- Email: [email protected]
Thermodynamic properties of actinide complexes. Uranyl(VI)— and thorium(IV)—aspartate systems
Po/yMron Vol. 8, No. IS, pp. 2233-2236, Printed in Great Britain
1989 0
THERMODYNAMIC PROPERTIES OF ACTINIDE URANYL(VI)- AND THORIUM(IV)-ASPARTATE ARTURO
BISMONDO*
a2n-538l/a9 s3.00+.00 1989 Pergamon Press
plc
COMPLEXES. SYSTEMS
and LUIGI RIZZO
Istituto di Chimica e Tecnologia dei Radioelementi, Area della Ricerca de1 CNR, Corso Stati Uniti 4,35020 Padova, Italy (Received 29 September 1988 ; accepted after revision 22 March 1989)
Abstract-Stability constants, enthalpies and entropies in the formation of uranyl(VI) and thorium(W) with aspartic acid have been determined at 250°C in a 1.Omol dm- 3 aqueous solution of sodium perchlorate by means of the potentiometric and calorimetric techniques. Because of precipitation of solid compounds, only two complexes of 1: 1 and 1: 2 stoichiometry were observed in the uranyl(VI)-asparate system and only one complex of 1: 1 stoichiometry in the thorium(IVtaspartate system. Comparison of values of stability constants and enthalpy and entropy changes with the corresponding values of some mono- and di-carboxylate ligands suggests that the binding of both metal ions involves one carboxylate group only, the one furthest from the positive amino group.
In previous works we have reported the changes in the thermodynamic functions for the uranyl(VI) and thorium(IV) complex formation with some aminomonocarboxylate ligands. l-3 From the values of the stability constants and enthalpy and entropy changes, it was deduced that those ligands bind to metal ion through the carboxylate group only, whereas the amino group, in the experimental conditions used, is protonated. We have examined the coordination behaviour of some amino acids, which have two carboxylic groups in the chain, in order to evaluate the influence of another group on the capacity of amino acids to form stable bonds to actinide ions. For this purpose, we have chosen initially aspartic acid, an a-amino acid with an additional carboxylic acid group in the side chain. Complexation of the uranyl(V1) ion with aspartate in solution below pH 3.5 was studied by potentiometry,4 where it was reported that the uranyl(V1) was bound to one of the carboxylic groups only, and by NMR spectroscopy’ where it appears that the aspartate is bound via two carboxylic groups with the uranyl ion. A more recent study of uranyl(VI) ion by means of the solvent extraction technique6 suggests that, at pH 8, the binding of aspartate involves both the carboxylate groups, but there is no uranyl-amino group interaction.
In order to clarify the question whether aspartic acid coordinates via one or two carboxylate groups to uranyl(V1) and thorium(W) ions, in the range of pH below 3, we have examined these systems by -carrying out potentiometric and calorimetric measurements. EXPERIMENTAL
Stock solutions of uranyl(V1) and thorium(IV) perchlorate, containing an excess of perchloric acid, were prepared and standardized as reported before.’ gspartic acid (Baker product) was recrystallized from water and dried at lOO”C, before weighing to prepare the appropriate buffer solution. For each system several potentiometric and calorimetric titrations were carried out by adding a known amount of buffer solutions of ligand to a known volume of a solution containing the metal ion (Ck = 10-30 mm01 dm- 3, and perchloric acid (Ci = 10-20 mmol dme3). Potentiometric measurements were carried out with a Radiometer pH 64 ; the titration vessel was equipped with a glass electrode (Metrohm EA-157) and a double junction Ag/AgCl reference electrode (Metrohm EA-440). The reaction heats were determined by a Tronac model 450 calorimeter; the reaction vessel was a rapid response, silvered glass vacuum Dewar of 25 cm3 capacity. The burette *Author to whom wrrespondence should be addressed. capacity was 5 cm3 and the titrant delivery rate was 2233
2234
A. BISMONDO
and L. RIZZO _
0.21 I9 cm’ mitt-‘. The precision and accuracy of
ThoriumW-aspartate
the equipment were checked by standard methods. The procedures and titration techniques have been described earlier. ’ The calculations were performed by a CDC computer 6700. RESULTS AND DISCUSSION
Proton-ligand system Under the experimental conditions of this work, the ligand exists as zwitterion form and thus the pK, value of the amino group was not determined, because it is not needed for the calculations. The values calculated for the equilibrium constants and the enthalpy changes relative to the protonation of two carboxylic groups were determined by adding known volumes of HC104 or NaOH standard solutions to solutions containing different concentrations of the ligand. The values obtained (see Table 1) are sufficiently in agreement with the data available in the literature taking into account the different experimental conditions’ (log fiO,1 = 3.66 M-l, AH,,,, = -4.6 kJ mol-‘, log j.?,,2,= 5.56 M- ‘, AH021 = - 10.5 k.l mol-‘).
