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Rhenium—VI
J. inorg, nucl Chem.. 1972, Vol. 34, pp. 3479-3484.
Pergamon Press.
Printed in Great Britain
RHENIUM-
VI
MIXED L I G A N D O X O F L U O R O C Y A N O COMPLEXES OF QUINQUEVALENT RHENIUM M. C. C H A K R A V O R T I and M. K. C H A U D H U R I Department of Chemistry, Indian Institute of Technology, Kharagpur, West Bengal, India (Received 21 December 1971 )
A b s t r a c t - A mixed ligand oxofluorocyano complex of quinquevalent rhenium, viz. H[ReOF(H20)(CN):d has been prepared by boiling K3[ReO2(CN)4] with 40% hydrofuoric acid. Sodium, potassium and ammonium salts, M[[ReOF(H20)(CN)3] have been obtained by neutralising the acid with the corresponding alkali acetates and then precipitating with ethanol. TI *, NV ÷, [Co(NH~)o] :~÷ and [Coen3] 3' salts have been prepared by precipitating the acid with the salts of these ions. The acid loses half a molecule of hydrogen fluoride at 110°C and forms the dinuclear acid H[Re.~O..,F(H~Oh(CN),]. The potassium salt of this acid has been prepared by neutralising with caustic potash and precipitating with ethanol. The mononuclear acid is very weakly paramagnetic. The dissociation constant determined by Bjerrum's method is pK, = 3'3 at 30°C. The i.r. spectra of the acid and of its potassium salt have been reported. INTRODUCTION STUDIES o n the fluoro complexes of rhenium are very scanty. Except the fluorides and oxyfluorides, the few complex compounds isolated are of the types M,/[ReF~] [1-4], M~[ReFr] [5, 6], M2~[ReF8] [7], M'[ReO2F4] [8], M~[ReFT] [9], M~[ReF8] [9], MI[ReOF~] [9, 10] and M,/[ReO2F~] [7]. Very few salts of each series have been reported. All these complexes have been prepared either by dry reactions or from nonaqueous medium. No mixed ligand fluoro complex of rhenium has been reported. We have undertaken a programme to make detailed studies on the formation, isolation and characterisation of fluoro and mixed ligand fluoro complexes of rhenium in its various oxidation states from aqueous medium. In the present communication the isolation and the properties of a oxofluorocyano complex of quinquevalent rhenium, H[ReOF(H20)(CN)3] and of its various salts are reported. EXPERIMENTAL Potassium perrhenate used was of Messrs. Johnson Matthey & Co., London (99-8% pure). Hydrofluoric acid was E. Merck's G.R. quality (40%). Other chemicals were reagent quality of E. Merck or 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
R. D. Peacock, Chem. Ind. 1453 (1955). N. S. Nikolaev and E. G. lppolitov, Dokl. Adad. Nauk S S S R 140, 129 ( 196 I). R. D. Peacock,J. chem. Soc. 1291 (1956). E. Weise, Z. anorg, allg. Chem. 283, 377 (1956). R. D. Peacock,J. chem. Soc. 467 (1957). G. B. Hargreaves and R. D. Peacock, J. chem. Soc. 3776 (1958). N. S. Nikolaev and E. G. lppolitov, Dokl. A dad. Naul S S S R 136, 1 I 1 ( 1961 ). R. D. Peacock, J. chem. Soc. 602 (1955). E. G, Ippolitov and N. S. Nikolaev, lzv, Akad. Nauk SSSR. Otd. khim. Nauk 748 (1962). E. G. lppolitov, Zh. neorg. Khim. 7,940 (1962). 3479
3480
M . C . C H A K R A V O R T I and M. K. C H A U D H U R I
