- Email: [email protected]
Formation and thermodynamic properties of complexes of Ag(I) with thiourea as ligand
T&ma,
Vol. 20, pp. 1225-1228. Pergamon Press, 1973. Prmted in Great Britain
ANALYTICAL
FORMATION COMPLEXES
DATA
AND THERMODYNAMIC PROPERTIES OF OF Ag(1) WITH THIOUREA AS LIGAND*
(Received 30 December 1971. Revised 14 May 1973. Accepted 31 May 1973)
Thiourea (Tu) has a marked tendency to give co-ordinate bonds with many univalentid and multivalent’-9 ions and is therefore considered one of the most important masking agents having sulphur as donor atom. Tu has three potential co-ordination centres, i.e., the two nitrogen atoms and the sulphur atom. Tu almost always co-ordinates through the sulphur atom, although some authors do not exclude co-ordination through the nitrogen atoms for some metal ions such as Cu(I), I%(H) and Pd(II).‘-’ Preferential bonding through the sulphur atom is in agreement with some structural characteristics. Sulphur can form both (T and r bonds; moreover, the presence of empty anti-bonding ?T* orbitals in Tu leads to a further stabilization of the M-S bond through back-donation from the metal to the ligand. Finally, factors such as the complete planarity of Tu, including the hydrogen atoms,5 the C-S and C-N bond lengths, and restricted rotation round the C-N bonds, lo show a significant contribution of the polar structure to the ground state of Tu.
i?H, Y,CH
e/NH, I~--c,
‘NH,
(1)
VH2
The charge separation in this structure decreases the donor capacity of the nitrogen atoms and increases that of the sulphur atom. Some authors have studied the Ag(I)-Tu system and found two- and three-coordinate complexes’-4 for which only the stability constant of AgTu: is known. Diffractometric studie? on a his-Tu(AgC1) crystal showed it to have a very distorted tetrahedral structure with sulphur atoms contiguous to silver ions, at long and short distances alternately; in contrast the Ag-Cl distance was large and the chloride probably bonded ionically. Here we report the values of the formation constants, obtained in a potentiometric study on the Ag(I)-TuHz0 system. The results give definitive information about the chemical species present and their stability in dependence on ionic strength and temperature. EXPERIMENTAL
The method used by several authors ‘i-i3 is based on mathematical form only for a mononuclear system.
analysis of equation (2), valid in this
The l/a0 values were obtained by potentiometric measurements made by using a silver wire electrolytically coated with silver iodide as a measuring electrode, and a cell of the type:
* Part of this paper was presented at the 2nd Conference on Applied Physical Chemistry, Vesxprem, Hungary, August 1971. Research supported by the Consiglio Nazionale delle Richerche. 1225
1226
SHORT COMMUNICATIONS
for which the following relationship is valid: E = E” + E, + k log [Ag+ ]t,c.
(4)
The potential di&ence measurements free for solutions at constant ionic strength (KNOB = 0.25, 0.5 and 0*75&f)and containing different amounts of silver nitrate, show a constant value of E,. The k values used in equation (4) to determine -log ao are those experimentallyfoundfor thespecific ionic strengths used, and are in agreement with theoretical values in the Nernst equation. The reproductibility of E was & 2-3 mV. The -log a,, values were extrapolated to [Ag+llot = 0 at constant Tu concentration and variable [Ag+]$., concentration ranging from 5 x lo-” to 6.25 x 10-sM. The expression (2) is then valid and iT&.= r]ru],.,. The pH of the solutions, measured by glass-calomel electrode at 25”, was 6.8. At this value the reaction Tu + H+ = TuH+ and the hydrolysis of silver ion are negligible.‘*
Tu solutions were prepared just before use from Tu that had been recrystallized from ethanol and dried at 85Ym.p. 181-183”). Silver nitrate solutions were kept in dark flasks and standardized periodically by dielectrometric titration with potassium chloride. Measurements were made at 25, 35 and 45” with 100-m] samples with ionic strength kept constant by means of 0.25, 0.50 and 0.7SM potassium nitrate. When stoichiometric amounts of the two reagents were mixed, a white precipitate appeared which blackened immediately. This was accompanied by a smell of hydrogen sulphide and appearance of a metallic fihn on the surface of the solution. Later it was found that these phenomena arise when the potential of the silver electrode becomes 60-70 mV us. S.C.E. On the other hand when the silver nitrate solution was added to an excess of Tu, a clear colourless and stable solution was obtained. Before measurement the solutions were equilibrated for 5-6 hr in a thermostat. RESULTS
of the large number of experimental data and calculations, only the final resultsare given here. Figure 1 shows the dependence of -log a0 on pTu at 25” and [KNO,] = 0.25M. The value of tan 0 = Because
II
I
I -2-O
-I*0 -log
[Tu]
Fig. 1. Relation between -log ac and --log[Tu] at 2Y, [KNOJ ==0.25M and [Ag+] = 0 (tan eo = 3.4). a( -log ao)/a(pTu) = 3.4 indicates the formation of AgTut . The values of a0 are not quoted here, but mathematical analysis of l/a0 tu.~uk,,. gives the values of the overall formation constants (@,) of four mononuclear complexes AgTu.+. Table 1 gives the values of log & at different ionic strength and temperature together with thermodynamic values of log & extrapolated to KNOI = 0, and thermodynamic formation parameters.
