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The stability of metal complexes with 8-mercaptoquinoline and alkyl-substituted 8-mercaptoquinolines in dimethylformamide
Talanta, Vol. 31, No. 9, PP. 127-129, 1984 Printed in Great Britain. All rights reserved
0039-9140/84 $3.00+ 0.00 Copyright 0 1984Pergamon Press Ltd
ANALYTICAL
DATA
THE STABILITY OF METAL COMPLEXES WITH 8-MERCAPTOQUINOLINE AND ALKYL-SUBSTITUTED %MERCAPTOQUINOLINES IN DIMETHYLFORMAMIDE N. A. ULAKHOVICH, H. C. BUDNIKOV, T. S. GORBUNOVA and A. P. STURIS Faculty of Chemistry, V.I. Ul’yanov-Lenin State University, Lenina 18, Kazan, 420008, USSR (Received 13 March 1984. Accepted 5 April 1984)
Summary-The stoichiometry and stability constants of I-mercaptoquinoline and alkyl-8 mercaptoquinoline complexes of Zn(II), Cd(B), Pb(II), Ni(II), Bi(II1) and Ag(1) were determined potentiometrically in dimethylformamide. The stability of the 8-mercaptoquinolinates decreases in the order Ag(I) > Bi(II1) > Ni(I1) > Pb(I1) > Cd(I1) > Zn(I1). Metal 7-methyl-8-mercaptoquinolinates are the most stable. The presence of the alkyl group in the 2-position (which has a steric effect) lowers the strength of metal-ligand bonding.
%Mercaptoquinoline and alkyl-%mercaptoquinolines are well known as complexing reagents for heavy metals.’ These reagents are especially promising when
used for combining concentration and separation with a sensitive method of determination. Hence the stability constants of these complexes in non-aqueous solutions should prove of interest. As a rule the extracted species is a non-electrolyte. Therefore, in order to study the stability of the 8-mercaptoquinolinates by potentiometry it is necessary to add some polar solvent to the extract or to use only a polar solution. This article deals with measurement of the redox potentials of systems containing the ligand (8-mercaptoquinolinate anion), an oxidized form of the ligand (the disulphide of 8-mercaptoquinoline), and metal ion, at a constant concentration of the disulphide.2.3 The conditions for application of the method are that the electrode process is reversible and the metal complexes only with the reduced form of the ligand. The stability constants of Zn(II), Cd(H), Pb(II), Ni(II), Bi(II1) and Ag(1) alkyl-8-mercaptoquinolinates in dimethylformamide have been determined, the alkyl positions in the ligand being 2,4, 6,7 and 2,7. EXPERIMENTAL
electrolyte. Dimethylformamide (DMF) was vent. The purity of the electrolyte was checked polarograms with a platinum electrode. The DMF was run as a check on electrochemical
used as solby recording base-line for purity.
Apparatus and potentiometric measurements
Potentiometric measurements were made with an R363 potentiometer (USSR) at 25 + 0.2”. The indicator electrode was a platinum plate, the reference electrode a saturated calomel electrode. The solutions were deoxygenated, and a slow stream of argon was passed over the solution throughout the experiment. The logarithmic values obtained from a series of measurements differed by not more than kO.1, even in the most unfavourable conditions. The stability constants were obtained from the experimental data by the method of Schulman et al.’ with some slight alterations. Instead of the simultaneous titration of a ligand with a metal and a background electrolyte, the redox electrode potentials in the presence and absence of metal ions were measured to save time. The values of the stability constants were calculated according to Bjerrum, with the formulae
- log[L] = & r-,(1
n=
- log Cl_ -
~o-wo.o~~
Gi
’
where C, and C, are the total concentrations of the ligand and metal respectively, AE is the difference in the equilibrium potential of the ligand redox-system in the presence and absence of metal ions. In a number of cases, where the values of the stepwise stability constants were rather close, another method of calculating was used to correct the data.’
