitable aliquots of standard sodium hydroxide solution, in the
U.K. Thakur et al.
same ionic strength medium, were added progressively with constant time duration for a titration. The titrant volumes and the potentials/pH of the glass electrode were recorded and stored in the computer. For equilibrium constants determination, 30 ml of thorium perchlorate and DHB mixture adjusted to 1.0 M ionic strength using NaClO4 , was taken in the titration vessel. All measurements were made at room temperature (23 ± 0.5 ◦ C) in 1.0 M NaClO4 and the potential readings were stable within ±0.2 mV.
3. Results and discussions 3.1 Complex formation The protonation constants βnH for equilibrium reactions can be represented as βnH
nH + L Hn L (where n = 1, 2) βnH =
[Hn L] . [L][H]n
The complex formation between a metal ion and the ligand and/or hydroxide is represented by the reaction mMz+ + nL− + pOH− = Mm Ln (OH) p (mz−n−p)+ Therefore equilibrium constants for overall reaction can be written as [Mm Ln (OH) p (mz−n−p)+ ] βmn p = [Mz+ ]m [L− ]n [OH− ]p The equilibrium constants, βmn p relate to the differences in the Gibbs free energies of products and reactants in their standard states. The use of computer has revolutionized the calculation of stability constants from equilibrium data and very complex systems can now be handled with relative ease. A highly acclaimed Hyperquad suite of software programs (windows XP based version) was used for stability constant calculations. Hyperquad minimizes the sum of squares of differences between calculated and experimental parameters (mass balance equations) using non-linear least square technique. Several model species following coordination chemistry principles were used as initial input for iterative refinement cycles. Statistical parameters (e.g. goodness-of-fit, chi-squares) and other error analysis techniques were used in selecting the best model. However, complex model species were selected with utmost care so as to avoid the postulation of unjustified species.
3.2 Protonation constants Protonation constants needed to calculate the metal ligand equilibrium constants were evaluated under identical experimental conditions. Gran’s method of titration was performed each time to evaluate the formal electrode potential of the glass electrode, E 0 . The reliability of the electrode system was confirmed by determining the dissociation constant (pK a ) of acetic acid and ionization constant of water, pK w .
861
Studies on protonation and Th(IV) complexation behaviour of dihydroxybenzenes in aqueous 1 M NaClO4 medium Table 1. Protonation constants of dihydroxybenzene in 1.0 M NaClO4 medium. Temperature: 23 ± 0.5 ◦ C; Titrate: 25 ml of 10 mM dihydroxybenzene; Titrant: 0.1 M NaOH.
Dihydrobenzene
Set
log β1H
Mean log β1H
log β2H
Mean log β2H
Hydroquinone
I II III I II I II III IV V VI
11.46 ± 0.07 11.47 ± 0.01 11.47 ± 0.03 11.76 ± 0.01 11.76 ± 0.05 12.56 ± 0.08 12.56 ± 0.06 12.59 ± 0.03 12.57 ± 0.06 12.58 ± 0.09 12.65 ± 0.09
11.47 ± 0.05
21.46 ± 0.01 21.48 ± 0.01 21.42 ± 0.03 20.98 ± 0.01 20.98 ± 0.05 21.99 ± 0.08 21.84 ± 0.06 21.94 ± 0.06 21.84 ± 0.10 21.85 ± 0.13 21.78 ± 0.10
21.45 ± 0.06
Resorcinol Catechol
11.76 ± 0.04 12.58 ± 0.03
20.98 ± 0.04 21.87 ± 0.08
Table 2. Comparison of protonation constants of dihydroxybenzenes with literature values. Dihydrobenzene Hydroquinone
Resorcinol
Catechol
Method a
I Medium (M)
T K
log β1
log β2
Reference
gl gl gl gl gl gl gl gl gl gl gl sp gl gl gl gl gl gl gl gl gl gl gl gl sp gl gl gl gl gl gl gl gl gl gl gl
1 (NaClO4 ) 1 (NaClO4 ) 1 (NaClO4 ) 1.0(NaClO4 ) 1.0(NaClO4 ) 1.0(NaClO4 ) 0.10(KNO3 ) 0.20(NaClO4) 1.0(NaClO4 ) 0.1(KNO3 ) 0.1(NaClO4 ) 0.1(KCl) 0.1(NaClO4 ) 0.1(KNO3 ) 1 (KNO3 ) 0.1(oth/un) 0.1(KNO3 ) 0.1(NaClO4 ) 0.2(NaClO4 ) 0.1(NaClO4 ) 0.1(oth/un) 0.2(NaClO4 ) 0.1(NaClO4 ) 0.1(KNO3 ) 0.1(oth/un) 0.2(NaClO4 ) 0.2(KCl) 0.1(KNO3 ) 0.2(NaClO4 ) 0.1(oth/un) un(NaClO4 ) 0.2(KNO3 ) 0.1(NaClO4 ) 0.05(KNO3 ) 0.2(NaClO4 ) 0.2(NaClO4 )
296 298 273 298 298 296 293 301 296 298 303 298 298 295 298 298 298 298 298 298 298 298 298 298 368 298 298 298 298 300 298 298 298 298 298 308
11.47 ± 0.05 11.45 11.50 11.39 8.7 11.76 ± 0.04 11.07 11.06 12.58 ± 0.03 11.93 12.6 13.1 12.62 12.60 13.05 13.00 11.66 13.00 11.93 12.8 12.6 11.72 13.00 12.98 13.00 11.78 13.00 11.49 11.65 13.00 11.49 13.00 11.75 11.80 11.70 11.61
21.45 ± 0.06 21.30 21.39
This Work [10] [11] [12] [13] This Work [14] [15] This Work [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42]
14.20 20.98 ± 0.04 20.14 20.36 21.87 ± 0.08 21.13 21.94 22.49 21.95 21.94 22.28 22.32 20.92 22.32 21.13 22.25 21.94 20.94 22.32 22.18 22.32 21.05 22.28 20.76 20.75 22.22 21.09 22.23 21.01 21.25 21.00 20.69
a: gl: Glass Electrode, sp: spectrophotometry.