SHORT COMMUNICATIONS
1227
Table 1. Over-all stability constants and thermodynamx data. T, “C
log 81
25 :::
log Bz
25 ::
log Bs
:: 45
log 84
:: 45
KNOa =@75M
KNO$ zD5M
7.30 6.40 560 1060 9-92 9.34 12.68 11.98 11.53 13.78 12895 12.16
Y-18 6.69 560 10.60 *::z 13.0 1190 11.67 13.75 13.04 12.24
KNOs =@ZM
KNOs -0
7*30 6.48 5.60 10.36 9.92 9*34 12.74 12.22 11.56 13.64 :2t::
7.30 6.52 5.60 ‘E 9:34 :2:E 11.59 13.72 Z
AG” kc&/mole
AH” kcoljmole
ASO cal/mole.deg.
r;; -8:l -14.4 1;::‘:
-36.9
-!Xl
-25.3
-36
-26.1
-29
-34.1
-51
-17.4 -16.9 -16.9 -18.7 -18.3 -17.7
CONCLUSIONS
Four mononuclear complexes of rather high stability have been found in the Ag(I)-T’u-Hz0 system. The overall formation constants were 7-10 orders of magnitude greater than those for complexes of silver with organic sulphides as ligand15 and were similar to those for phosphine ligands.16 The order KI > K2 c & > K4 indicates a probable change in co-ordination on entrance of the third l&and, confirmed by the sequence of partial heats of formation (AH, < AH, > AH,); the first two groups are probably co-ordinated linearly to the silver. The thermodynamic data for the reaction Ag(Hz0):
+ n Tu = AgTu.(HzO):
+ (x - v)HzO
indicate a markedly higher stability of the Ag-S bond in AgTuz than of the Ag-0 bond in the aquo-ion. The large negative entropy term can be explained by a partial release of water molecules in the complexation process. A. BELLOMO D. DE MARCO
Institute of Analytical Chemistry University of Messinu Messina, Italy
A. DE ROBERTIS
REFERENCES 1. N. L. Strelets, E. A. Gymmer,
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
A. K. Orlyankaya and T. V. Yuganova, Zh. Neorgun. Khim., 1967, 12,
2407. L. V. Nazarova and L. V. Prizhilevskaya, ibid., 1967,12, 3051. M. Nardelli and A. Braibanti, Guzz. Chim. Ital., 1957, 87, 907. W. S. Fyfe, J. Chem. SOL, 1955, 1032. E. A. Viuini, I. F. Taylor and E. L. Amma, Znorg. Chem., 1968,7, 1351. G. Pfrepper, Z. Anorg. Allgem. Chem., 1966,347,160. J. H. Yoe and L. G. Overhalser, Ind. Eng. Chem., Anal. Ed., 1942, 14,435. T. J. Lane, A. Yamaguchi, J. V. Quagliano, J. A. Ryan and S. Mizushima, J. Am. Chem. Sot., 1959,81, 3824. A. Korkzynski and J. Ziolkowski, Roczniki Chem., 1968,42, 377. J. W. Emsley and J. A. S. Smith, Proc. Chem. Sot., 1968, 53. J. Bjerrum, Metal Ammine Formation in Aqueous Solutions. Haase, Copenhagen, 1941. F. J. C. Rossotti and H. Rossotti, The Determination of Sfabilfty Constants, McGraw-Hill, New York, 1961. A. Bellomo, Rassegna Chim., 1964, 4, 167. L. G. Sill& and A. E. Martell, Stability Constants of Metal-ion Complexes. The Chemical Society, London, 1964. S. Ahrland, J. Chatt, N. R. Davies and A. A. Williams, J. Chem. Sot., 1958, 264. Idem, ibid., 1958, 273,
1228
SHORT COMMUNICATIONS