B.D.H. Potassium dioxotetracyanorhenate(V), Ka[ReO2(CN)4] was prepared by the method previously described [11, 12]. Rhenium was estimated by decomposing the complexes with sodium peroxide and then precipitating rhenium heptasulphide. The precipitate was dissolved in aqueous sodium peroxide and from sulphuric acid solution rhenium was determined by electrodeposition [13]. After decomposition of the complexes with sodium peroxide, fluorine was estimated by titration with thorium nitrate solution using sodium alizarin sulphonate as indicator. Nitrogen was estimated by Kjeidahrs method. Sodium, potassium and cobalt were estimated as their suiphates after fuming the compounds with sulphuric acid in a platinum curcible. Nickel and thallium(I) were estimated as dimethyl glyoximate and chromate respectively after decomposing the compounds by fuming with sulphuric acid. The determination of the oxidation state of rhenium was carded out by oxidising a known amount of the substance with dichromate in very dilute sulphuric acid medium. Cr(III) was then separated as hydroxide and estimated iodometrically [ 14]. I.R. spectra were recorded in KBr pellet using a Perkin-Elmer lnfracord (No. 137). Conductivity measurements were carried out in aqueous medium with a Philip's conductivity bridge (No. PR9500) using platinised platinum electrodes sealed in polythene stems. Magnetic susceptibility measurements were made by Gouy method, the calibrant being Hg[Co(CNS)4]. The diamagnetic corrections were calculated from the data of Figgis and Lewis [ 15]. The dissociation constant of the acid H[ReOF(H20)(CN)a] was determined potentiometrically by Bjerrum's method applying Kelvin-Wilson titration technique. The formation function was calculated by using the equation A - - P - [H +] + [OH-] A where A is the total initial concentration of the acid taken and P is the concentration of alkali added at any stage of titration. The pKa value was graphically obtained from the pH value at the point where h = 0.5. The potentiometric titration was carried out in a polythene beaker using quinhydrone electrode and a saturated calomel electrode connected by a salt bridge (KC! in Agar Agar) made ofpolythene as the reference electrode. The potential measurements were made with a Toshniwal potentiometer (manufactured under iicence from W. G. Pye & Co. Ltd., Cambridge), S. No. 173, reading up to 0.1 mV. 50 ml of 3"706 × 10-a M solution of the acid was titrated with 0.05407 M caustic soda at 30°C, the ionic strength being maintained 0"50 with sodium perchlorate. The pH values were calculated from the observed e.m.f, using the known standard potential of the quinhydrone electrode and the potential of the saturated calomel electrode.
PREPARATION OF THE COMPLEXES Hydrogen oxofluorotricyanoaquorhenate(V), H[ReOF(HzO)(CN)a]. 1.4 g of powdered K3[ReOz(CN)4] was boiled with 15-20 ml of hydrofluoric acid (40%) in a platinum crucible for 40-45 min. Acid lost by boiling was replaced twice. The cooled mixture was separated into a colourless centrifugate and a deep pink residue by centrifugation using a polythene cone. The residue was washed with 40% hydrofluoric acid and allowed to dry in a desiccator over sulphuric acid and caustic soda. The dried mass was extracted with ethanol leaving behind a little white residue. The ethanol extract was dried over sulphuric acid. The yield was 0.7 g. Sodium-Potassium-ammonium oxofluorotricyanoaquorhenate(V), M~[ReOF(H20)(CN)a], M l = Na, K, and NH4. 11. 12. 13. 14. 15.
M.C. Chakravorti, J. Ind. chem. Soc. 40, 81 (1963). M.C. Chakravorti, J. inorg, nucl. Chem. 34, 893 (1972). B. K. Sen and P. Bandyopadhyay, J. Ind. chem. Soc. 40, 813 (1963). M. C. Chakravorti, J. Ind. chem. Soc. 47, 838 (1970). B.N. Figgis and J. Lewis, Modern Coordination Chemistry (Edited by J. Lewis and R. G. Wilkins), Chap. 6. Interscience, New York (1960).
