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Stability constants of zinc halide complexes in DMSO—water and DMF—water mixtures
Polyhedron Vol. 4, No. 8, pp. 1467-1470, Printed in Great Britain
1985 0
STABILITY CONSTANTS OF ZINC HALIDE COMPLEXES DMSO-WATER AND DMF-WATER MIXTURES
0277-5387/W $3.00+ .OO 1985 pcrgsmon Press Ltd
IN
F. GAIZER* Department of Inorganic and Analytical Chemistry, A. Jozsef University, Szeged, P.O. Box 440, Hungary
II. B. SILBER Division of Earth and Physical Sciences, The University of Texas at San Antonio, TX 78285, U.S.A. and J. LAZAR
Department for Pharmaceutical
Chemistry, The Medical University, Szeged, Hungary
(Received 6 December 1984; accepted 6 February 1985) Abstract-The
overall stability constants of zinc bromide and iodide complexes in DMSOwater and those of zinc chloride, bromide and iodide in DMF-water mixtures have been determined potentiometrically at mole ratios about 0.2-l for the organic solvents at 25°C and in a 0.5 M ammonium perchlorate ionic medium. The complex formation is much stronger in DMF and its mixtures than in DMSO and the log j3sare generally higher in solvent mixtures than those expected from the values measured in the pure components.
In our previous study1 the change of the stability constants of the zinc chloride complexes has been observed in DMSO-water mixtures. It was pointed out that the values of the constants change in response to the change of the composition of the solvent mixtures, but the constants are greater than could be expected from the values measured in pure DMSO and water. The above studies were extended to zinc bromide and zinc iodide systems in DMSO and to all the three zinc halides in DMF-water mixtures. EXPERIMENTAL Chemicals. Zinc perchlorate was examined according to our previous descriptions.’ The ammonium halides were of analytical grade. DMSO was purified by vacuum distillation and DMF was purified by fractional distillation after the addition of 10% benzol and 5% water. Titrations and computer evaluation of the results were carried out by the MAXIPOT program* corresponding to the previous paper.’
* Author to whom correspondence should be addressed.
RESULTS
The titrations were performed at constant zinc ion concentration. Since only small differences were observed between the shapes of potentiometric curves obtained in either system, for the sake of conciseness, potentiometric results of the study of the zinc chloride and zinc iodide systems in DMF-water mixtures are presented as two extreme cases (Fig. 1). The log /3 values showing the best agreement between the measured and calculated potentials are summarized in Tables 1 and 2. The standard deviations of the constants calculated from the residuals of the individual titration curves are given in brackets. However, it must be noted that these are the standard deviations of the fit of only one experimental curve and, considering reproducibility, the errors of the log jJ2 and & values amount to approximately 0.05-0.1. As can be seen from the data, the uncertainty of the constants of complexes (1,l) is considerably greater than for the other complexes. Therefore, in Figure 2 illustrating the data of Table 2, the log /I1 values are displayed with their errors calculated from individual titration curves. It is worth mentioning that, because of the
1467
F. GAIZER
1468 cm3 0.48
I 0
I 0.05
I 0.1 q
(M
cm3 0.48
I 0.05
I 01 T,,-
I
0.2
dm-‘) M NHdCl
I 0 15 (M
solutions of 0.5 molar ionic strength with NH,ClO, at 25°C are summarized in Table 3. The values obtained in DMSO approximately
M NH31
I 0.15
et al.
correspond to the literature results,3 the deviations are probably due to the difference in ionic strengths. Accessible data4 are not available for the constants measured in DMF. The difference between the measured values of equilibrium constants and those expected from the composition of the solvent mixtures is never negative in any system, i.e. the solvent mixtures usually have a stabilizing effect on the complex formation. In mixtures in which there are detectable deviations between the measured values and those expected from constants obtained in pure solvents, generally the maximum of the deviations is approximately at a = 0.5 and the shapes of difference-curves are symmetrical. Deviations from this were detected
I 0.2
dme3)
Fig. 1. Potentiometric titration curves of 50.0 cm3 0.01 M zinc perchlorate in DMF-water mixtures, A : with 0.48 M ammonium chloride and B: with 0.48 M ammonium iodide. One half of the experimental points chosen randomly are plotted. The solid curves are the calculated ones using the formation constants summarized in Table 2.