Reagents
The sodium salts of the alkyl-8-mercaptoquinohnes and corresponding disulphides were prepared as described in the literature.4 The analytical (total) concentrations of the ligand and disulphidk were constant in all experiments, and 5 x 10e4M and 2.5 x 10e4M resoectivelv. The metal ion concentrations were varied in ihe range 1 x 10e41 x IO-‘M. Stock solutions of the anhydrous metal perchlorates (twice recrystallized) were prepared from the solids and standardized by EDTA titration.’ Lithium perchlorate solution (O.lM) was used as the background 727
RESULTS AND DISCUSSION
The relations between the average ligand number, 6, and the corresponding concentration of the free, uncomplexed, ligand are shown in Fig. 1 for some metal-ligand systems. Logarithmic values of the stability constants are given in Table 1, and show that the metal alkylmercaptoquinolinates have high
128
ANALYTICAL
DATA
Table 1. Logarithmic values of the stability constants of the alkyl-8-mercaptoquinolinate complexes
I
I
1
4
6 -log
I
I
6
10
Metal
R
logk,
logk,
log82
Zn(I1)
H 2-CH, CCH, 6-CH, 7-CH, 2,7-(cH,), 2-i-C,H,
8.1 8.5 8.3 8.3 10.8 9.1 5.9
4.3 6.4 6.2 6.2 8.3 6.9 5.4
12.4 14.9 14.5 14.5 19.1 16.0 11.3
Cd(I1)
H 2-CH, 4-CH, 6_CH, 7-CH, 2,7-(cH,), 2-i-C,H,
8.3 8.4 8.1 9.3 9.8 9.2 5.1
4.9 6.1 6.1 7.1 6.2 6.9 4.6
13.2 14.5 14.2 16.4 16.0 16.1 9.7
Pb(I1)
H 2-CH, 4-CH, 6-CH, 7-CH, 2,7-(cH,), 2i-C,H,
8.6 8.8 8.4 9.4 10.6 10.0 4.5
5.5 5.3 5.1 6.9 6.7 6.5 3.9
14.1 14.1 13.5 16.3 17.3 16.5 8.4
Ni(I1)
H 2-CH, 4-CH, 6-CH, 7-CH, 2,7-(cH,), 2i-C,H,
9.5 8.1 9.7 9.2 11.3 8.2 3.9
6.6 5.3 7.3 7.3 7.9 6.0 3.6
16.1 13.4 17.0 16.5 19.2 14.2 7.5
WI)
H 2-CH, 4-CH, 6-CH, 7-CH, 2,7-(cH,), 2-i-C,H,
13.8 14.7 15.7 14.9 15.1 14.1 8.9
4.4 4.6 5.3 5.5 5.8 6.7 8.5
18.2 19.3 21.0 20.4 20.9 20.8 17.4
Bi(II1)
H 2-CH, 4-CH, 6-CH, 7-CH, 2,7-(CH,), 2i-C,H,
12.7 12.4 13.3 13.6 13.5 13.7 7.2
8.3 9.2 10.8 11.4 9.8 9.4 7.0
26.2 28.5 30.5 32.4 29.5 30.0 20.8
CL1
Fig. 1. Formation curves of zinc 8-mercaptoquinolinate (l), cadmium 2-methyl-8-mercaptoquinolinate (2) and nickel 6-methyl-8-mercaptoquinolinate (3). stability. The chelating power of an alkylmercaptoquinoline is accounted for by the donor heterocyclic nitrogen atom, the sulphur atom conjugated with the quinoline ring, the dative n-bonding of the central atom and finally by the formation of five-membered rings. In the case of Ag(I), Zn(II),
Cd(II), Pb(I1) and Bi(II1) stepwise complex formation takes place. Simultaneous addition of two molecules of the ligand to nickel occurs in accordance with the values of the stepwise constants. When a metal with occupied d-orbitals forms a cationic complex with an alkylmercaptoquinoline, bonding characteristics do not vary apart from a slight decrease in the conjugation of the chelating ring, which makes the metal-sulphur bonding more ionic. The situation is more complicated for the complexes formed by a metal with empty d-orbitals. It is well known that the stability of complexes depends on the possibility for charge levelling by a-bonding and n-back-bonding. Like many polyatomic ligands the alkylquinolinate ligands can have the special features of a-donation and n-back-donation, the canonical structures 6, c, d providing the greatest contribution (Scheme l).’ The donor-acceptor interaction MtS which more or less increases the stability of the dative M-3 n-bonding, and the existence of the dative n-bonding formed by the d-electrons of the central atom and the delocalized p-orbitals of the quinoline nucleus, are a
characteristic feature of the meso-ionic structures of the complexes. This strengthening of bonding leads to greater stability of the transition metal chelates, e.g., in the case of nickel. The introduction of an alkyl group into the
-I b
C Scheme 1.