R h e n i u m - VI
3481
To a concentrated aqueous solution of H[ReOF(H20)(CN)a] excess (acid:alkali acetate = 1 : 10) of concentrated solution of sodium-potassium-ammonium acetate was separately added and stirred. Ethanol was gradually added with thorough stirring when a pink brown precipitate appeared. This was separated by centrifugation. The residue was dissolved in minimum volume of water and reprecipitated with ethanol. The process was repeated twice. Finally, the residue was thoroughly washed with ethanol and dried over sulphuric acid. Thallous-nickel-hexamminecobalt(lII)-tris(ethylenediamine) cobalt(Ill) oxofluorotricyanoaquorhenate(V), M[ReOF(H20)(CN)3] where M = TI +, ½Ni2+, ,~[Co(NH:0~] 34 and :~[Coen3] :~ . These were prepared by precipitating concentrated aqueous solutions of H[ReOF(H~O)(CN):;] with excess of concentrated solutions of thallous nitrate, nickel nitrate, hexammine cobalt(Ill) chloride and tris(ethylene diamine) cobalt(Ill) chloride respectively. The precipitation was immediate and almost complete. The precipitates were washed successively with water, ethanol and acetone and then dried over sulphuric acid. H[Re20..,F(H20).o(CN)6]. 0.3 g of H[ReOF(H20)(CN)a] was heated in a platinum crucible at 110°C and occasionally weighed after cooling till constant weight. The issuing vapour contained hydrogen fluoride. The substance took one and a half hours to reach constant weight and the loss suffered was 2.75%. K[Re2OzF(H~O)2(CN)d. Excess (5 times of the theoretical amount) of a concentrated solution of potassium hydroxide was added to H[RezO2F(H20)2(CN)6] when the substance dissolved readily. From this solution the potassium salt was precipitated by ethanol and purified by dissolution in water and reprecipitation with ethanol, as in the case of potassium salt of the mononuclear acid. It was then dried over sulphuric acid. All the compounds were stored in polythene tubes. The analytical results are given in Table 1.
DISCUSSION
The analytical data of the product obtained by reacting K3[ReO2(CN)4] with 40% hydrofluoric acid fit into the empirical formula ReO(CN)3" H 2 0 - H F (the oxidation number of rhenium determined by dichromate oxidation being 5.04). This is a deep pink crystalline substance soluble in water, ethanol and acetone. The aqueous solution reacts acidic. It has been formulated as H[ReOF(H20)(CN)3] on the basis of the stoichiometry in various salts prepared from the acid, conductivity measurement, potentiometric titration, pyrolysis and i.r. spectral studies. The alternative formulation H[Re(OH)2F(CN)3] is not preferred since the i.r. spectra for the acid and its potassium salt gave H - O - H bands (vide infra). In the potentiometric titration of the complex with caustic soda solution a fairly sharp inflection at the point complex-alkali = 1 : 1 was observed in the titration curve (Table 2). From its solution nickel, thallium (1), hexamminecobalt(III) and tris(ethylene diamine) cobalt(III) salts gave sparingly soluble precipitates and from solutions containing alkali acetates and the pink substance ethanol gave precipitates the analysis of which conforms to the formulation M[ReOF(H20)(CN)3], where M = ½Ni, TI, ½[Co(NHa)6] ~+, ~[Coen3] :~+, Na, K and NH4 (vide Table l). The molar conductance of the sodium salt at the dilutions 64, 128, 256, 512 and 1024 l/g mol were 36.7, 47-0, 61.8, 75.7 and 109.1 f~-I respectively at 3 I°C, indicating that it is a uni-univalent type of electrolyte. The acid H[ReOF(H20)(CN)3] did not lose any weight on keeping in vacuum in a polythene container in a desiccator containing sulphuric acid and caustic alkali. On heating to constant weight at 110°C it lost 2.75% in weight and the issuing vapour contained hydrogen fluoride. The loss in weight corresponds to the loss of half a molecule of hydrogen fluoride (calc. 2.96%). The residue was dirty pink in colour and very sparingly soluble in water, ethanol and acetone. An aqueous suspension
3482
M . C . C H A K R A V O R T I and M. K. C H A U D H U R I
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Rhenium- VI
3483
Table 2. Titration of H[ReOF(H~O)(CN).3] (50.0 ml 3-706 × 10-:~ M) with N a O H (0-05407 M) at 30°C
Volume of alkali added
(V)
pH