minor solubility of ammonium chloride, measurements could be made in DMF only in mixtures containing more than 5% water. From the experimental data it could be stated that the values of all the equilibrium constants are greater by several orders of magnitude in DMF than in DMSO. It is also obvious that the zinc iodide system
can well be measured in DMF, but in DMSO a rather slight interaction can be experienced : the value of AE is only about 2 mV in response to the 0.2 mol iodide. Systems containing chloride or bromide can definitely be described by the supposition of the formation of complexes (1, l), (1,2) and (1,3). Among systems containing iodide the formation of these three species could definitely be detected only in DMF and values with relatively great experimental errors were only obtained in the case oflog j?i values. However, in DMSO the formation of the diiodo complex was found to be minor and that of the triiodo complex cannot be substantiated on the basis of measurements. DISCUSSION The stability constants of zinc halide complexes obtained in the course of measurements made in
0 IO
0.5 05
1.0 0 0 %MSO’O cl&O
05 0.5
I 0 IO
05 05
C
‘090 0 U&F
B
A
i;4 10 QH,O 0 aDMF
I 0 I.0
0.5 0 5
\ 1.0 aH,O o “DMF
D
Fig. 2. The plot of the measured log /.I1 (lines); log & (points) and log p3 (crosses) values of zinc halide complexes as a function of molar fraction of organic solvent. A : Zn2 ‘-Br - in DMSG-water mixtures and B : Zn’+-Cl-, C: Zn’+-Brand D: Zn’+--I- in DMFwater mixtures. The dotted lines are the probable runnings of the constants.
Stability constants of zinc halide complexes
1469
Table 1. Formation constants and their errors (in parentheses) of zinc halide complexes in DMF-water solvent mixtures
System
log B1
log A
108 83 9.57 (0.04) 8.22 (0.02) 6.39 (0.05) 4.87 (0.07) 4.66 (0.03) 3.47 (0.02)
u, mV
No. of expt. points
324 282 219 95.5 45 25.1 -
0.87 0.55 1.13 0.26 0.4 0.12 0.11
55 97 99 87 80 77 62
K3
Zr?+-Cl-
0.81 0.67 0.48 0.35 0.35 0.26 0.13
2 (0.15) 1.42 (0.05) 0.5 (0.42) 0.72 (0.05) - -0.13 (0.1)
7.06 (0.02) 5.77 (0.02) 4.05 (0.04) 2.89 (0.13) 3.01 (0.09) 2.07 (0.07) 1.21 (0.04)
Zn2+-Br-
1 1 0.96 0.91 0.88 0.85 0.67 0.56 0.48 0.35
1.90 (0.4) 2.78 (0.54) 1.19 (0.3) 0.67 (0.2) 1.77 (0.17) 1.63 (0.5) 1.63 (0.11) 1.30 (0.08) 1.03 (0.07) 0.46 (0.03)
7.24 (0.1) 7.03 (0.06) 6.22 (0.03) 5.78 (0.07) 5.49 (0.06) 5.12 (0.05) 4.12 (0.09) 3.55 (0.05) 2.77 (0.05) 1.77 (0.04)
8.91 (0.1) 8.70 (0.07) 8.72 (0.01) 8.07 (0.02) 7.88 (0.03) 7.45 (0.01) 6.15 (0.02) 5.26 (0.02) 4.22 (0.02) 3.01 (0.01)
47 47 316 195 245 214 107 51 28 17.5
2.97 3.23 0.86 1.35 0.71 0.65 0.77 0.37 0.22 0.06
36 73 96 77 100 79 95 99 82 82
Zn2+-I-
1 0.91 0.84 0.78 0.68 0.55
0.79 (0.11) 0.59 (0.11) 0.55 (0.1) 0.36 (0.04) 0.36 (0.1) - -0.22 (0.1)
2.85 (0.04) 2.72 (0.04) 2.30 (0.06) 2.13 (0.02) 1.77 (0.05) 1.38 (0.1)
4.30 (0.01) 3.94 (0.02) 3.61 (0.02) 3.0 (0.15) 2.65 (0.1) -
28 17 20 7 8
0.27 0.18 0.17 0.1 0.08 0.12
90 90 93 84 100 82
only in zinc bromide and zinc iodide systems in DMF, but only in the case of log /&. In order to aid in the interpretation of the constants, we have measured the relative permittivity of the solvent mixtures. They differ from additivity only to such a small extent, that the
