from Hydrochloric Acid Solutions

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1 ISS , Russian Journal of Inorganic hemistry, 203, Vol. 58, o. 4, pp Pleiades Publishing, Ltd., 203. riginal Russian Text L.G. Golubyatnikova, R.A. Khisamutdinov, A.. Lobov, Yu.I. Murinov, 203, published in Zhurnal eorganicheskoi Khimii, 203, Vol. 58, o. 4, pp PYSIAL EMISTRY F SLUTIS Extraction of Gold(III) with (RS)--(4-hlorophenyl)-4,4- dimethyl-3-(-,2,4-triazol--yl-methyl)-pentan-3-ol from ydrochloric Acid Solutions L. G. Golubyatnikova, R. A. Khisamutdinov, A.. Lobov, and Yu. I. Murinov Institute of rganic hemistry, Ufa Scientific enter, Russian Academy of Sciences, pr. ktyabrya 7, Ufa, Russia hisam@anrb.ru Received March 26, 202 Abstract Extraction of gold(iii) with (RS)--(4-chlorophenyl)-4,4-dimethyl-3-(-,2,4-triazol--ylmethyl)-pentan-3-ol from 3 M hydrochloric acid solutions (with chloroform as a diluent) has been studied. ptimal extraction conditions have been found. The reagent has been shown to extract efficiently metal ion from solutions containing 3 M hydrochloric acid due to formation of coordination bond between gold(iii) and the 4 atom of the triazole ring. The coordination mechanism of gold(iii) extraction has been proposed on the basis of the data obtained. oncentration constants of extraction have been calculated, and the thermodynamic parameters of extraction have been determined. DI: 0.34/S Recently, interest in coordination compounds of transition metals with many derivatives of,2,4-triazole has been growing due to their high biological activity. It was shown that the complexing properties of derivatives of,2,4-triazole towards transition metals, provided that their molecules contain no other heteroatoms, are conditioned by the presence of a triazole ring in their structures [, 2]. Previously, we have studied the extraction properties of some,2,4-triazole derivatives towards a number of metals including the noble ones. It was found that -{[2-(2,4-dichlorophenyl)-4-propyl-,3-dioxolan-2-yl]-methyl}--,2,4-triazole (propiconazole) and (RS)--(4-chlorophenyl)-4,4-dimethyl-3-(-,2,4-triazol--yl-methyl)-pentan-3-ol (tebuconazole) are highly efficient extractants for some nonferrous, ferrous, and rare metals, as well as for platinumgroup metals [3, 4]. An unambiguous advantage of these compounds consists in their availability, nontoxicity, and chemical stability in strongly acidic media. Lately gold hydrometallurgy has especially bloomed due to the application of novel efficient sulfur- and nitrogen-containing reagents [5, 6]. At the same time, the selection of extractants which do not have reductive properties towards metal ions is a special problem in the case of gold(iii) extraction. Aiming to find novel and selective gold(iii) extractants, we here report the first study of the extraction properties of (RS)--(4-chlorophenyl)-4,4-dimethyl-3-(-,2,4-triazol--yl-methyl)-pentan-3-ol. EXPERIMETAL (RS)--(4-hlorophenyl)-4,4-dimethyl-3-(-,2,4-triazol--yl-methyl)-pentan-3-ol reagent (purity of not less than 98%), hereafter denoted as S, was used in the work: l 2 2 ( 3 ) 3 2 This compound is produced on an industrial scale, in particular under tebuconazole name, by Bayer rop Science AG. This is a white crystalline substance in appearance (mp С) having low solubility in water ( g/l at 20 С and p 5 9) and good one in low-polar solvents, and is a nonvolatile and low-toxic compound (LD 50 = 4 g/kg for rats) [7]. Therefore, the reagent properties meet the requirements for industrial extractants well enough. The individuality and purity of the reagent were verified by elemental analysis and MR spectroscopy. The structures of the reagent and its compounds with gold(iii) were determined on the basis of, 3 and 5 MR spectroscopy data using two-dimensional SY, ESY, SQ, and MB techniques. Stock solutions of tebuconazole were prepared from precisely weighed amounts, while working solutions were prepared by dilution of the stock solutions. hloroform of pure for analysis grade was used as a diluent. Solutions of gold(iii) were prepared from 49