The values of the protonation constants of dihydroxybenzenes obtained in the present work along with literature values are listed in Tables 1 and 2. Hydraph a module of Hyperquad is very useful for generating calculated titration curves using refined equilibrium constants. A good agreement between experimental and calculated titration curves (Fig. 1) indicated the reliability of the experimental data. The values of protonation constants β1H obtained in this work follow the order, hydroquinone < resorcinol < catechol which differs from the order displayed by litera-
ture values given in Table 2. It should be mentioned that the lower literature values obtained for resorcinol may be attributed to different medium and the temperature in which the data obtained. The lower basicity of hydroquinone is a consequence of the fact that the protonation necessarily occurs in a position para to the hydroxyl group. Whereas resorcinol is the more effective base due to the participation of both oxygen atoms in the stabilization of the protonated form. Higher basicity of resorcinol was revealed by ab initio gas-phase
862
U.K. Thakur et al.
Fig. 1. Experimental and calculated titration curves. Titrate: 25 ml of 10 mM dihydroxybenzene; Titrant: 0.1 M NaOH, Medium: 1 M NaClO4 .
Fig. 2. Plots of pH vs. Volume of NaOH along with ∆ pH. Titrate: 30 ml of 1.07 mM Thorium + 8.3 mM dihydroxybenzene; Titrant: 0.1 M NaOH; Mole ratio (Th : DHB) = 1 : 8; Medium: 1 M NaClO4 .
quantum chemical calculations [7] at the MP2/6-31G(d) and B3LYP/6-31G(d) levels due to participation of both oxygen atoms in the stabilization of protonated form. Furthermore, the protonation reaction was found to decrease the entropy by freezing of the two internal rotations. Predominantly intra molecular hydrogen bonding of hydroxyl group at ortho- position in catechol in liquid-phase effectively retards the first de-protonation resulting in higher β1H . Alternately the de-protonation should be expected to follow the reverse order namely hydroquinone > resorcinol > catechol and the corresponding ∆G values calculated from thermodynamic data should also show similar trend. Fujio et al. [8] did report similar trend of ∆r G values for the de-protonation reactions of the DHBs (∆r G = 1436 kJ/mol for hydroquinone ∆r G = 1422 kJ/mol for resorcinol ∆r G = 1392 kJ/mol for catechol). These thermodynamic data thus support the β1H values reported in the present work. The β2H , however, follow the order, resorcinol < hydroquinone < catechol. The phenoxide ion, (resonance hybrid) of resorcinol allows some delocalizing of electrons in the structure, resulting in less electron density at meta positions
that probably explains the lower magnitude of β2H for resorcinol. The dihydroxybenzenes are very weak acids and the values of log β1H indicate that the phenolic group H+ is dissociable only above pH 11. But the hard cations like tetravalent metal ions can replace the phenolic group hydrogen even below pH 6.