equilibrium constants variations could not be interpreted in terms of relative permittivity. By means of the equilibrium constants the number of halide ions coordinated to the zinc ion (@ was calculated and plotted against the composition of the solvent mixtures. In the case of all the systems studied
Table 2. Formation constants and their errors (in parentheses) of zinc halide complexes in DMSOwater solvent mixtures
System
aDMsO
hit
Bi
Zn’+-Br-
1.0 0.96 0.96 0.92 0.92 0.85 0.69 0.58 0.5 0.47 0.27
1.68 (0.2) 1.38 (0.2) 1.45 (0.12) 1.46 (0.15) 1.44 (0.15) 0.98 (0.20) 1.11 (0.07) 0.85 (0.13) 0.94 (0.10) 0.69 (0.10) 0.1 (0.06)
Zn2+-I+
1.0
0.12 (0.05)
loi3
B2
4.27 (0.05) 4.05 (0.06) 4.13 (0.02) 4.02 (0.03) 4.02 (0.02) 3.90 (0.03) 3.38 (0.03) 2.88 (0.03) 2.52 (0.03) 1.41 (0.06) 0.56 (0.14) -
1%
lb
5.8 (0.03) 5.69 (0.04) 5.78 (0.02) 5.64 (0.02) 5.73 (0.02) 5.28 (0.03) 4.69 (0.10) 3.86 (0.04) 3.69 (0.04) 2.85 (0.02) 1.91 (0.06) -
KS
a,
mV
No. of expt. points
34 43.6 44.7 41.7 51 24 20 9.5 15 27 22
0.75 0.82 0.20 0.37 0.23 0.40 0.15 0.36 0.18 0.1 0.08
37 36 44 35 44 37 44 35 37 29 22
-
0.19
20
1470
F. GAIZER
et al.
Table3.Thelogarithmofoverallstabilityconstantsofzinchaiidecomplexesin DMF and DMSO in 0.5 M (NH&IO,) ionic medium Solvent
cl-
Br-
DMF
log B1 log 82 log 83
3f0.25 8.2 fO.l 10.7kO.15
DMSO
log/I, log Bz log BJ
6.16kO.l 8.76kO.l
a curve similar to that of the zinc chloride system was obtained,’ the limiting value of which cannot exceed fi = 3 even in the case of the most stable systems. On this basis, in accordance with our earlier supposition, for the geometry of the complexes a tetrahedral configuration is predicted in which three of the coordination sites are occupied by one halide ion each and the fourth coordination site is occupied by a solvent molecule. This result is consistent with the ultrasonic absorption measurements in ZnCl, systems in aqueous methanol, DMSO and DMF.5*6 This supposition is also supported by electrondiffraction measurements recently made on the LiZnBr, solution.7 The experimental technique used in the course of experiments provided reliable values only for the stability constants of the (1,2) and (1,3) complexes. Generally, the stability constants of the (1,l) complex were determined only with relatively great experimental errors, moreover, in solutions in which the complex formation is rather strong they could not be treated as parameters of fits and the determination of their values was not possible by
2kO.5 7.1 fO.l 8.8kO.12 1.7kO.2 4.27 + 0.07 5.8kO.l
I0.8 f 0.3 2.8kO.l 4.3kO.l 0.12f0.5 -
graphical treatment either. In spite of this, it would be ditkult to exclude the possibility of the formation of this species. Acknowledgement-The authors graciously acknowledge the Robert A. Welch Foundation of Houston, Texas, U.S.A. for partial support under Grant AX-659.
REFERENCES 1. F. Gaizer and H. B. Silber, J. Znorg. Nucl. Chem. 1980, 42, 1317. 2. F. Gaizer, Acta Chim. Acad. Sci. Hung. 1980,103,397. 3. S. Ahrland and N. 0. Bjork, Acta Chem. Scand. A 1965, 30,265. 4. K. G. Ashurst, Natl. Inst. Metall., S. Afi. Rep. 1974, 1626; Chem. Abstr. 1974,81,159816q. 5. H. B. Silber, L. U. Kromer and F. Gaizer, Znorg. Chem. 1981,20,3323. 6. H. B. Silber, D. Simon and F. Gaizer, Znorg. Chem. 1984,23,2844. 7. G. Kabisch, E. Kalmaq G. P&link&, T. Radnai and F. Gaizer, Chem. Phys. Letts. 1984, 107,463.