1.5
a@
Urany WIbaspartate “8
00 3.5
u
I 2.5
30 -
I 2.0
log [L]_
Metal-ligand systems
Fig. 1. The complex formation curves of uranyl(VI)- and thorium(IL+aspartate systems. Concentrations in 10m3
For the uranyl(VI)-aspartate system the potentiometric measurements were limited to small concentrations of ligand in relation to the small solubility in aqueous solution of the acidic form and to the limitation of using the salt of the ligand because of the high tendency of free uranyl ion to hydrolyse. In addition, precipitation of uranyl(V1) compounds occurred at low pH values and for this reason the measurements were limited to the pH range up to about 3.4. The elemental analysis of precipitates indicated the formation of hydrolysis products. As a result, the ri range of complex formation was limited to values of 1.6 and calculations were made for complexes with no more than two
mol dir- 3; 6 = HL/L- . For uranyl(V1) system : 0, 0, V denote CL = 20.59, Cfi = 14.67, CL = 30.89, Cfi = 15.50 and C& = 10.30, Ci = 13.97 titrated with 6 = 0.12 buffer; for thorium(IV) system: 0, 0, A, V denote Ch = 30.24, Cj?, = 22.50, C& = 20.16, C$ = 22.55, C& = 27.21, Cfi = 20.25 and C& = 10.08, Ci = 10.86 titrated with 6 = 0.15 buffer.
ligands. This is shown in Fig. 1, where the complex formation curves are plotted. The data show that 6 values are not dependent on C,, therefore no polynuclear complexes are formed and, in the pH range used, hydrolytic reaction of the uranyl(V1) is negligible.
Table 1. The stability constants and changes in enthalpy and entropy for the formation of proton-, uranyl(VI)--, and thorium(IVkaspartate complexes at 250°C in 1.0 mol dm-’ sodium perchlorate medium
P
4
r
0 0 1 1 1
1 2 0 0 0
1 1 1 2 1
Reaction H+L#HL 2H+L+H2L uo*+L P UOZL UO*+2L Ft UOZL, Th+L#ThL
log B,r
AH (kJ mall ‘)
AS (J mall ’ K- ‘)
3.72kO.02 5.77 f 0.03 2.41&-0.03 4.14kO.04 4.2lkO.03
-5.9f0.2 - 10.3 kO.2 8.9f0.4 10.5 +0.6 10.9kO.4
51 76 76 114 117
Thermodynamic
properties of actinide complexes
For the thorium(N)-aspartate system, the experimental conditions were more restrictive. For this system, because of precipitation at low pH values (pH < 2.3), the A range of complex formation (fi < 0.7) was too low to allow any reliable calculation of complex formation with more than one ligand. As can be seen in Fig. 1, the complex formation curves for this system too, indicate that both polynuclear and hydrolytic reactions are negligible. The calorimetric measurements for both systems were limited by the above mentioned experimental conditions and by relatively small amounts of complexation heat in the titrations. Hence, no very accurate information could be obtained about the AH values of both systems. The obtained values of the thermodynamic functions log j&,, AH and AS for the examined systems are reported in Table 1. The limits of error refer to three standard deviations as obtained from the computer programs Miniquad 75 and Letagrop Kalle. In order to perform a comparison, the available data on the complex formation of uranyl(V1) and thorium(IV) with some mono- and di-carboxylate ligands are reported in Table 2. The value of the stability constant for the uranyl(VI)-monoaspartate system, obtained by us, is in fairly good agreement with that reported in the literature,4 i.e. log /I, ,,, = 2.63, taking into account the different experimental conditions. In that work it was reported that uranyl(V1) ion is bound to aspartate ligand by one of the carboxylate groups only, the one furthest from the positive amino group. Our data confirm that hypothesis. In fact we can notice that the value of the stability constant for the first complex of the uranyl(V1) with aspartate is relatively smaller than that for the 1: 1 uranyl(VI)monosuccinate system (see Table 2). These values would indicate that there is a possible loss of the chelate effect with respect to succinate system (the succinate behaves as a bidentate chelating agent through the two carboxylate groups coordinated