d
ANALYTICAL
8-mercaptoquinoline molecule generally increases the complex stability (Table 1). This is connected with strengthening of the basic properties of the donor nitrogen atom and weakening of the acid properties of the mercapto group. An alkyl group in the 7-position causes further weakening of the acidity of the mercapto group. The reduced acidity strengthens the covalent bonding M-S, distorts the n-d-conjugation of the sulphur atom with the quinoline ring and finally increases the basicity of the nitrogen atom. The possibility of coplanarity being destroyed in the case of the 7-alkyl ligand, together with the factors mentioned above, brings about the greater stability of the metal 7-alkyl-8mercaptoquinolinates. A greater influence of the 7-alkyl group appears for the nickel chelates. The increase in basicity of the nitrogen atom results in a greater stabilization of the meso-ionic structure. The greatest stability of the meso-ionic structure and a thus higher stability constant is characteristic of the complex. nickel 4-methyl-8-mercaptoquinolinate This is likely to be a result of the hyperconjugation of the methyl group with the quinoline ring and increase in the basicity of the nitrogen atom. The lower stability of the nickel complex with the 2-alkylmercaptoquinolines is notable in the range of metal complexes studied. This is likely to be a result of the steric effects of an alkyl group in the 2-position, which cause the nickel complex to take the configuration of a distorted tetrahedron, whereas all the other nickel complexes have a square-planar configuration.’ The deviation from the square-planar configuration is assumed to decrease the strength of the dative d,-p, bonding formed by the d-electrons of the central atom and the delocalized p-orbitals of the quinoline nucleus. The alkyl group in the 2-position influences the stability of other metal 8-mercaptoquinolinates in a similar manner, although to a lesser extent (because these metal 8-mercaptoquinolinates do not have a square-planar configuration). Therefore the chelates with 2,7dimethyl-8-mercaptoquinoline are the most stable. In spite of the steric hindrance the methyl group in the 2-position strengthens the basicity of the nitrogen atom. Furthermore the methyl group in the 7-position helps to strengthen the metal-ligand bonding. Attention should be drawn to the determination of the stability constants of silver alkyl-8mercaptoquinolinates in DMF containing 2.5% water. The character of the relation between the stability constant and the water content has been clarified for the silver 6-methyl-8-mercaptoquinolinate complex. The stability of this complex was found to increase with increase in the DMF content. A plot of log k us. the reciprocal of the dielectric constant of the medium is linear (Fig. 2) and extrapolation to 100% DMF (points A, and A, in Fig. 2) makes it possible to compare the stability constant for the silver complex with those for other
729
DATA
154
57
14 8 -t’ 5 14 2
51
I 255
I 2 60 100
I 265 16
Fig. 2. The relation between stepwise stability constants of silver 6-methyl-8-mercaptoquinolinates and the composition of the medium.
8-mercaptoquinolinates.