0 0.1 0.2 0.3 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2-4 2-6 2.8 3.0 3.2 3-4
0.277 0.277 0-274 0-273 0.272 0.269 0.264 0.260 0.256 0-253 0.245 0.240 0.231 0.224 0.208 0.193 0.177 0.162 0.145 0.165
2.95 2-95 3.00 3.01 3.03 3.08 3-16 3-23 3.30 3.35 3.48 3.56 3-71 3.83 4-09 4.34 4.60 5.02 5.60 6.55
E
[H +] × 10:'
g ion/I 1.13 1.13 1.007 0-9683 0.9311 0-8318 0.6871 0.5902 0.5058 0-4508 0-3334 0-2754 0.1950 0.1496 0-08110 0.04581 0.02489 0.00955 0-002512 0.0002818
A × 10:~ g mol/I
P × 10:' g mol/l
3.706 3.699 3.692 3.684 3.677 3.662 3.648 3.640 3-618 3-604 3.591 3.578 3.563 3-550 3.537 3-523 3-510 3-496 3.483 3.470
0.0000 0.1079 0.2154 0.3225 0.4292 0.6412 0.8515 1.062 1-268 1-472 1-677 1-882 2-080 2.279 2.477 2.673 2.868 3.061 3.253 3.443
0.695 0.665 0.669 0.650 0.630 0.598 0-578 0.546 0.509 0.466 0.440 0.397 0.361 0.316 0.277 0.228 0.176 0.122 0.065 0-008
of the residue was acidic to litmus. It did not dissolve in alkali acetate solution, but dissolved completely in caustic potash solution from which ethanol gave a brownish pink precipitate. The analysis (see Table 1) of the residue and of the product obtained from it with caustic potash indicated that the residue may be formulated as H[Re202F(HzO)z(CN)6]. The oxidation number of rhenium in it was found to be +4.98. That water is not lost from the complex at 110°C indicates that it is strongly held to Re(V) in the complex. It appears that in the dinuclear complex H[Re202F(H20)2(CN)6] fluoride is acting as the bridging group. The i.r. spectra of the substance H[ReOF(H20)(CN):d gave rio-H, ~C~N) and 8(a-o-H) bands at 3350 (broad), 2170(s) and 1620(s) cm-' respectively. The bands observed in Re=O or R e - O - H region were at 1120(m), 1015(s) and 920(m) cm-'. Besides, a few other bands at 1550(m), 1475(m), 1250(w), 800 (broad) and 715 (broad) were observed. The spectra of the potassium salt gave V(o-r~), U(C=N~and 6(~-o-H) band at 3350(m), 2130(s) and 1620(s) cm-' respectively. The other bands were generally similar to those of the acid. An analogous hydroxo complex H[ReO(OH)(HzO)(CN)3] was previously reported by one of us[12, 16]. This was obtained by reacting K3[ReO2(CN)4] with dilute hydrochloric acid in cold. Replacement of hydrochloric acid by hydrofluoric acid, has thus led to the substitution of the hydroxide group by fluoride. 16. M. C. Chakravorti, J. Ind. chem. Soc. 41,477 (1964); 43, 381 ( 1966); 44, 734 (1967).
3484
M . C . C H A K R A V O R T I and M. K. C H A U D H U R I
No definite compound could be obtained by reacting K3[ReO2(CN)4] with 40% (and also 20%) hydrofluoric acid in cold or at steam bath temperature. This indicates the reluctance of fluoride to combine with rhenium under mild conditions. The colour, solubility and other properties of the two acids are generally similar. The hydroxo complex also gives an insoluble dinuclear product H2[(CN)a" O" (HzO)" Re. O" Re(H~O). O • (CN)3] by the loss of half a molecule of water. T h e sodium, potassium and ammonium salts of the acid are soluble in water, the other isolated salts being sparingly soluble. Aged samples of the acid and also of its alkali metal salts are difficultly soluble. When kept in a glass container the acid attacks glass very slowly. The dissociation constant of the acid H[ReOF(H20)(CN)3] was pKa = 3-3 at 30°C at an ionic strength of 0.5. The pKa value of hydrofluoric acid determined by the same method and under the same experimental conditions was 3.2. The acid is thus almost equal in strength to hydrofluoric acid. The pKa value of H[ReO(OH)(H20)(CN)3] was 3"5 at 29°C. The acid is very weakly paramagnetic. The molecular susceptibility corrected for diamagnetism of other groups was 68.6 × 10-6 c.g.s, units at 30°C (/zeff = 0.40 B.M.). This agrees with the weak paramagnetism or diamagnetism of other quinquevalent rhenium complexes [ 15]. The/zeff value of the analogous H[ReO(OH)(H20)(CN)3] was 0.25 B.M. at 27°C. It is interesting to note that the replacement of one hydroxide group by a fluoride has not altered the paramagnetism of the complex to any significant extent. Acknowledgements-The authors express their heartiest thanks to Dr. P. Bandyopadhyay for his keen interest and kind cooperation. Thanks are also due to the Council of Scientific and Industrial Research for financing the scheme.