2235
to the metal ion). Moreover, the value of the 1: 1 stability constant for the uranyl(VIbaspartate complex is comparable with that for the succinic monoprotonated acid log K,i, = 2.28. This analogous behaviour is contirmed by the enthalpy changes, as their values are relatively low and both close to the value for the uranyl(VI)-acetate system (see Table 2). As the value of the stability-constant of the 1: 1 uranyl(VI)--glycine complex is smaller than that of the aspartate complex, it should be possible to show that the b-carboxylate group is bound to the metal ion. The values of the entropy changes suggest this hypothesis. In Table 2, the value for the succinate system (AS = 146) shows that this ligand binds through both carboxylate groups, having a value comparable with that for the diacetate system,’ while the values for acetate, succinate monoprotonate and aspartate (AS = 87,79, 76 respectively) show that these ligands bind through one carboxylate group only. Moreover, as the AS value of the glycine system (AS = 35) is smaller than that of the aspartate system (AS = 76) it is reasonable to assume that the uranyl(V1) ion is bound to the /?carboxylate group, while the a-carboxylate group is deprotonated in correspondence with the protonation of the amino group. Similar results seem to hold for the thorium(IV)aspartate system. Comparing the values of the 1: 1 stability constant for the aspartate complex (log /I,,,, = 4.21) with that for the succinate complex (log /3iol = 6.44) it would seem that there is no interaction of thorium with both of the carboxylate groups. By analogy with uranyl(VI), the values of enthalpy and entropy changes for the thorium(N) aspartate system are comparable in magnitude with those of the complex with the succinic monoprotonated acid. In fact the values of AH and AS for the thorium-succinate complex, where the ligand binds via both carboxylate groups, are higher than the values for aspartate complex, while the last values (AH = 10.9, AS = 117) are comparable to
Table 2. The stability constants and the changes in enthalpy and entropy for the formation of uranyl(V1) and thorium(W) 1: 1 complexes at 250°C in 1.0 mol dm-’ sodium perchlorate medium
Ligands
log /I
Acetate Diacetate Succinate H-Succinate Glycine Aspartate
2.46 4.38 3.85 2.28 1.16 2.41
Uranyl(V1) AH AS (kJ mol- ‘) (J mol- ’ K- ‘) 11.8 17.9 21.7 10.0 3.9 8.9
87 144 146 79 35 76
Ref.
log /I
9 9 11 11 1 -
3.86 6.97 6.44 3.60 2.55 4.21
Thorium(IV) AH AS (kJ mol- ‘) (J mol- ’ K- ‘) 11.3 15.8 18.6 8.5 4.2 10.9
112 186 186 97 63 117
Ref. 10 10 12 12 1 -
2236
A. BISMONDO
the values for the thorium monoacetate complex (AH = 11.3, AS = 112). Therefore it is reasonable to presume that, for the aspartate, one carboxylate group only binds to the metal ion. The fact that the values of thermodynamic functions for the thorium glycine system are all smaller than those for the aspartate system and, on the other hand, the last values are comparable to the acetate system, can show that the aspartate ligand binds through the furthest carboxylate group from the amino group.