3.3 Complex formation of thorium(IV) Table 3 lists the stability constants of dihydroxybenzenes with Th(IV). The typical titration profiles of volume vs. pH along with ∆pH (∆pH = observed pH − calculated pH) for three DHBs are shown in Fig. 2. The maximum spread of ∆pH is well within ±0.08 of pH. No data on the stability constants of Th(IV) with DHBs are available in the literature except the single publication on catechol [9] where the authors presumed single mononuclear species and evaluated the stability constants employing Bjerrum’s enbar method. No attempt was made to analyse the data with the hydrolytic species. The present study was carried out for two different ligand to metal ratios of 8 : 1
863
Studies on protonation and Th(IV) complexation behaviour of dihydroxybenzenes in aqueous 1 M NaClO4 medium
Table 3. Stability Constants of Thorium(IV)-dihydroxybenzenes. Medium: 1 M NaClO4 Temperature: 23 ± 0.5 ◦ C; Titrate: 30 ml of 1.07 & 2.14 mM Thorium + 8.3 mM dihydroxybenzene; Titrant: 0.1 M NaOH; Mole ratio (Th : DHB) = 1 : 8 and 1 : 4 Ligand
Set No.
Hydroquinone
I II III IV V VI VII VIII IX X Mean I II III IV Mean I II III IV V VI VII Mean
Resorcinol
Catechol
log β1 1 0
log β1 1 −1
log β1 3 −1
log β1 4 −1
log β1 4 −2
– – – – – – – – – –
– – – – – – – – – – – – – – – 13.94 ± 0.03 13.98 ± 0.07 14.01 ± 0.09 14.04 ± 0.07 14.12 ± 0.04 14.03 ± 0.02 14.14 ± 0.04 14.06 ± 0.10
63.07 ± 0.14 63.14 ± 0.95 63.32 ± 0.13 65.78 ± 0.08 66.19 ± 0.06 66.10 ± 0.06 65.14 ± 0.08 65.52 ± 0.05 65.74 ± 0.08 64.55 ± 0.11 64.86 ± 1.25 – – – – – – – –
– – – – – – – – – –
17.05 ± 0.07 16.77 ± 0.06 17.12 ± 0.09 16.99 ± 0.09 16.98 ± 0.16 – – –
47.62 ± 0.04 47.87 ± 0.08 47.56 ± 0.06 48.72 ± 0.05 49.43 ± 0.08 49.34 ± 0.04 48.62 ± 0.07 48.70 ± 0.04 49.05 ± 0.06 48.22 ± 0.06 48.51 ± 0.67 46.47 ± 0.06 46.24 ± 0.08 46.67 ± 0.11 46.46 ± 0.11 46.46 ± 0.18 – – –
59.78 ± 0.09 59.42 ± 0.05 60.06 ± 0.12 59.33 ± 0.12 59.65 ± 0.20 – – –
–
–
–
–
and 4 : 1 for all the three systems. Various models conforming to species Mm Ln (OH) p (mz−n−p)+ (where m = 1 to 3, n = 0 to 6 and p = −4 to 2) were submitted to Hyperquad. Model consisting of hydrolyzed species converged with better statistics (Table 3). Others converged with either poor statistics or rejected or did not lead to convergence at all. The stability constants β1 3 −1 , β1 4 −1 corresponding to [Th(L)3 (OH)]3− and [Th(L)4 (OH)]5− were refined among the various model species invoked for hydroquinone. Probably, the formation of 1 : 1 and 1 : 2 complexes might occur in lower pH region. Formation of the above species was consistent even with varying metal and ligand concentrations. No polymeric species were indicated under the present experimental conditions. The best models obtained for resorcinol consisted of β1 1 0 β1 3 −1 , β1 4 −2 for corresponding species [Th(L)]2+ , [Th(L)3 (OH)]3− and [Th(L)4 (OH)2 ]6− species [Th(L)3 (OH)]3− were common to both hydroquinone and resorcinol. Hydroquinone favored the formation of 1 : 3 complex species as hydrolysed quinoid structure is more stable. Catechol formed a hydrolysed 1 : 1 complex species corresponding to [Th(L)(OH)]+ with Th(IV). On the basis of its higher basicity, the magnitude of stability constant of catechol-Th(IV) 1 : 1 complex was expected to be higher than that with the resorcinol. However, hydrolytic changes offset electron density around the metal ion resulting in relatively smaller values of first stability constants of catechol. Higher magnitude of hydroquinone as compared with resorcinol may be attributed to steric hindrance caused by later. Speciation curves are given in Fig. 3 for three DHBs. Speciation curves drawn using hyperquad routine give the % formation of various species at different pH.
Fig. 3. Typical speciation curves of Th(IV)-DHBs. Titrate: 30 ml of 1.07 Thorium + 8.3 mM dihydroxybenzene; Titrant: 0.1 M NaOH; Mole ratio (Th : DHB) = 1 : 8; Medium: 1 M NaClO4 .
864 Acknowledgment. Authors are grateful to Dr. V. Venugopal, Director, Radiochemistry and Isotope Group, his interest in this work.