2 492 GLUBYATIKVA et al. E, % c l, mol/l Fig.. Degree of gold(iii) extraction with the S reagent versus the hydrochloric acid concentration in aqueous phase (c (III) = mol/l, c S = mol/l), T =20 С. y (III), mol/l mol/l S mol/l S 0.00 mol/l S mol/l S x (III), mol/l Fig. 2. Isotherms of gold(iii) extraction with the S reagent from 3 M hydrochloric acid solutions at 20 С. gold metal with purity of 99.99% by dissolving a precisely weighed amount in aqua regia followed by transfer into hydrochloric solution. Gold(III) concentration in the starting solutions and raffinates was determined using spectrophotometrically from l4 ion own color at 33 nm [8]. oncentration of metal ions in extracts was determined from the difference in its concentrations in the starting solutions and raffinates. Extraction was performed in glass separation funnels at the temperature of 20 ± С. The ratio of aqueous and organic phases was maintained constant and equal to :. The electronic absorption spectra (EAS) of the reagent, the compound being extracted, and the synthesized complex (specially pure grade acetonitrile was used as a solvent) were recorded on a Specord M40 spectrophotometer; IR spectra of the compounds (as ujol mulls) were obtained on a Specord M80 spectrophotometer. The melting points of the compounds were measured on a Boethius hot stage. MR spectra were recorded on a Bruker Avance III pulsed spectrometer with working frequencies of ( ), ( 3 ) and ( 5 ) Mz using a 5-mm probe with Z-gradient PABB at a constant sample temperature of 298 K. hemical shifts in the and 3 MR spectra are given in ppm relative to the signal of the internal standard (TMS), while in the 5 MR spectra, relative to the signal of the external standard ( 3 ). hloroform-d 6 was used as a solvent. The energetic parameters of complexes were determined using the PRIRDA-06 program within the frames of the density functional theory (DFT) and the second order Moller Plesset perturbation theory (MP2) [9, 0]. The work was carried out using the cluster supercomputer of the Institute of rganic hemistry, Ufa Scientific enter of the Russian Academy of Sciences. alculations of thermodynamic corrections [] for the full energy of compounds were conducted by PBE/Λ and PBE/Λ2 methods [2, 3] in gas phase for standard conditions. By the example of relativistic bases of double (Λ) and triple (Λ2) valence splitting, it was found that an increase in the basis set does no lead to any significant improvement in the results of calculations. The RI-MP2 method [4, 5], on which basis a correction of the full energy was performed, was chosen as an alternative method taking into account electron correlations. The differences in the relative thermodynamic values obtained using different methods and bases are small, that is why the RI-MP2/Λ method was chosen as an optimal one to calculate the full energies of compounds. RESULTS AD DISUSSI The optimal phase contact time required to achieve extraction equilibrium in the case of extraction of gold(iii) with the reagent under study was ascertained preliminary and was equal to 20 min. Phase separation after agitation occurred practically immediately; the formation of a third phase was not observed. Figure shows the influence of hydrochloric acid concentration in aqueous phase on the degree of gold(iii) extraction with the S reagent, from which one can see that metal ion is extracted over a wide range of acid concentrations, but the most efficient extraction takes place at the aqueous phase acidity of mol/l of l. We have previously studied the interaction of the S reagent with hydrochloric acid under extraction conditions and have shown that the acid is extracted to form a non-dissociated compound in organic phase [6]. All further experiments were conducted at the aqueous phase acidity of 3.0 mol/l of l. The selection of chloroform as a diluent for the reagent being studied is conditioned by the fact that its use does not result in the formation of a third phase as it take place in the case of application of other RUSSIA JURAL F IRGAI EMISTRY Vol. 58 o