REFERENCES
A. Bismondo, L. Rizzo, G. Tomat, D. Curto, P. Di Bernard0 and A. Cassol, Znorg. Chim. Acta 1983,74, 21. A. Bismondo, L. Rizzo, P. Di Bernard0 and P. L. Zanonato, J. Chem. Sot., Dalton Trans. 1987,695. A. Bismondo and L. Rizzo, Radiochim. Acta 1989, 47,47.
and L. RIZZO 4. I. Feldmann and L. Koval, Znorg. Chem. 1963, 2, 145. 5. H. Wieczorek and H. Kozlowski, Znorg. Nucl. Chem. Lett. 1980, 16,401. 6. A. Sarto and G. R. Choppin, Radiochim. Acta 1984, 36,135. 7. A. E. Martell and R. M. Smith, Critical Stability Constants, Vol. 5. Plenum Press, New York (1982). 8. Y. Hasegava and G. R. Choppin, Znorg. Chem. 1977, 16,293 1. 9. R. Portanova, P. Di Bemardo, A. Cassol, E. Tondello and L. Magon, Znorg. Nucl. Chem. 1974, 8, 233. 10. R. Portanova, P. Di Bemardo, 0. Traverso, G. A. Mazzocchin and L. Magon, J. Znorg. Nucl. Chem. 1975,37,2177. 11. A. Bismondo, A. Cassol, P. Di Bemardo, L. Magon and G. Tomat, Znorg. Nucl. Chem. L&t. 1981, 17, 79. 12. P. Di Bemardo, A. Cassol, G. Tomat, A. Bismondo and L. Magon, J. Chem. Sot., Dalton Trans. 1983, 733.
1989 0
THERMODYNAMIC PROPERTIES OF ACTINIDE URANYL(VI)- AND THORIUM(IV)-ASPARTATE ARTURO
BISMONDO*
a2n-538l/a9 s3.00+.00 1989 Pergamon Press
plc
COMPLEXES. SYSTEMS
and LUIGI RIZZO
Istituto di Chimica e Tecnologia dei Radioelementi, Area della Ricerca de1 CNR, Corso Stati Uniti 4,35020 Padova, Italy (Received 29 September 1988 ; accepted after revision 22 March 1989)
Abstract-Stability constants, enthalpies and entropies in the formation of uranyl(VI) and thorium(W) with aspartic acid have been determined at 250°C in a 1.Omol dm- 3 aqueous solution of sodium perchlorate by means of the potentiometric and calorimetric techniques. Because of precipitation of solid compounds, only two complexes of 1: 1 and 1: 2 stoichiometry were observed in the uranyl(VI)-asparate system and only one complex of 1: 1 stoichiometry in the thorium(IVtaspartate system. Comparison of values of stability constants and enthalpy and entropy changes with the corresponding values of some mono- and di-carboxylate ligands suggests that the binding of both metal ions involves one carboxylate group only, the one furthest from the positive amino group.
In previous works we have reported the changes in the thermodynamic functions for the uranyl(VI) and thorium(IV) complex formation with some aminomonocarboxylate ligands. l-3 From the values of the stability constants and enthalpy and entropy changes, it was deduced that those ligands bind to metal ion through the carboxylate group only, whereas the amino group, in the experimental conditions used, is protonated. We have examined the coordination behaviour of some amino acids, which have two carboxylic groups in the chain, in order to evaluate the influence of another group on the capacity of amino acids to form stable bonds to actinide ions. For this purpose, we have chosen initially aspartic acid, an a-amino acid with an additional carboxylic acid group in the side chain. Complexation of the uranyl(V1) ion with aspartate in solution below pH 3.5 was studied by potentiometry,4 where it was reported that the uranyl(V1) was bound to one of the carboxylic groups only, and by NMR spectroscopy’ where it appears that the aspartate is bound via two carboxylic groups with the uranyl ion. A more recent study of uranyl(VI) ion by means of the solvent extraction technique6 suggests that, at pH 8, the binding of aspartate involves both the carboxylate groups, but there is no uranyl-amino group interaction.