Summary-A potentiometric study has been made of the Ag(I)-CSNIH,-H,O system. Mathematical analysis of the formation functions reveals the existence of the complexes AgCSN2H:. Ag(CSN,H,)t, Ag(CSN,H& and Ag(CSNIH& for which the stability constants have been calculated at different ionic strengths and temperatures. No evidence was found for the formation of polynuciear complexes. Zm-Das System Ag(I)-CSN,H4-Ha0 wurde potentiometrisch untersucht. Die mathematische Analyse der Bildun~funktionen zeigt die Existenx der Komplexe AgCSN2H4+, Ag(CSNzH&+, Ag(CSN,H&+ und Ag(CSN2H4>4+ an. Deren Stabilitiitskonstanten wurden bei vemchiedenen Ionenstarken und Temperaturen berechnet. Es fand sich kein Hinweis auf die Bildung mehrkemiger Komplexe. R6smr&-On a effectu6 une etude potentiometrique du systeme Ag(I)-CSN2H4-H20. L’anaiyse mathbmatique des fonctions de formation revtIe l’existence des complexes AgCSNtH4+, Ag(CSN2H&+, Ag(CSN,H& + et Ag(CSN2H&+ pour lesquels on a calcule les constantes de stabilite a differerentes forces ioniques et temperatures. On n’a pas trouve de preuve de la formation de complexes polynu&aires.
Vol. 20, pp. 1225-1228. Pergamon Press, 1973. Prmted in Great Britain
ANALYTICAL
FORMATION COMPLEXES
DATA
AND THERMODYNAMIC PROPERTIES OF OF Ag(1) WITH THIOUREA AS LIGAND*
(Received 30 December 1971. Revised 14 May 1973. Accepted 31 May 1973)
Thiourea (Tu) has a marked tendency to give co-ordinate bonds with many univalentid and multivalent’-9 ions and is therefore considered one of the most important masking agents having sulphur as donor atom. Tu has three potential co-ordination centres, i.e., the two nitrogen atoms and the sulphur atom. Tu almost always co-ordinates through the sulphur atom, although some authors do not exclude co-ordination through the nitrogen atoms for some metal ions such as Cu(I), I%(H) and Pd(II).‘-’ Preferential bonding through the sulphur atom is in agreement with some structural characteristics. Sulphur can form both (T and r bonds; moreover, the presence of empty anti-bonding ?T* orbitals in Tu leads to a further stabilization of the M-S bond through back-donation from the metal to the ligand. Finally, factors such as the complete planarity of Tu, including the hydrogen atoms,5 the C-S and C-N bond lengths, and restricted rotation round the C-N bonds, lo show a significant contribution of the polar structure to the ground state of Tu.
i?H, Y,CH
e/NH, I~--c,
‘NH,
(1)
VH2
The charge separation in this structure decreases the donor capacity of the nitrogen atoms and increases that of the sulphur atom. Some authors have studied the Ag(I)-Tu system and found two- and three-coordinate complexes’-4 for which only the stability constant of AgTu: is known. Diffractometric studie? on a his-Tu(AgC1) crystal showed it to have a very distorted tetrahedral structure with sulphur atoms contiguous to silver ions, at long and short distances alternately; in contrast the Ag-Cl distance was large and the chloride probably bonded ionically. Here we report the values of the formation constants, obtained in a potentiometric study on the Ag(I)-TuHz0 system. The results give definitive information about the chemical species present and their stability in dependence on ionic strength and temperature. EXPERIMENTAL
The method used by several authors ‘i-i3 is based on mathematical form only for a mononuclear system.