3 EXTRATI F GLD(III) 493 known diluents, such as toluene and carbon tetrachloride. It should be noted that chloroform is a conventional diluent in gold hydrometallurgy [7 20]. Isotherms of gold(iii) extraction with the S reagent obtained at 20 С are shown in Fig. 2. The slopes of the initial regions of isotherms indicate efficient extraction of metal ion. Solvate number q was unity as determined both by the saturation method (0.97.3) and by the method of shift of extraction equilibrium (.08.28) (Fig. 3). Therefore, gold(iii) is extracted over the entire region of concentrations studied to form an extractable compound in which the : S ratio is :. The recovery of metal ion decreases with an increase in temperature. This is indicated by the dependence given in Fig. 4. The extraction process is an exothermic one. The calculated values of the thermodynamic parameters of the gold(iii) extraction process are as follows: ΔН = 24.3 kj/mol, ΔG = 6.0 kj/mol, and ΔS = 28.2 J/(mol K); the extraction proceeds due to the enthalpy factor. Figure 5 shows the logd (III) = f(log[ + ]) plot at ionic strength of aqueous phase of 2 M (l and al), from which one can see that the gold(iii) extraction practically does not change with an increase in hydrogen ion concentration (the slope of the plot is 0.059). Figure 6 shows the logd (III) = f(log[l ]) plot, from which it follows that the gold(iii) extraction does not depend on the salt background. To study in more detail the composition and the structure of the extractable gold(iii) compound, the compound was prepared by the method of saturation of organic phase followed by evaporation of the diluent. The isolated extractable compound (I) is a longterm storage stable, solid, glassy brittle mass with yellow color, which is triturable to form a powder (mp 49 5 С), and soluble in conventional organic solvents excluding saturated hydrocarbons. To compare the complexing of gold(iii) with the S reagent under extraction conditions and in a neutral medium, the synthesis of coordination compound of gold(iii) chloride in the neutral medium with the specified : S ratio of : (compound II) was carried out. Potassium tetrachloroaurate(iii) was used as a starting compound. A solution of Kl 4 (0.350 g, 0.92 mmol) in 3 ml of acetone was added to a solution of the S reagent (0.285 g, 0.92 mmol) in 0 ml of acetone in a pear-shaped 25 ml flask having ground glass joint. The mixture obtained was stirred on a rotary evaporator at room temperature for 3 h, then the solution was filtered from a potassium chloride precipitate formed and was evaporated to remove acetone. Being in dry state, the compound obtained is a bright yellow solid substance (mp ), insoluble in water and soluble in conventional organic solvents excluding saturated hydrocarbons. logd (III) q =.28 R = q =.05 R = log[s free ] Fig. 3. Effect of the concentration of free S extractant on gold(iii) extraction from 3 M hydrochloric acid solutions. logd (III) /T 0 3, K Fig. 4. Effect of temperature on gold(iii) extraction with the S reagent from 3 M hydrochloric acid solutions (c (III) = mol/l, c S = mol/l). For l 3 S (I) anal. calcd., %:, 32.2; l, 23.2;, 6.9. Found, %:, 30.5; l, 2.6;, 6.7. For l 3 S (II) anal. calcd., %:, 32.2; l, 23.2;, 6.9. Found, %:, 30.4; l, 24.3;, 7.0. The complexes of gold(iii) with the reagent under study are long-term storage stable. haracteristics of the and 3 MR spectra of the S reagent as well as its compounds with gold(iii) are presented in Tables and 2. Assignment of the signals of atoms of,2,4-triazole ring in the 3 and 5 MR spectra of the S reagent was performed using the data of - 3 and - 5 MB spectra. Methine groups of the heterocyclic moiety (δ 5.64 ppm, RUSSIA JURAL F IRGAI EMISTRY Vol. 58 o