In order to clarify the question whether aspartic acid coordinates via one or two carboxylate groups to uranyl(V1) and thorium(W) ions, in the range of pH below 3, we have examined these systems by -carrying out potentiometric and calorimetric measurements. EXPERIMENTAL
Stock solutions of uranyl(V1) and thorium(IV) perchlorate, containing an excess of perchloric acid, were prepared and standardized as reported before.’ gspartic acid (Baker product) was recrystallized from water and dried at lOO”C, before weighing to prepare the appropriate buffer solution. For each system several potentiometric and calorimetric titrations were carried out by adding a known amount of buffer solutions of ligand to a known volume of a solution containing the metal ion (Ck = 10-30 mm01 dm- 3, and perchloric acid (Ci = 10-20 mmol dme3). Potentiometric measurements were carried out with a Radiometer pH 64 ; the titration vessel was equipped with a glass electrode (Metrohm EA-157) and a double junction Ag/AgCl reference electrode (Metrohm EA-440). The reaction heats were determined by a Tronac model 450 calorimeter; the reaction vessel was a rapid response, silvered glass vacuum Dewar of 25 cm3 capacity. The burette *Author to whom wrrespondence should be addressed. capacity was 5 cm3 and the titrant delivery rate was 2233
2234
A. BISMONDO
and L. RIZZO _
0.21 I9 cm’ mitt-‘. The precision and accuracy of
ThoriumW-aspartate
the equipment were checked by standard methods. The procedures and titration techniques have been described earlier. ’ The calculations were performed by a CDC computer 6700. RESULTS AND DISCUSSION
Proton-ligand system Under the experimental conditions of this work, the ligand exists as zwitterion form and thus the pK, value of the amino group was not determined, because it is not needed for the calculations. The values calculated for the equilibrium constants and the enthalpy changes relative to the protonation of two carboxylic groups were determined by adding known volumes of HC104 or NaOH standard solutions to solutions containing different concentrations of the ligand. The values obtained (see Table 1) are sufficiently in agreement with the data available in the literature taking into account the different experimental conditions’ (log fiO,1 = 3.66 M-l, AH,,,, = -4.6 kJ mol-‘, log j.?,,2,= 5.56 M- ‘, AH021 = - 10.5 k.l mol-‘).
1.5
a@
Urany WIbaspartate “8
00 3.5
u
I 2.5
30 -
I 2.0
log [L]_
Metal-ligand systems
Fig. 1. The complex formation curves of uranyl(VI)- and thorium(IL+aspartate systems. Concentrations in 10m3
For the uranyl(VI)-aspartate system the potentiometric measurements were limited to small concentrations of ligand in relation to the small solubility in aqueous solution of the acidic form and to the limitation of using the salt of the ligand because of the high tendency of free uranyl ion to hydrolyse. In addition, precipitation of uranyl(V1) compounds occurred at low pH values and for this reason the measurements were limited to the pH range up to about 3.4. The elemental analysis of precipitates indicated the formation of hydrolysis products. As a result, the ri range of complex formation was limited to values of 1.6 and calculations were made for complexes with no more than two
mol dir- 3; 6 = HL/L- . For uranyl(V1) system : 0, 0, V denote CL = 20.59, Cfi = 14.67, CL = 30.89, Cfi = 15.50 and C& = 10.30, Ci = 13.97 titrated with 6 = 0.12 buffer; for thorium(IV) system: 0, 0, A, V denote Ch = 30.24, Cj?, = 22.50, C& = 20.16, C$ = 22.55, C& = 27.21, Cfi = 20.25 and C& = 10.08, Ci = 10.86 titrated with 6 = 0.15 buffer.
ligands. This is shown in Fig. 1, where the complex formation curves are plotted. The data show that 6 values are not dependent on C,, therefore no polynuclear complexes are formed and, in the pH range used, hydrolytic reaction of the uranyl(V1) is negligible.