Pergamon Press.
Printed in Great Britain
RHENIUM-
VI
MIXED L I G A N D O X O F L U O R O C Y A N O COMPLEXES OF QUINQUEVALENT RHENIUM M. C. C H A K R A V O R T I and M. K. C H A U D H U R I Department of Chemistry, Indian Institute of Technology, Kharagpur, West Bengal, India (Received 21 December 1971 )
A b s t r a c t - A mixed ligand oxofluorocyano complex of quinquevalent rhenium, viz. H[ReOF(H20)(CN):d has been prepared by boiling K3[ReO2(CN)4] with 40% hydrofuoric acid. Sodium, potassium and ammonium salts, M[[ReOF(H20)(CN)3] have been obtained by neutralising the acid with the corresponding alkali acetates and then precipitating with ethanol. TI *, NV ÷, [Co(NH~)o] :~÷ and [Coen3] 3' salts have been prepared by precipitating the acid with the salts of these ions. The acid loses half a molecule of hydrogen fluoride at 110°C and forms the dinuclear acid H[Re.~O..,F(H~Oh(CN),]. The potassium salt of this acid has been prepared by neutralising with caustic potash and precipitating with ethanol. The mononuclear acid is very weakly paramagnetic. The dissociation constant determined by Bjerrum's method is pK, = 3'3 at 30°C. The i.r. spectra of the acid and of its potassium salt have been reported. INTRODUCTION STUDIES o n the fluoro complexes of rhenium are very scanty. Except the fluorides and oxyfluorides, the few complex compounds isolated are of the types M,/[ReF~] [1-4], M~[ReFr] [5, 6], M2~[ReF8] [7], M'[ReO2F4] [8], M~[ReFT] [9], M~[ReF8] [9], MI[ReOF~] [9, 10] and M,/[ReO2F~] [7]. Very few salts of each series have been reported. All these complexes have been prepared either by dry reactions or from nonaqueous medium. No mixed ligand fluoro complex of rhenium has been reported. We have undertaken a programme to make detailed studies on the formation, isolation and characterisation of fluoro and mixed ligand fluoro complexes of rhenium in its various oxidation states from aqueous medium. In the present communication the isolation and the properties of a oxofluorocyano complex of quinquevalent rhenium, H[ReOF(H20)(CN)3] and of its various salts are reported. EXPERIMENTAL Potassium perrhenate used was of Messrs. Johnson Matthey & Co., London (99-8% pure). Hydrofluoric acid was E. Merck's G.R. quality (40%). Other chemicals were reagent quality of E. Merck or 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
R. D. Peacock, Chem. Ind. 1453 (1955). N. S. Nikolaev and E. G. lppolitov, Dokl. Adad. Nauk S S S R 140, 129 ( 196 I). R. D. Peacock,J. chem. Soc. 1291 (1956). E. Weise, Z. anorg, allg. Chem. 283, 377 (1956). R. D. Peacock,J. chem. Soc. 467 (1957). G. B. Hargreaves and R. D. Peacock, J. chem. Soc. 3776 (1958). N. S. Nikolaev and E. G. lppolitov, Dokl. A dad. Naul S S S R 136, 1 I 1 ( 1961 ). R. D. Peacock, J. chem. Soc. 602 (1955). E. G, Ippolitov and N. S. Nikolaev, lzv, Akad. Nauk SSSR. Otd. khim. Nauk 748 (1962). E. G. lppolitov, Zh. neorg. Khim. 7,940 (1962). 3479
3480
M . C . C H A K R A V O R T I and M. K. C H A U D H U R I