4 494 GLUBYATIKVA et al. logd (III) logd (III) log[ + ] Fig. 5. Gold(III) distribution coefficient versus hydrogen ion concentration at 20 С (c (III) = 0.00 mol/l, c S = 0.00 mol/l) log[l ] Fig. 6. Gold(III) distribution coefficient versus chloride ion concentration at 8 С (c (III) = 0.00 mol/l, c S = mol/l). δ 8.00 ppm, and (δ ppm, δ 8.22 ppm) are assigned on the basis of a - 3 MB cross-peak between ' protons of 2 group and 5'' carbon atom: 4.35/44.60 ppm. The - 5 MB correlations of methylene 2 -' protons: 4.35/23.77 and 4.35/ ppm, as well as correlations of methine protons of the,2,4-triazole ring: 8.00/249.78, 8.00/ ppm and 8.22/249.78, 8.22/23.77 ppm, were found for the signals (δ 23.77, and ppm) in the 5 MR spectrum. The scheme below shows values of the chemical shifts of signals from the triazole ring atoms and MB (heteronuclear multiple-bond correlation) (with the solid arrows representing - 5 correlations and the dotted arrows representing - 3 correlations): p-l ( 3 ) 3 5'' 4'' '' 5.64 ' 2'' '' The assignment of signals in the spectra of the extractable gold(iii) (compound I) and the complex obtained by the direct method (compound II) was carried out in an analogous manner. Thus, there are - 3 MB correlations of 2 -' protons with 3 and 5'' carbon atoms in the spectra of compound II: 4.53, 4.54/76.44 ppm and 4.53, 4.54/ 44.8 ppm. In the - 5 spectra there are observed the cross-peaks between 5'' proton (δ 9.33 ppm) Table. haracteristics of MR spectra of the S reagent, extracted compound (I), and gold(iii) complex (II) obtained by direct method. Dl 3 was used as a solvent* l ' 5 5'' 4'' '' 2'' 3'' ompound Parameter Values of chemical shifts (δ) of proton signals and their change (Δ), in ppm, as well as their multiplicity a b 2 a 2 b 5, 6, 7 9, 3 0, 2 ' b 3'' 5'' ' а S δ 2.45 td.76 td.69 td.83 td.03 s 6.95 d 7.9 d 4.35 s 8.00 s 8.22 s 3.07 br s I δ 2.0 td 2.49 td.77 ddd.87 ddd d 7.24 d 4.52 d 4.53 d 8.64 s 9.32 s 2.35 br s Δ II δ 2.09 td 2.5 td.76 ddd.86 ddd d 7.22 d 4.53 d 4.54 d 8.62 s 9.33 s 2.38 br s Δ * ddd stands for doublet of doublets of doublets; td stands for triplet of doublets. RUSSIA JURAL F IRGAI EMISTRY Vol. 58 o

5 EXTRATI F GLD(III) 495 Table 2. haracteristics of 3 and 5 MR spectra of the reagent S, extracted compound (I) and gold(iii) complex (II) obtained by direct method. Dl 3 was used as a solvent Pa- ompounrameter Values of chemical shifts (δ) of carbon atom signals and their change (Δ), ppm , 6, 7 8 (ipso-) 9, 3 (ortho-) 0, 2 (meta-) (para-) Values of chemical shifts (δ) of nitrogen atom signals and their change (Δ), ppm ' 3'' 5'' '' 2'' 4'' S δ I δ Δ II δ Δ and 4'' (δ ppm) and '' (δ ppm) nitrogen atoms, between 3'' proton (δ 8.62 ppm) and 4'' (δ ppm), 2'' (δ ppm), and '' (δ ppm) nitrogen atoms, as well as between ' protons of 2 group and '' (δ ppm) and 2'' (δ ppm) nitrogen atoms (with the solid arrows representing - 5 correlations and the dotted arrows representing - 3 correlations): p-l ( 3 ) 3 5'' 4'' '' '' '' ' Shifts of signals of carbon atoms in a strong field as compared to the spectrum of the S reagent are observed in the 3 MR spectra of the extractable compound (I) and the complex (II) synthesized by the direct method from the neutral medium: Δδ 3'' is 5.82 ppm for I and 5.85 ppm for II, whereas the change in position of the signals does not practically occur for the 5'' atom. In the MR spectra of these