1985 0
STABILITY CONSTANTS OF ZINC HALIDE COMPLEXES DMSO-WATER AND DMF-WATER MIXTURES
0277-5387/W $3.00+ .OO 1985 pcrgsmon Press Ltd
IN
F. GAIZER* Department of Inorganic and Analytical Chemistry, A. Jozsef University, Szeged, P.O. Box 440, Hungary
II. B. SILBER Division of Earth and Physical Sciences, The University of Texas at San Antonio, TX 78285, U.S.A. and J. LAZAR
Department for Pharmaceutical
Chemistry, The Medical University, Szeged, Hungary
(Received 6 December 1984; accepted 6 February 1985) Abstract-The
overall stability constants of zinc bromide and iodide complexes in DMSOwater and those of zinc chloride, bromide and iodide in DMF-water mixtures have been determined potentiometrically at mole ratios about 0.2-l for the organic solvents at 25°C and in a 0.5 M ammonium perchlorate ionic medium. The complex formation is much stronger in DMF and its mixtures than in DMSO and the log j3sare generally higher in solvent mixtures than those expected from the values measured in the pure components.
In our previous study1 the change of the stability constants of the zinc chloride complexes has been observed in DMSO-water mixtures. It was pointed out that the values of the constants change in response to the change of the composition of the solvent mixtures, but the constants are greater than could be expected from the values measured in pure DMSO and water. The above studies were extended to zinc bromide and zinc iodide systems in DMSO and to all the three zinc halides in DMF-water mixtures. EXPERIMENTAL Chemicals. Zinc perchlorate was examined according to our previous descriptions.’ The ammonium halides were of analytical grade. DMSO was purified by vacuum distillation and DMF was purified by fractional distillation after the addition of 10% benzol and 5% water. Titrations and computer evaluation of the results were carried out by the MAXIPOT program* corresponding to the previous paper.’
* Author to whom correspondence should be addressed.
RESULTS
The titrations were performed at constant zinc ion concentration. Since only small differences were observed between the shapes of potentiometric curves obtained in either system, for the sake of conciseness, potentiometric results of the study of the zinc chloride and zinc iodide systems in DMF-water mixtures are presented as two extreme cases (Fig. 1). The log /3 values showing the best agreement between the measured and calculated potentials are summarized in Tables 1 and 2. The standard deviations of the constants calculated from the residuals of the individual titration curves are given in brackets. However, it must be noted that these are the standard deviations of the fit of only one experimental curve and, considering reproducibility, the errors of the log jJ2 and & values amount to approximately 0.05-0.1. As can be seen from the data, the uncertainty of the constants of complexes (1,l) is considerably greater than for the other complexes. Therefore, in Figure 2 illustrating the data of Table 2, the log /I1 values are displayed with their errors calculated from individual titration curves. It is worth mentioning that, because of the
1467
F. GAIZER
1468 cm3 0.48
I 0
I 0.05
I 0.1 q
(M
cm3 0.48
I 0.05
I 01 T,,-
I
0.2
dm-‘) M NHdCl
I 0 15 (M
solutions of 0.5 molar ionic strength with NH,ClO, at 25°C are summarized in Table 3. The values obtained in DMSO approximately
M NH31
I 0.15
et al.
correspond to the literature results,3 the deviations are probably due to the difference in ionic strengths. Accessible data4 are not available for the constants measured in DMF. The difference between the measured values of equilibrium constants and those expected from the composition of the solvent mixtures is never negative in any system, i.e. the solvent mixtures usually have a stabilizing effect on the complex formation. In mixtures in which there are detectable deviations between the measured values and those expected from constants obtained in pure solvents, generally the maximum of the deviations is approximately at a = 0.5 and the shapes of difference-curves are symmetrical. Deviations from this were detected
I 0.2
dme3)
Fig. 1. Potentiometric titration curves of 50.0 cm3 0.01 M zinc perchlorate in DMF-water mixtures, A : with 0.48 M ammonium chloride and B: with 0.48 M ammonium iodide. One half of the experimental points chosen randomly are plotted. The solid curves are the calculated ones using the formation constants summarized in Table 2.