The values obtained are log the values for the other silver complexes is difficult, because of the low solubilities in DMF. The complexes are also adsorbed on the platinum electrode when there are traces of water in the DMF, and this causes unsatisfactory results. According to the data obtained, the number of ligands in the complex is usually the same as the oxidation state of the central atom, over a large range of concentrations. The reason for this is considered to be the large size of the 8-mercaptoquinoline molecule and the ease of polarization of sulphur, which transfers charge to the central atom and therefore prevents the appearance of a high co-ordination number. However, in silver 8-mercaptoquinolinate two ligands are bonded to the metal, forming the species [AgL,]-. The possibility of existence of analogous complexes has been reported by Suprunovich and Shevchenko.’ The central atom is likely to use the electron-acceptor 5s-, Sp- and Sd-orbitals for bonding the second molecule of 8-mercaptoquinoline. The stability of the complexes decreases in general in the order Bi(II1) > Ag(I) > Ni(I1) > Pb(I1) h Cd(I1) N Zn(I1). k, = 15.4; log k2 = 5.7. To extrapolate
REFERENCES 1. Yu. A. Bankovskii, The Chemistry of the Mercaptoquinoline Chelates and their Derivatives, Zinatne, Riga, 1978. Izv. Akad. Nauk 2. V. M. Shulman and T. V. Kramareva, USSR (Siberia Dept.), 1961, 55. 3. S. V. Larionov. V. M. Shulman and L. A. Podolskava. Zh. Neorgan. Khim., 1967, 12, 1253. 4. Yu. A. Bankovskii, D. E. Zaruma, E. A. Luksha and A. P. Sturis, Izv. Akad. Nauk. Lutv. SSR, 1966, 387. and H. Flaschka, Die kom5. G. Schwarzenbach plexomerrische Titration, Enke Verlag, Stuttgart, 1965. and H. Rossotti, The Determination of 6. F. Rossotti Stability Constants and other Equilibrium Constants in Solution, McGraw-Hill, New York 1961. I. L. Ya. Pech, Ya. K. Ozols, A. P. Sturis and A. F. Ievinsh, Izv. Akad. Nauk. Latv. SSR, 1974, 621. and Yu. I. Shevchenko. Koord. 8. V. I. Suprunovich Khim., 1979, 5, 1167.
0039-9140/84 $3.00+ 0.00 Copyright 0 1984Pergamon Press Ltd
ANALYTICAL
DATA
THE STABILITY OF METAL COMPLEXES WITH 8-MERCAPTOQUINOLINE AND ALKYL-SUBSTITUTED %MERCAPTOQUINOLINES IN DIMETHYLFORMAMIDE N. A. ULAKHOVICH, H. C. BUDNIKOV, T. S. GORBUNOVA and A. P. STURIS Faculty of Chemistry, V.I. Ul’yanov-Lenin State University, Lenina 18, Kazan, 420008, USSR (Received 13 March 1984. Accepted 5 April 1984)
Summary-The stoichiometry and stability constants of I-mercaptoquinoline and alkyl-8 mercaptoquinoline complexes of Zn(II), Cd(B), Pb(II), Ni(II), Bi(II1) and Ag(1) were determined potentiometrically in dimethylformamide. The stability of the 8-mercaptoquinolinates decreases in the order Ag(I) > Bi(II1) > Ni(I1) > Pb(I1) > Cd(I1) > Zn(I1). Metal 7-methyl-8-mercaptoquinolinates are the most stable. The presence of the alkyl group in the 2-position (which has a steric effect) lowers the strength of metal-ligand bonding.