References 1. Meizhen, S., Zhaohua, Y., Zhenli, N., Chen-Guibao, Liu-Shijing, Chen-Honghong, Luo-Meichu: Chinese-J. Radiological Med. protec. 19, 183 (1999). 2. Divicenzo, G. D., Hamilton, M. L., Reynolds, R. C., Ziegler, D. A.: Toxicology 33, 9 (1984). 3. Talamoni, J., Corderio, P. J. M.: J. Radioanal. Nucl. Chem. Lett. 145, 145 (1990). 4. Kirk, Othmer: Encyclopedia Chem. Technol. 13, 996 (1994). 5. Gans, P.: HYPERQUAD suit of software programs was acquired from Depts. of Inorganic and Structural Chemistry. University of Leed, LS2 9JT, U.K. 6. Gans, P., O’Sullivan, B.: Talanta 51, 33 (2000). 7. Bouchoux, G., Defaye, D., McMahon, T., Likholyot, A., Otilia, M., Yanez, M.: Chem. Eur. J. 8 2900 (2002). 8. Fujio, M., McIver Jr., R. T., Taft, R. W.: J. Am. Chem. Soc. 103, 4017 (1981). 9. Athavale, V. T., Prabhu, L. H., Vartak, D. G.: J. Inorg. Nucl. Chem. 66 1237 (1966). 10. Christian, E., Yngve, A., Josick, M. C, Brown, P. L.: J. Sol. Chem. 29, 63 (2000). 11. Baxendale, J. H., Hardy, H. R.: Trans. Faraday Soc. 49, 1140 (1953). 12. Silvestri, P., Talarico, F., Vicedomini, M.: Ann. Chim (Rome) 69, 19 (1979). 13. Raut, A., Mhaske, T.: J. Indian Chem. Soc. 62, 782 (1985). 14. Bartusek, M., Sommer, L.: J. Inorg. Nucl. Chem. 27, 2397 (1965). 15. Limaye, S., Saxena, M.: J. Indian Chem. Soc. 59, 916 (1982). 16. Heureux, G. L., Martell, A.: J. Inorg. Nucl. Chem. 28, 481 (1966). 17. Athavale, V., Prabhu, L., Vartak, D.: J. Inorg. Nucl. Chem. 28, 1237 (1966).
U.K. Thakur et al. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42.
Pichet, P., Benoit, R.: Inorg. Chem. 6, 1505 (1967). Oka, Y., Harada, H.: Nippon Kagaku Kaishi 88, 441 (1967). Zakharova, E., Kumok, V.: Zh. Obshch. Khim. 38(9) 1922 (1968). Tyson, C., Martell, A.: J. Am. Chem. Soc. 90, 3379 (1968). Huber, P., Griesser, R., Prijs, B., Sigel, H.: Eur. J. Biochem. 10, 238 (1969). Condike, G., Martell, A.: J. Inorg. Nucl. Chem. 31, 2455 (1969). Griesser, R., Sigel, H.: Inorg. Chem. 9, 1238 (1970). Chidambaran, M., Bhattacharya, P.: J. Inorg. Nucl. Chem. 32, 3271 (1970). Anderegg, G., Malik, S.: Helv. Chim. Acta 53, 564, 577 (1970). Usherenko, L., Kulikova, N., Skorik, N. et al.: Zh. Neorg. Khim. 16(11) 3230 (1971). Patel, D., Bhattacharya, P.: J. Inorg. Nucl. Chem. 33, 529 (1971). Griesser, R., Sigel, H.: Inorg. Chem. 10, 2229 (1971). Jameson, R., Wilson, M.: J. Chem. Soc. Dalton Trans. 2607, 2610/4/7 (1972). Sigel, H., Huber, P., Griesser, R., Prijs, B.: Inorg. Chem. 12, 1198 (1973). Mhaske, T., Munshi, K.: Indian J. Chem. A 14, 421 (1976). Gergely, A., Kiss, T.: Inorg. Chim. Acta 16, 51 (1976). Pizer, R., Babcock, L.: Inorg. Chem. 16, 1677 (1977). Makhijani, S., Sangal, S.: Ann. Chim. (Rome) 68, 461 (1978). Avdeef, A., Sofen, S., Bregante, T. et al.: J. Am. Chem. Soc. 100, 5362 (1978). Nagpal, S., Dubey, S., Kalra, H., Puri, D.: Indian J. Chem. A 18, 270 (1979). Mohan, M., Bancroft, D., Abbott, E.: Inorg. Chem. 18, 344 (1979). Marathe, D., Munshi, K.: Indian J. Chem. A 18, 1142 (1981). Bhuyan, B., Dubey, S.: J. Indian Chem. Soc. 58, 613 (1981). Ambulkar, R., Marathe, D., Munshi, K.: Indian J. Chem. A 20, 1044 (1981). Lakhani, S., Thakur, G., Sangal S.: J. Indian Chem. Soc. 59, 801 (1982).