compounds, the signals of 3'' and 5'' protons shift to the low field by approximately. and 0.6 ppm, respectively, as compared to the spectrum of the reagent. It should be noted that the signals of diastereotopic protons of the ' methylene group are isochronous in the MR spectra of the S reagent, whereas in the spectra of the compounds these protons form two resonance signals with the value of diastereotopic difference on the order of 0. ppm. In the 5 MR spectra of gold(iii) complexes I and II, the signals of the '' ( 4.5 ppm and 5.20 ppm, respectively) and 2'' ( 6.20 and 6.2 ppm) nitrogen atoms shift downfield as compared to the reagent spectrum. owever, the most essential change in the chemical shift values takes l l l place for the signal of the 4'' atom (66.39 and ppm), which serves as direct evidence for the coordination of the reagent with gold(iii) through the 4'' atom of the triazole ring. The spectroscopic data set indicates the essential change in the electronic structure of,2,4-triazole ring which gives evidence of the formation of a stable complex between the S reagent and gold(iii). For the purpose of studying the possibility of formation of other complexes of R- and S-enantiomers of the reagent with gold(iii) through coordination with the 2'' nitrogen atom and the oxygen atom of the group along with the compound prepared, their thermodynamic parameters in a gas phase were calculated [2] (Fig. 7). According to the quantum-chemical calculations in the RI-MP2/Λ//PBE/Λ2 approximation, the formation of gold(iii) complex through coordination of R- and S-enantiomers of the reagent with the 4'' nitrogen atom of triazole ring in accordance with the Maxwell Boltzmann distribution calculated on the basis of the Δ 298 is more thermodynamically preferable than that with the 2'' atom or oxygen atom of the hydroxy group by. and 30.9 kcal/mol, respectively. omparison of the thermodynamic parameters of the complexes formed from S- and R-enantiomers indicates the absence of differences in these parameters between the enantiomers because it is probable that the square-planar inner coordination sphere of gold(iii) is not a source of asymmetry and does not lead to the formation of diastereomeric pairs (Table 3). The EAS of compound I isolated from the gold(iii) extract and of gold(iii) chloride coordination compound II both have a strong absorption band at and cm, respectively, as well as a weak band in the region of cm, the combination of which is characteristic of the spectra of l 3 S type complexes. The shift of the absorption band peaks as compared to the spectrum of starting Kl 4 salt ( l4 ion) and of the intensity ratios indicates some distortion of the RUSSIA JURAL F IRGAI EMISTRY Vol. 58 o

6 496 GLUBYATIKVA et al. l l l l 2.72 l l l l l l l R-4'' R- R-2'' l S-4'' l l l l l l 2.76 S- l l S-2'' Fig. 7. Structures of complexes of R- and S-enantiomers of the S reagent with gold(iii) with coordination to 4'' and 2'' atoms of,2,4-triazole ring and to oxygen of hydroxy group according to the quantum-chemical calculations in PBE/Λ2 approximation (the designation of each complex includes designation of enantiomer and donor atom of the reagent). l l l planarity of the inner coordination sphere of the complex. The absorption band of the stretching/bending vibrations of triazole ring, ν ring, located at 508 cm in the IR spectrum of the reagent shifts to 537 and 540 cm, respectively, in the spectra of compounds I and II, which provides evidence for the coordination of the reagent with gold(iii) through the nitrogen atom of Table 3. hange in thermodynamic parameters upon complexation of gold(iii) with R- and S-enantiomers of the S reagent at 298 K alculation method Donor atom of the reagent ΔG, kcal/mol Δ, kcal/mol ΔS, cal/mol K R- S- R- S- R- S- PBE/Λ 4'' '' PBE/Λ2 4'' '' RI-MP2/Λ//PBE/Λ2 4'' '' RI-MP2/Λ2//PBE/Λ2 4'' '' RUSSIA JURAL F IRGAI EMISTRY Vol. 58 o