analysis of equation (2), valid in this
The l/a0 values were obtained by potentiometric measurements made by using a silver wire electrolytically coated with silver iodide as a measuring electrode, and a cell of the type:
* Part of this paper was presented at the 2nd Conference on Applied Physical Chemistry, Vesxprem, Hungary, August 1971. Research supported by the Consiglio Nazionale delle Richerche. 1225
1226
SHORT COMMUNICATIONS
for which the following relationship is valid: E = E” + E, + k log [Ag+ ]t,c.
(4)
The potential di&ence measurements free for solutions at constant ionic strength (KNOB = 0.25, 0.5 and 0*75&f)and containing different amounts of silver nitrate, show a constant value of E,. The k values used in equation (4) to determine -log ao are those experimentallyfoundfor thespecific ionic strengths used, and are in agreement with theoretical values in the Nernst equation. The reproductibility of E was & 2-3 mV. The -log a,, values were extrapolated to [Ag+llot = 0 at constant Tu concentration and variable [Ag+]$., concentration ranging from 5 x lo-” to 6.25 x 10-sM. The expression (2) is then valid and iT&.= r]ru],.,. The pH of the solutions, measured by glass-calomel electrode at 25”, was 6.8. At this value the reaction Tu + H+ = TuH+ and the hydrolysis of silver ion are negligible.‘*
Tu solutions were prepared just before use from Tu that had been recrystallized from ethanol and dried at 85Ym.p. 181-183”). Silver nitrate solutions were kept in dark flasks and standardized periodically by dielectrometric titration with potassium chloride. Measurements were made at 25, 35 and 45” with 100-m] samples with ionic strength kept constant by means of 0.25, 0.50 and 0.7SM potassium nitrate. When stoichiometric amounts of the two reagents were mixed, a white precipitate appeared which blackened immediately. This was accompanied by a smell of hydrogen sulphide and appearance of a metallic fihn on the surface of the solution. Later it was found that these phenomena arise when the potential of the silver electrode becomes 60-70 mV us. S.C.E. On the other hand when the silver nitrate solution was added to an excess of Tu, a clear colourless and stable solution was obtained. Before measurement the solutions were equilibrated for 5-6 hr in a thermostat. RESULTS
of the large number of experimental data and calculations, only the final resultsare given here. Figure 1 shows the dependence of -log a0 on pTu at 25” and [KNO,] = 0.25M. The value of tan 0 = Because
II
I
I -2-O
-I*0 -log
[Tu]
Fig. 1. Relation between -log ac and --log[Tu] at 2Y, [KNOJ ==0.25M and [Ag+] = 0 (tan eo = 3.4). a( -log ao)/a(pTu) = 3.4 indicates the formation of AgTut . The values of a0 are not quoted here, but mathematical analysis of l/a0 tu.~uk,,. gives the values of the overall formation constants (@,) of four mononuclear complexes AgTu.+. Table 1 gives the values of log & at different ionic strength and temperature together with thermodynamic values of log & extrapolated to KNOI = 0, and thermodynamic formation parameters.
SHORT COMMUNICATIONS
1227
Table 1. Over-all stability constants and thermodynamx data. T, “C
log 81
25 :::
log Bz
25 ::
log Bs
:: 45
log 84
:: 45
KNOa =@75M
KNO$ zD5M
7.30 6.40 560 1060 9-92 9.34 12.68 11.98 11.53 13.78 12895 12.16
Y-18 6.69 560 10.60 *::z 13.0 1190 11.67 13.75 13.04 12.24
KNOs =@ZM
KNOs -0
7*30 6.48 5.60 10.36 9.92 9*34 12.74 12.22 11.56 13.64 :2t::
7.30 6.52 5.60 ‘E 9:34 :2:E 11.59 13.72 Z
AG” kc&/mole
AH” kcoljmole
ASO cal/mole.deg.