minor solubility of ammonium chloride, measurements could be made in DMF only in mixtures containing more than 5% water. From the experimental data it could be stated that the values of all the equilibrium constants are greater by several orders of magnitude in DMF than in DMSO. It is also obvious that the zinc iodide system
can well be measured in DMF, but in DMSO a rather slight interaction can be experienced : the value of AE is only about 2 mV in response to the 0.2 mol iodide. Systems containing chloride or bromide can definitely be described by the supposition of the formation of complexes (1, l), (1,2) and (1,3). Among systems containing iodide the formation of these three species could definitely be detected only in DMF and values with relatively great experimental errors were only obtained in the case oflog j?i values. However, in DMSO the formation of the diiodo complex was found to be minor and that of the triiodo complex cannot be substantiated on the basis of measurements. DISCUSSION The stability constants of zinc halide complexes obtained in the course of measurements made in
0 IO
0.5 05
1.0 0 0 %MSO’O cl&O
05 0.5
I 0 IO
05 05
C
‘090 0 U&F
B
A
i;4 10 QH,O 0 aDMF
I 0 I.0
0.5 0 5
\ 1.0 aH,O o “DMF
D
Fig. 2. The plot of the measured log /.I1 (lines); log & (points) and log p3 (crosses) values of zinc halide complexes as a function of molar fraction of organic solvent. A : Zn2 ‘-Br - in DMSG-water mixtures and B : Zn’+-Cl-, C: Zn’+-Brand D: Zn’+--I- in DMFwater mixtures. The dotted lines are the probable runnings of the constants.
Stability constants of zinc halide complexes
1469
Table 1. Formation constants and their errors (in parentheses) of zinc halide complexes in DMF-water solvent mixtures
System
log B1
log A
108 83 9.57 (0.04) 8.22 (0.02) 6.39 (0.05) 4.87 (0.07) 4.66 (0.03) 3.47 (0.02)
u, mV
No. of expt. points
324 282 219 95.5 45 25.1 -
0.87 0.55 1.13 0.26 0.4 0.12 0.11
55 97 99 87 80 77 62
K3
Zr?+-Cl-
0.81 0.67 0.48 0.35 0.35 0.26 0.13
2 (0.15) 1.42 (0.05) 0.5 (0.42) 0.72 (0.05) - -0.13 (0.1)
7.06 (0.02) 5.77 (0.02) 4.05 (0.04) 2.89 (0.13) 3.01 (0.09) 2.07 (0.07) 1.21 (0.04)
Zn2+-Br-
1 1 0.96 0.91 0.88 0.85 0.67 0.56 0.48 0.35
1.90 (0.4) 2.78 (0.54) 1.19 (0.3) 0.67 (0.2) 1.77 (0.17) 1.63 (0.5) 1.63 (0.11) 1.30 (0.08) 1.03 (0.07) 0.46 (0.03)
7.24 (0.1) 7.03 (0.06) 6.22 (0.03) 5.78 (0.07) 5.49 (0.06) 5.12 (0.05) 4.12 (0.09) 3.55 (0.05) 2.77 (0.05) 1.77 (0.04)
8.91 (0.1) 8.70 (0.07) 8.72 (0.01) 8.07 (0.02) 7.88 (0.03) 7.45 (0.01) 6.15 (0.02) 5.26 (0.02) 4.22 (0.02) 3.01 (0.01)
47 47 316 195 245 214 107 51 28 17.5
2.97 3.23 0.86 1.35 0.71 0.65 0.77 0.37 0.22 0.06
36 73 96 77 100 79 95 99 82 82
Zn2+-I-
1 0.91 0.84 0.78 0.68 0.55
0.79 (0.11) 0.59 (0.11) 0.55 (0.1) 0.36 (0.04) 0.36 (0.1) - -0.22 (0.1)
2.85 (0.04) 2.72 (0.04) 2.30 (0.06) 2.13 (0.02) 1.77 (0.05) 1.38 (0.1)
4.30 (0.01) 3.94 (0.02) 3.61 (0.02) 3.0 (0.15) 2.65 (0.1) -
28 17 20 7 8
0.27 0.18 0.17 0.1 0.08 0.12
90 90 93 84 100 82
only in zinc bromide and zinc iodide systems in DMF, but only in the case of log /&. In order to aid in the interpretation of the constants, we have measured the relative permittivity of the solvent mixtures. They differ from additivity only to such a small extent, that the