%Mercaptoquinoline and alkyl-%mercaptoquinolines are well known as complexing reagents for heavy metals.’ These reagents are especially promising when
used for combining concentration and separation with a sensitive method of determination. Hence the stability constants of these complexes in non-aqueous solutions should prove of interest. As a rule the extracted species is a non-electrolyte. Therefore, in order to study the stability of the 8-mercaptoquinolinates by potentiometry it is necessary to add some polar solvent to the extract or to use only a polar solution. This article deals with measurement of the redox potentials of systems containing the ligand (8-mercaptoquinolinate anion), an oxidized form of the ligand (the disulphide of 8-mercaptoquinoline), and metal ion, at a constant concentration of the disulphide.2.3 The conditions for application of the method are that the electrode process is reversible and the metal complexes only with the reduced form of the ligand. The stability constants of Zn(II), Cd(H), Pb(II), Ni(II), Bi(II1) and Ag(1) alkyl-8-mercaptoquinolinates in dimethylformamide have been determined, the alkyl positions in the ligand being 2,4, 6,7 and 2,7. EXPERIMENTAL
electrolyte. Dimethylformamide (DMF) was vent. The purity of the electrolyte was checked polarograms with a platinum electrode. The DMF was run as a check on electrochemical
used as solby recording base-line for purity.
Apparatus and potentiometric measurements
Potentiometric measurements were made with an R363 potentiometer (USSR) at 25 + 0.2”. The indicator electrode was a platinum plate, the reference electrode a saturated calomel electrode. The solutions were deoxygenated, and a slow stream of argon was passed over the solution throughout the experiment. The logarithmic values obtained from a series of measurements differed by not more than kO.1, even in the most unfavourable conditions. The stability constants were obtained from the experimental data by the method of Schulman et al.’ with some slight alterations. Instead of the simultaneous titration of a ligand with a metal and a background electrolyte, the redox electrode potentials in the presence and absence of metal ions were measured to save time. The values of the stability constants were calculated according to Bjerrum, with the formulae
- log[L] = & r-,(1
n=
- log Cl_ -
~o-wo.o~~
Gi
’
where C, and C, are the total concentrations of the ligand and metal respectively, AE is the difference in the equilibrium potential of the ligand redox-system in the presence and absence of metal ions. In a number of cases, where the values of the stepwise stability constants were rather close, another method of calculating was used to correct the data.’
Reagents
The sodium salts of the alkyl-8-mercaptoquinohnes and corresponding disulphides were prepared as described in the literature.4 The analytical (total) concentrations of the ligand and disulphidk were constant in all experiments, and 5 x 10e4M and 2.5 x 10e4M resoectivelv. The metal ion concentrations were varied in ihe range 1 x 10e41 x IO-‘M. Stock solutions of the anhydrous metal perchlorates (twice recrystallized) were prepared from the solids and standardized by EDTA titration.’ Lithium perchlorate solution (O.lM) was used as the background 727
RESULTS AND DISCUSSION
The relations between the average ligand number, 6, and the corresponding concentration of the free, uncomplexed, ligand are shown in Fig. 1 for some metal-ligand systems. Logarithmic values of the stability constants are given in Table 1, and show that the metal alkylmercaptoquinolinates have high
128
ANALYTICAL
DATA
Table 1. Logarithmic values of the stability constants of the alkyl-8-mercaptoquinolinate complexes
I
I
1
4
6 -log
I
I
6
10
Metal
R
logk,
logk,
log82
Zn(I1)
H 2-CH, CCH, 6-CH, 7-CH, 2,7-(cH,), 2-i-C,H,
8.1 8.5 8.3 8.3 10.8 9.1 5.9
4.3 6.4 6.2 6.2 8.3 6.9 5.4