Table 1. The stability constants and changes in enthalpy and entropy for the formation of proton-, uranyl(VI)--, and thorium(IVkaspartate complexes at 250°C in 1.0 mol dm-’ sodium perchlorate medium
P
4
r
0 0 1 1 1
1 2 0 0 0
1 1 1 2 1
Reaction H+L#HL 2H+L+H2L uo*+L P UOZL UO*+2L Ft UOZL, Th+L#ThL
log B,r
AH (kJ mall ‘)
AS (J mall ’ K- ‘)
3.72kO.02 5.77 f 0.03 2.41&-0.03 4.14kO.04 4.2lkO.03
-5.9f0.2 - 10.3 kO.2 8.9f0.4 10.5 +0.6 10.9kO.4
51 76 76 114 117
Thermodynamic
properties of actinide complexes
For the thorium(N)-aspartate system, the experimental conditions were more restrictive. For this system, because of precipitation at low pH values (pH < 2.3), the A range of complex formation (fi < 0.7) was too low to allow any reliable calculation of complex formation with more than one ligand. As can be seen in Fig. 1, the complex formation curves for this system too, indicate that both polynuclear and hydrolytic reactions are negligible. The calorimetric measurements for both systems were limited by the above mentioned experimental conditions and by relatively small amounts of complexation heat in the titrations. Hence, no very accurate information could be obtained about the AH values of both systems. The obtained values of the thermodynamic functions log j&,, AH and AS for the examined systems are reported in Table 1. The limits of error refer to three standard deviations as obtained from the computer programs Miniquad 75 and Letagrop Kalle. In order to perform a comparison, the available data on the complex formation of uranyl(V1) and thorium(IV) with some mono- and di-carboxylate ligands are reported in Table 2. The value of the stability constant for the uranyl(VI)-monoaspartate system, obtained by us, is in fairly good agreement with that reported in the literature,4 i.e. log /I, ,,, = 2.63, taking into account the different experimental conditions. In that work it was reported that uranyl(V1) ion is bound to aspartate ligand by one of the carboxylate groups only, the one furthest from the positive amino group. Our data confirm that hypothesis. In fact we can notice that the value of the stability constant for the first complex of the uranyl(V1) with aspartate is relatively smaller than that for the 1: 1 uranyl(VI)monosuccinate system (see Table 2). These values would indicate that there is a possible loss of the chelate effect with respect to succinate system (the succinate behaves as a bidentate chelating agent through the two carboxylate groups coordinated
2235
to the metal ion). Moreover, the value of the 1: 1 stability constant for the uranyl(VIbaspartate complex is comparable with that for the succinic monoprotonated acid log K,i, = 2.28. This analogous behaviour is contirmed by the enthalpy changes, as their values are relatively low and both close to the value for the uranyl(VI)-acetate system (see Table 2). As the value of the stability-constant of the 1: 1 uranyl(VI)--glycine complex is smaller than that of the aspartate complex, it should be possible to show that the b-carboxylate group is bound to the metal ion. The values of the entropy changes suggest this hypothesis. In Table 2, the value for the succinate system (AS = 146) shows that this ligand binds through both carboxylate groups, having a value comparable with that for the diacetate system,’ while