B.D.H. Potassium dioxotetracyanorhenate(V), Ka[ReO2(CN)4] was prepared by the method previously described [11, 12]. Rhenium was estimated by decomposing the complexes with sodium peroxide and then precipitating rhenium heptasulphide. The precipitate was dissolved in aqueous sodium peroxide and from sulphuric acid solution rhenium was determined by electrodeposition [13]. After decomposition of the complexes with sodium peroxide, fluorine was estimated by titration with thorium nitrate solution using sodium alizarin sulphonate as indicator. Nitrogen was estimated by Kjeidahrs method. Sodium, potassium and cobalt were estimated as their suiphates after fuming the compounds with sulphuric acid in a platinum curcible. Nickel and thallium(I) were estimated as dimethyl glyoximate and chromate respectively after decomposing the compounds by fuming with sulphuric acid. The determination of the oxidation state of rhenium was carded out by oxidising a known amount of the substance with dichromate in very dilute sulphuric acid medium. Cr(III) was then separated as hydroxide and estimated iodometrically [ 14]. I.R. spectra were recorded in KBr pellet using a Perkin-Elmer lnfracord (No. 137). Conductivity measurements were carried out in aqueous medium with a Philip's conductivity bridge (No. PR9500) using platinised platinum electrodes sealed in polythene stems. Magnetic susceptibility measurements were made by Gouy method, the calibrant being Hg[Co(CNS)4]. The diamagnetic corrections were calculated from the data of Figgis and Lewis [ 15]. The dissociation constant of the acid H[ReOF(H20)(CN)a] was determined potentiometrically by Bjerrum's method applying Kelvin-Wilson titration technique. The formation function was calculated by using the equation A - - P - [H +] + [OH-] A where A is the total initial concentration of the acid taken and P is the concentration of alkali added at any stage of titration. The pKa value was graphically obtained from the pH value at the point where h = 0.5. The potentiometric titration was carried out in a polythene beaker using quinhydrone electrode and a saturated calomel electrode connected by a salt bridge (KC! in Agar Agar) made ofpolythene as the reference electrode. The potential measurements were made with a Toshniwal potentiometer (manufactured under iicence from W. G. Pye & Co. Ltd., Cambridge), S. No. 173, reading up to 0.1 mV. 50 ml of 3"706 × 10-a M solution of the acid was titrated with 0.05407 M caustic soda at 30°C, the ionic strength being maintained 0"50 with sodium perchlorate. The pH values were calculated from the observed e.m.f, using the known standard potential of the quinhydrone electrode and the potential of the saturated calomel electrode.
PREPARATION OF THE COMPLEXES Hydrogen oxofluorotricyanoaquorhenate(V), H[ReOF(HzO)(CN)a]. 1.4 g of powdered K3[ReOz(CN)4] was boiled with 15-20 ml of hydrofluoric acid (40%) in a platinum crucible for 40-45 min. Acid lost by boiling was replaced twice. The cooled mixture was separated into a colourless centrifugate and a deep pink residue by centrifugation using a polythene cone. The residue was washed with 40% hydrofluoric acid and allowed to dry in a desiccator over sulphuric acid and caustic soda. The dried mass was extracted with ethanol leaving behind a little white residue. The ethanol extract was dried over sulphuric acid. The yield was 0.7 g. Sodium-Potassium-ammonium oxofluorotricyanoaquorhenate(V), M~[ReOF(H20)(CN)a], M l = Na, K, and NH4. 11. 12. 13. 14. 15.
M.C. Chakravorti, J. Ind. chem. Soc. 40, 81 (1963). M.C. Chakravorti, J. inorg, nucl. Chem. 34, 893 (1972). B. K. Sen and P. Bandyopadhyay, J. Ind. chem. Soc. 40, 813 (1963). M. C. Chakravorti, J. Ind. chem. Soc. 47, 838 (1970). B.N. Figgis and J. Lewis, Modern Coordination Chemistry (Edited by J. Lewis and R. G. Wilkins), Chap. 6. Interscience, New York (1960).