12.4 14.9 14.5 14.5 19.1 16.0 11.3
Cd(I1)
H 2-CH, 4-CH, 6_CH, 7-CH, 2,7-(cH,), 2-i-C,H,
8.3 8.4 8.1 9.3 9.8 9.2 5.1
4.9 6.1 6.1 7.1 6.2 6.9 4.6
13.2 14.5 14.2 16.4 16.0 16.1 9.7
Pb(I1)
H 2-CH, 4-CH, 6-CH, 7-CH, 2,7-(cH,), 2i-C,H,
8.6 8.8 8.4 9.4 10.6 10.0 4.5
5.5 5.3 5.1 6.9 6.7 6.5 3.9
14.1 14.1 13.5 16.3 17.3 16.5 8.4
Ni(I1)
H 2-CH, 4-CH, 6-CH, 7-CH, 2,7-(cH,), 2i-C,H,
9.5 8.1 9.7 9.2 11.3 8.2 3.9
6.6 5.3 7.3 7.3 7.9 6.0 3.6
16.1 13.4 17.0 16.5 19.2 14.2 7.5
WI)
H 2-CH, 4-CH, 6-CH, 7-CH, 2,7-(cH,), 2-i-C,H,
13.8 14.7 15.7 14.9 15.1 14.1 8.9
4.4 4.6 5.3 5.5 5.8 6.7 8.5
18.2 19.3 21.0 20.4 20.9 20.8 17.4
Bi(II1)
H 2-CH, 4-CH, 6-CH, 7-CH, 2,7-(CH,), 2i-C,H,
12.7 12.4 13.3 13.6 13.5 13.7 7.2
8.3 9.2 10.8 11.4 9.8 9.4 7.0
26.2 28.5 30.5 32.4 29.5 30.0 20.8
CL1
Fig. 1. Formation curves of zinc 8-mercaptoquinolinate (l), cadmium 2-methyl-8-mercaptoquinolinate (2) and nickel 6-methyl-8-mercaptoquinolinate (3). stability. The chelating power of an alkylmercaptoquinoline is accounted for by the donor heterocyclic nitrogen atom, the sulphur atom conjugated with the quinoline ring, the dative n-bonding of the central atom and finally by the formation of five-membered rings. In the case of Ag(I), Zn(II),
Cd(II), Pb(I1) and Bi(II1) stepwise complex formation takes place. Simultaneous addition of two molecules of the ligand to nickel occurs in accordance with the values of the stepwise constants. When a metal with occupied d-orbitals forms a cationic complex with an alkylmercaptoquinoline, bonding characteristics do not vary apart from a slight decrease in the conjugation of the chelating ring, which makes the metal-sulphur bonding more ionic. The situation is more complicated for the complexes formed by a metal with empty d-orbitals. It is well known that the stability of complexes depends on the possibility for charge levelling by a-bonding and n-back-bonding. Like many polyatomic ligands the alkylquinolinate ligands can have the special features of a-donation and n-back-donation, the canonical structures 6, c, d providing the greatest contribution (Scheme l).’ The donor-acceptor interaction MtS which more or less increases the stability of the dative M-3 n-bonding, and the existence of the dative n-bonding formed by the d-electrons of the central atom and the delocalized p-orbitals of the quinoline nucleus, are a
characteristic feature of the meso-ionic structures of the complexes. This strengthening of bonding leads to greater stability of the transition metal chelates, e.g., in the case of nickel. The introduction of an alkyl group into the
-I b
C Scheme 1.
d
ANALYTICAL
8-mercaptoquinoline molecule generally increases the complex stability (Table 1). This is connected with strengthening of the basic properties of the donor nitrogen atom and weakening of the acid properties of the mercapto group. An alkyl group in the 7-position causes further weakening of the acidity of the mercapto group. The reduced acidity strengthens the covalent bonding M-S, distorts the n-d-conjugation of the sulphur atom with the quinoline ring and finally increases the basicity of the nitrogen atom. The possibility of coplanarity being destroyed in the case of the 7-alkyl ligand, together with the factors mentioned above, brings about the greater stability of the metal 7-alkyl-8mercaptoquinolinates. A greater influence of the 7-alkyl group appears for the nickel chelates. The increase in basicity of the nitrogen atom results in a greater stabilization of the meso-ionic structure. The greatest stability of the meso-ionic structure and a thus higher stability constant is characteristic of the complex. nickel 4-methyl-8-mercaptoquinolinate This is likely to be a result of the hyperconjugation of the methyl group with the quinoline ring and increase in the basicity of the nitrogen atom. The lower stability of the nickel complex with the 2-alkylmercaptoquinolines is notable in the range of metal complexes studied. This is likely to be a result of the steric effects of an alkyl group in the 2-position, which cause the nickel complex to take the configuration of a distorted tetrahedron, whereas all the other nickel complexes have a square-planar configuration.’ The deviation from the square-planar configuration is assumed to decrease the strength of the dative d,-p, bonding formed by the d-electrons of the central atom and the delocalized p-orbitals of the quinoline nucleus. The alkyl group in the 2-position influences the stability of other metal 8-mercaptoquinolinates in a similar manner, although to a lesser extent (because these metal 8-mercaptoquinolinates do not have a square-planar configuration). Therefore the chelates with 2,7dimethyl-8-mercaptoquinoline are the most stable. In spite of the steric hindrance the methyl group in the 2-position strengthens the basicity of the nitrogen atom. Furthermore the methyl group in the 7-position helps to strengthen the metal-ligand bonding. Attention should be drawn to the determination of the stability constants of silver alkyl-8mercaptoquinolinates in DMF containing 2.5% water. The character of the relation between the stability constant and the water content has been clarified for the silver 6-methyl-8-mercaptoquinolinate complex. The stability of this complex was found to increase with increase in the DMF content. A plot of log k us. the reciprocal of the dielectric constant of the medium is linear (Fig. 2) and extrapolation to 100% DMF (points A, and A, in Fig. 2) makes it possible to compare the stability constant for the silver complex with those for other
729
DATA
154
57
14 8 -t’ 5 14 2
51
I 255
I 2 60 100
I 265 16
Fig. 2. The relation between stepwise stability constants of silver 6-methyl-8-mercaptoquinolinates and the composition of the medium.
8-mercaptoquinolinates.
The values obtained are log the values for the other silver complexes is difficult, because of the low solubilities in DMF. The complexes are also adsorbed on the platinum electrode when there are traces of water in the DMF, and this causes unsatisfactory results. According to the data obtained, the number of ligands in the complex is usually the same as the oxidation state of the central atom, over a large range of concentrations. The reason for this is considered to be the large size of the 8-mercaptoquinoline molecule and the ease of polarization of sulphur, which transfers charge to the central atom and therefore prevents the appearance of a high co-ordination number. However, in silver 8-mercaptoquinolinate two ligands are bonded to the metal, forming the species [AgL,]-. The possibility of existence of analogous complexes has been reported by Suprunovich and Shevchenko.’ The central atom is likely to use the electron-acceptor 5s-, Sp- and Sd-orbitals for bonding the second molecule of 8-mercaptoquinoline. The stability of the complexes decreases in general in the order Bi(II1) > Ag(I) > Ni(I1) > Pb(I1) h Cd(I1) N Zn(I1). k, = 15.4; log k2 = 5.7. To extrapolate
REFERENCES 1. Yu. A. Bankovskii, The Chemistry of the Mercaptoquinoline Chelates and their Derivatives, Zinatne, Riga, 1978. Izv. Akad. Nauk 2. V. M. Shulman and T. V. Kramareva, USSR (Siberia Dept.), 1961, 55. 3. S. V. Larionov. V. M. Shulman and L. A. Podolskava. Zh. Neorgan. Khim., 1967, 12, 1253. 4. Yu. A. Bankovskii, D. E. Zaruma, E. A. Luksha and A. P. Sturis, Izv. Akad. Nauk. Lutv. SSR, 1966, 387. and H. Flaschka, Die kom5. G. Schwarzenbach plexomerrische Titration, Enke Verlag, Stuttgart, 1965. and H. Rossotti, The Determination of 6. F. Rossotti Stability Constants and other Equilibrium Constants in Solution, McGraw-Hill, New York 1961. I. L. Ya. Pech, Ya. K. Ozols, A. P. Sturis and A. F. Ievinsh, Izv. Akad. Nauk. Latv. SSR, 1974, 621. and Yu. I. Shevchenko. Koord. 8. V. I. Suprunovich Khim., 1979, 5, 1167.