equilibrium constants variations could not be interpreted in terms of relative permittivity. By means of the equilibrium constants the number of halide ions coordinated to the zinc ion (@ was calculated and plotted against the composition of the solvent mixtures. In the case of all the systems studied
Table 2. Formation constants and their errors (in parentheses) of zinc halide complexes in DMSOwater solvent mixtures
System
aDMsO
hit
Bi
Zn’+-Br-
1.0 0.96 0.96 0.92 0.92 0.85 0.69 0.58 0.5 0.47 0.27
1.68 (0.2) 1.38 (0.2) 1.45 (0.12) 1.46 (0.15) 1.44 (0.15) 0.98 (0.20) 1.11 (0.07) 0.85 (0.13) 0.94 (0.10) 0.69 (0.10) 0.1 (0.06)
Zn2+-I+
1.0
0.12 (0.05)
loi3
B2
4.27 (0.05) 4.05 (0.06) 4.13 (0.02) 4.02 (0.03) 4.02 (0.02) 3.90 (0.03) 3.38 (0.03) 2.88 (0.03) 2.52 (0.03) 1.41 (0.06) 0.56 (0.14) -
1%
lb
5.8 (0.03) 5.69 (0.04) 5.78 (0.02) 5.64 (0.02) 5.73 (0.02) 5.28 (0.03) 4.69 (0.10) 3.86 (0.04) 3.69 (0.04) 2.85 (0.02) 1.91 (0.06) -
KS
a,
mV
No. of expt. points
34 43.6 44.7 41.7 51 24 20 9.5 15 27 22
0.75 0.82 0.20 0.37 0.23 0.40 0.15 0.36 0.18 0.1 0.08
37 36 44 35 44 37 44 35 37 29 22
-
0.19
20
1470
F. GAIZER
et al.
Table3.Thelogarithmofoverallstabilityconstantsofzinchaiidecomplexesin DMF and DMSO in 0.5 M (NH&IO,) ionic medium Solvent
cl-
Br-
DMF
log B1 log 82 log 83
3f0.25 8.2 fO.l 10.7kO.15
DMSO
log/I, log Bz log BJ
6.16kO.l 8.76kO.l
a curve similar to that of the zinc chloride system was obtained,’ the limiting value of which cannot exceed fi = 3 even in the case of the most stable systems. On this basis, in accordance with our earlier supposition, for the geometry of the complexes a tetrahedral configuration is predicted in which three of the coordination sites are occupied by one halide ion each and the fourth coordination site is occupied by a solvent molecule. This result is consistent with the ultrasonic absorption measurements in ZnCl, systems in aqueous methanol, DMSO and DMF.5*6 This supposition is also supported by electrondiffraction measurements recently made on the LiZnBr, solution.7 The experimental technique used in the course of experiments provided reliable values only for the stability constants of the (1,2) and (1,3) complexes. Generally, the stability constants of the (1,l) complex were determined only with relatively great experimental errors, moreover, in solutions in which the complex formation is rather strong they could not be treated as parameters of fits and the determination of their values was not possible by
2kO.5 7.1 fO.l 8.8kO.12 1.7kO.2 4.27 + 0.07 5.8kO.l
I0.8 f 0.3 2.8kO.l 4.3kO.l 0.12f0.5 -
graphical treatment either. In spite of this, it would be ditkult to exclude the possibility of the formation of this species. Acknowledgement-The authors graciously acknowledge the Robert A. Welch Foundation of Houston, Texas, U.S.A. for partial support under Grant AX-659.
REFERENCES 1. F. Gaizer and H. B. Silber, J. Znorg. Nucl. Chem. 1980, 42, 1317. 2. F. Gaizer, Acta Chim. Acad. Sci. Hung. 1980,103,397. 3. S. Ahrland and N. 0. Bjork, Acta Chem. Scand. A 1965, 30,265. 4. K. G. Ashurst, Natl. Inst. Metall., S. Afi. Rep. 1974, 1626; Chem. Abstr. 1974,81,159816q. 5. H. B. Silber, L. U. Kromer and F. Gaizer, Znorg. Chem. 1981,20,3323. 6. H. B. Silber, D. Simon and F. Gaizer, Znorg. Chem. 1984,23,2844. 7. G. Kabisch, E. Kalmaq G. P&link&, T. Radnai and F. Gaizer, Chem. Phys. Letts. 1984, 107,463.