R h e n i u m - VI
3481
To a concentrated aqueous solution of H[ReOF(H20)(CN)a] excess (acid:alkali acetate = 1 : 10) of concentrated solution of sodium-potassium-ammonium acetate was separately added and stirred. Ethanol was gradually added with thorough stirring when a pink brown precipitate appeared. This was separated by centrifugation. The residue was dissolved in minimum volume of water and reprecipitated with ethanol. The process was repeated twice. Finally, the residue was thoroughly washed with ethanol and dried over sulphuric acid. Thallous-nickel-hexamminecobalt(lII)-tris(ethylenediamine) cobalt(Ill) oxofluorotricyanoaquorhenate(V), M[ReOF(H20)(CN)3] where M = TI +, ½Ni2+, ,~[Co(NH:0~] 34 and :~[Coen3] :~ . These were prepared by precipitating concentrated aqueous solutions of H[ReOF(H~O)(CN):;] with excess of concentrated solutions of thallous nitrate, nickel nitrate, hexammine cobalt(Ill) chloride and tris(ethylene diamine) cobalt(Ill) chloride respectively. The precipitation was immediate and almost complete. The precipitates were washed successively with water, ethanol and acetone and then dried over sulphuric acid. H[Re20..,F(H20).o(CN)6]. 0.3 g of H[ReOF(H20)(CN)a] was heated in a platinum crucible at 110°C and occasionally weighed after cooling till constant weight. The issuing vapour contained hydrogen fluoride. The substance took one and a half hours to reach constant weight and the loss suffered was 2.75%. K[Re2OzF(H~O)2(CN)d. Excess (5 times of the theoretical amount) of a concentrated solution of potassium hydroxide was added to H[RezO2F(H20)2(CN)6] when the substance dissolved readily. From this solution the potassium salt was precipitated by ethanol and purified by dissolution in water and reprecipitation with ethanol, as in the case of potassium salt of the mononuclear acid. It was then dried over sulphuric acid. All the compounds were stored in polythene tubes. The analytical results are given in Table 1.
DISCUSSION
The analytical data of the product obtained by reacting K3[ReO2(CN)4] with 40% hydrofluoric acid fit into the empirical formula ReO(CN)3" H 2 0 - H F (the oxidation number of rhenium determined by dichromate oxidation being 5.04). This is a deep pink crystalline substance soluble in water, ethanol and acetone. The aqueous solution reacts acidic. It has been formulated as H[ReOF(H20)(CN)3] on the basis of the stoichiometry in various salts prepared from the acid, conductivity measurement, potentiometric titration, pyrolysis and i.r. spectral studies. The alternative formulation H[Re(OH)2F(CN)3] is not preferred since the i.r. spectra for the acid and its potassium salt gave H - O - H bands (vide infra). In the potentiometric titration of the complex with caustic soda solution a fairly sharp inflection at the point complex-alkali = 1 : 1 was observed in the titration curve (Table 2). From its solution nickel, thallium (1), hexamminecobalt(III) and tris(ethylene diamine) cobalt(III) salts gave sparingly soluble precipitates and from solutions containing alkali acetates and the pink substance ethanol gave precipitates the analysis of which conforms to the formulation M[ReOF(H20)(CN)3], where M = ½Ni, TI, ½[Co(NHa)6] ~+, ~[Coen3] :~+, Na, K and NH4 (vide Table l). The molar conductance of the sodium salt at the dilutions 64, 128, 256, 512 and 1024 l/g mol were 36.7, 47-0, 61.8, 75.7 and 109.1 f~-I respectively at 3 I°C, indicating that it is a uni-univalent type of electrolyte. The acid H[ReOF(H20)(CN)3] did not lose any weight on keeping in vacuum in a polythene container in a desiccator containing sulphuric acid and caustic alkali. On heating to constant weight at 110°C it lost 2.75% in weight and the issuing vapour contained hydrogen fluoride. The loss in weight corresponds to the loss of half a molecule of hydrogen fluoride (calc. 2.96%). The residue was dirty pink in colour and very sparingly soluble in water, ethanol and acetone. An aqueous suspension