7 EXTRATI F GLD(III) 497 Table 4. oncentration constants of extraction, K, for extraction of gold(iii) with the S reagent from a 3 mol/l hydrochloric acid solutions at 20 Initial concentration of the extractant, S 0, mol/l Equilibrium gold(iii) concentration, mol/l x K av pyridine type [2, 22]. The strong absorption band in the Kl 4 spectrum corresponding to the stretching vibrations ν( l) in l ion appears at 362 cm (at 367 cm 4 according to the data of [24, 25]). This band is slightly shifted to 372 and 366 cm, respectively, in the spectra of complexes I and II. The set of the obtained results makes it possible to assume that the extraction of gold(iii) with the S reagent proceeds according to the coordination mechanism owing to the formation of the 4'' coordination bond. This is also indicated by a relatively long time of establishment of the extraction equilibrium because it would not exceed several minutes in the case of (III) extraction according to the ion-association mechanism. The gold(iii) extraction reaction is described by the following equation: 4 aq + Sorg 3Sorg + aq. l l l The concentration constant of extraction is expressed by the equation: [ l3 ][ l ] y( y xн) K S + = =, l 4 [ S] x( S0 y) where x is an equilibrium gold(iii) concentration in aqueous phase, mol/l; y is an equilibrium gold(iii) y K = (5.2 ± 0.5) 0 3 concentration in organic phase, mol/l; and S 0 is an initial concentration of the extractant, mol/l. alculated values of the concentration constants of extraction are given in Table 4. An average value of the concentration constant of extraction K ~ is (5.2 ± 0.5) 0 3. Preliminary experiments showed that tebuconazole has low extraction ability towards cobalt(ii) and nickel(ii) at the aqueous phase acidity of 0. 6 M and towards iron(iii) in the range of 0. 3 M of l. Summing up the results obtained, this reagent may be recommended for use in selective separation of gold(iii) from iron(iii), cobalt(ii), and nickel(ii). Repeated usage of extractants is technologically favorable. The results show that tebuconazole may be repeatedly used in a lopped process of gold(iii) extraction. As stripping solutions, 5% thiourea aqueous solution (83.5% stripping) as well as M thiourea solution in M l solution (8.3% stripping) was used. LUSIS In summary, (RS)--(4-chlorophenyl)-4,4-dimethyl-3-(-,2,4-triazol--yl-methyl)-pentan-3-ol is an efficient extractant for gold(iii) from hydrochloric acid solutions, not reducing it to gold(i) or metal. Extraction and complexing take place due to the formation of the coordination bond with participation of the 4'' nitrogen atom of the triazole ring, whereas other heteroatoms including the oxygen atom of hydroxy group do not participate in complexing. The reagent may be eventually recommended for combined extraction of gold and palladium from composite technological and analytical solutions, as well as for separation of gold(iii) from cobalt(ii), nickel(ii) and iron(ii) in the case of joint presence in solutions. AKWLEDGMETS This work was supported by the KhM 5 Program of the hemistry and Material Science Department of the Russian Academy of Sciences. REFEREES. X. hen and. Yang, J. Agric. Food hem. 57, 244 (2009). 2. P. D. Evans, K. J. Schmalzl,. M. Forsyth, et al., J. Wood hem. Technol. 27 (3-4), 243 (2007). 3. R. A. Khisamutdinov, Yu. I. Murinov, and. V. Shitikova, Russ. J. Inorg. hem. 52, 969 (2007). 4. R. A. Khisamutdinov, G. R. Anpilogova, Yu. I. Murinov, and L. V. Spirikhin, Russ. J. Inorg. hem. 55, 205 (200). 5. J.. Marsden and. I. ouse, hemistry of Gold Extraction, 2nd ed. (Soc. for Mining, Metallurgy, and Exploration (SME), Littleton, olorado, 2006). RUSSIA JURAL F IRGAI EMISTRY Vol. 58 o

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