the values for acetate, succinate monoprotonate and aspartate (AS = 87,79, 76 respectively) show that these ligands bind through one carboxylate group only. Moreover, as the AS value of the glycine system (AS = 35) is smaller than that of the aspartate system (AS = 76) it is reasonable to assume that the uranyl(V1) ion is bound to the /?carboxylate group, while the a-carboxylate group is deprotonated in correspondence with the protonation of the amino group. Similar results seem to hold for the thorium(IV)aspartate system. Comparing the values of the 1: 1 stability constant for the aspartate complex (log /I,,,, = 4.21) with that for the succinate complex (log /3iol = 6.44) it would seem that there is no interaction of thorium with both of the carboxylate groups. By analogy with uranyl(VI), the values of enthalpy and entropy changes for the thorium(N) aspartate system are comparable in magnitude with those of the complex with the succinic monoprotonated acid. In fact the values of AH and AS for the thorium-succinate complex, where the ligand binds via both carboxylate groups, are higher than the values for aspartate complex, while the last values (AH = 10.9, AS = 117) are comparable to
Table 2. The stability constants and the changes in enthalpy and entropy for the formation of uranyl(V1) and thorium(W) 1: 1 complexes at 250°C in 1.0 mol dm-’ sodium perchlorate medium
Ligands
log /I
Acetate Diacetate Succinate H-Succinate Glycine Aspartate
2.46 4.38 3.85 2.28 1.16 2.41
Uranyl(V1) AH AS (kJ mol- ‘) (J mol- ’ K- ‘) 11.8 17.9 21.7 10.0 3.9 8.9
87 144 146 79 35 76
Ref.
log /I
9 9 11 11 1 -
3.86 6.97 6.44 3.60 2.55 4.21
Thorium(IV) AH AS (kJ mol- ‘) (J mol- ’ K- ‘) 11.3 15.8 18.6 8.5 4.2 10.9
112 186 186 97 63 117
Ref. 10 10 12 12 1 -
2236
A. BISMONDO
the values for the thorium monoacetate complex (AH = 11.3, AS = 112). Therefore it is reasonable to presume that, for the aspartate, one carboxylate group only binds to the metal ion. The fact that the values of thermodynamic functions for the thorium glycine system are all smaller than those for the aspartate system and, on the other hand, the last values are comparable to the acetate system, can show that the aspartate ligand binds through the furthest carboxylate group from the amino group.
REFERENCES
A. Bismondo, L. Rizzo, G. Tomat, D. Curto, P. Di Bernard0 and A. Cassol, Znorg. Chim. Acta 1983,74, 21. A. Bismondo, L. Rizzo, P. Di Bernard0 and P. L. Zanonato, J. Chem. Sot., Dalton Trans. 1987,695. A. Bismondo and L. Rizzo, Radiochim. Acta 1989, 47,47.
and L. RIZZO 4. I. Feldmann and L. Koval, Znorg. Chem. 1963, 2, 145. 5. H. Wieczorek and H. Kozlowski, Znorg. Nucl. Chem. Lett. 1980, 16,401. 6. A. Sarto and G. R. Choppin, Radiochim. Acta 1984, 36,135. 7. A. E. Martell and R. M. Smith, Critical Stability Constants, Vol. 5. Plenum Press, New York (1982). 8. Y. Hasegava and G. R. Choppin, Znorg. Chem. 1977, 16,293 1. 9. R. Portanova, P. Di Bemardo, A. Cassol, E. Tondello and L. Magon, Znorg. Nucl. Chem. 1974, 8, 233. 10. R. Portanova, P. Di Bemardo, 0. Traverso, G. A. Mazzocchin and L. Magon, J. Znorg. Nucl. Chem. 1975,37,2177. 11. A. Bismondo, A. Cassol, P. Di Bemardo, L. Magon and G. Tomat, Znorg. Nucl. Chem. L&t. 1981, 17, 79. 12. P. Di Bemardo, A. Cassol, G. Tomat, A. Bismondo and L. Magon, J. Chem. Sot., Dalton Trans. 1983, 733.