r;; -8:l -14.4 1;::‘:
-36.9
-!Xl
-25.3
-36
-26.1
-29
-34.1
-51
-17.4 -16.9 -16.9 -18.7 -18.3 -17.7
CONCLUSIONS
Four mononuclear complexes of rather high stability have been found in the Ag(I)-T’u-Hz0 system. The overall formation constants were 7-10 orders of magnitude greater than those for complexes of silver with organic sulphides as ligand15 and were similar to those for phosphine ligands.16 The order KI > K2 c & > K4 indicates a probable change in co-ordination on entrance of the third l&and, confirmed by the sequence of partial heats of formation (AH, < AH, > AH,); the first two groups are probably co-ordinated linearly to the silver. The thermodynamic data for the reaction Ag(Hz0):
+ n Tu = AgTu.(HzO):
+ (x - v)HzO
indicate a markedly higher stability of the Ag-S bond in AgTuz than of the Ag-0 bond in the aquo-ion. The large negative entropy term can be explained by a partial release of water molecules in the complexation process. A. BELLOMO D. DE MARCO
Institute of Analytical Chemistry University of Messinu Messina, Italy
A. DE ROBERTIS
REFERENCES 1. N. L. Strelets, E. A. Gymmer,
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
A. K. Orlyankaya and T. V. Yuganova, Zh. Neorgun. Khim., 1967, 12,
2407. L. V. Nazarova and L. V. Prizhilevskaya, ibid., 1967,12, 3051. M. Nardelli and A. Braibanti, Guzz. Chim. Ital., 1957, 87, 907. W. S. Fyfe, J. Chem. SOL, 1955, 1032. E. A. Viuini, I. F. Taylor and E. L. Amma, Znorg. Chem., 1968,7, 1351. G. Pfrepper, Z. Anorg. Allgem. Chem., 1966,347,160. J. H. Yoe and L. G. Overhalser, Ind. Eng. Chem., Anal. Ed., 1942, 14,435. T. J. Lane, A. Yamaguchi, J. V. Quagliano, J. A. Ryan and S. Mizushima, J. Am. Chem. Sot., 1959,81, 3824. A. Korkzynski and J. Ziolkowski, Roczniki Chem., 1968,42, 377. J. W. Emsley and J. A. S. Smith, Proc. Chem. Sot., 1968, 53. J. Bjerrum, Metal Ammine Formation in Aqueous Solutions. Haase, Copenhagen, 1941. F. J. C. Rossotti and H. Rossotti, The Determination of Sfabilfty Constants, McGraw-Hill, New York, 1961. A. Bellomo, Rassegna Chim., 1964, 4, 167. L. G. Sill& and A. E. Martell, Stability Constants of Metal-ion Complexes. The Chemical Society, London, 1964. S. Ahrland, J. Chatt, N. R. Davies and A. A. Williams, J. Chem. Sot., 1958, 264. Idem, ibid., 1958, 273,
1228
SHORT COMMUNICATIONS
Summary-A potentiometric study has been made of the Ag(I)-CSNIH,-H,O system. Mathematical analysis of the formation functions reveals the existence of the complexes AgCSN2H:. Ag(CSN,H,)t, Ag(CSN,H& and Ag(CSNIH& for which the stability constants have been calculated at different ionic strengths and temperatures. No evidence was found for the formation of polynuciear complexes. Zm-Das System Ag(I)-CSN,H4-Ha0 wurde potentiometrisch untersucht. Die mathematische Analyse der Bildun~funktionen zeigt die Existenx der Komplexe AgCSN2H4+, Ag(CSNzH&+, Ag(CSN,H&+ und Ag(CSN2H4>4+ an. Deren Stabilitiitskonstanten wurden bei vemchiedenen Ionenstarken und Temperaturen berechnet. Es fand sich kein Hinweis auf die Bildung mehrkemiger Komplexe. R6smr&-On a effectu6 une etude potentiometrique du systeme Ag(I)-CSN2H4-H20. L’anaiyse mathbmatique des fonctions de formation revtIe l’existence des complexes AgCSNtH4+, Ag(CSN2H&+, Ag(CSN,H& + et Ag(CSN2H&+ pour lesquels on a calcule les constantes de stabilite a differerentes forces ioniques et temperatures. On n’a pas trouve de preuve de la formation de complexes polynu&aires.