3482
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Rhenium- VI
3483
Table 2. Titration of H[ReOF(H~O)(CN).3] (50.0 ml 3-706 × 10-:~ M) with N a O H (0-05407 M) at 30°C
Volume of alkali added
(V)
pH
0 0.1 0.2 0.3 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2-4 2-6 2.8 3.0 3.2 3-4
0.277 0.277 0-274 0-273 0.272 0.269 0.264 0.260 0.256 0-253 0.245 0.240 0.231 0.224 0.208 0.193 0.177 0.162 0.145 0.165
2.95 2-95 3.00 3.01 3.03 3.08 3-16 3-23 3.30 3.35 3.48 3.56 3-71 3.83 4-09 4.34 4.60 5.02 5.60 6.55
E
[H +] × 10:'
g ion/I 1.13 1.13 1.007 0-9683 0.9311 0-8318 0.6871 0.5902 0.5058 0-4508 0-3334 0-2754 0.1950 0.1496 0-08110 0.04581 0.02489 0.00955 0-002512 0.0002818
A × 10:~ g mol/I
P × 10:' g mol/l
3.706 3.699 3.692 3.684 3.677 3.662 3.648 3.640 3-618 3-604 3.591 3.578 3.563 3-550 3.537 3-523 3-510 3-496 3.483 3.470
0.0000 0.1079 0.2154 0.3225 0.4292 0.6412 0.8515 1.062 1-268 1-472 1-677 1-882 2-080 2.279 2.477 2.673 2.868 3.061 3.253 3.443
0.695 0.665 0.669 0.650 0.630 0.598 0-578 0.546 0.509 0.466 0.440 0.397 0.361 0.316 0.277 0.228 0.176 0.122 0.065 0-008
of the residue was acidic to litmus. It did not dissolve in alkali acetate solution, but dissolved completely in caustic potash solution from which ethanol gave a brownish pink precipitate. The analysis (see Table 1) of the residue and of the product obtained from it with caustic potash indicated that the residue may be formulated as H[Re202F(HzO)z(CN)6]. The oxidation number of rhenium in it was found to be +4.98. That water is not lost from the complex at 110°C indicates that it is strongly held to Re(V) in the complex. It appears that in the dinuclear complex H[Re202F(H20)2(CN)6] fluoride is acting as the bridging group. The i.r. spectra of the substance H[ReOF(H20)(CN):d gave rio-H, ~C~N) and 8(a-o-H) bands at 3350 (broad), 2170(s) and 1620(s) cm-' respectively. The bands observed in Re=O or R e - O - H region were at 1120(m), 1015(s) and 920(m) cm-'. Besides, a few other bands at 1550(m), 1475(m), 1250(w), 800 (broad) and 715 (broad) were observed. The spectra of the potassium salt gave V(o-r~), U(C=N~and 6(~-o-H) band at 3350(m), 2130(s) and 1620(s) cm-' respectively. The other bands were generally similar to those of the acid. An analogous hydroxo complex H[ReO(OH)(HzO)(CN)3] was previously reported by one of us[12, 16]. This was obtained by reacting K3[ReO2(CN)4] with dilute hydrochloric acid in cold. Replacement of hydrochloric acid by hydrofluoric acid, has thus led to the substitution of the hydroxide group by fluoride. 16. M. C. Chakravorti, J. Ind. chem. Soc. 41,477 (1964); 43, 381 ( 1966); 44, 734 (1967).
3484
M . C . C H A K R A V O R T I and M. K. C H A U D H U R I
No definite compound could be obtained by reacting K3[ReO2(CN)4] with 40% (and also 20%) hydrofluoric acid in cold or at steam bath temperature. This indicates the reluctance of fluoride to combine with rhenium under mild conditions. The colour, solubility and other properties of the two acids are generally similar. The hydroxo complex also gives an insoluble dinuclear product H2[(CN)a" O" (HzO)" Re. O" Re(H~O). O • (CN)3] by the loss of half a molecule of water. T h e sodium, potassium and ammonium salts of the acid are soluble in water, the other isolated salts being sparingly soluble. Aged samples of the acid and also of its alkali metal salts are difficultly soluble. When kept in a glass container the acid attacks glass very slowly. The dissociation constant of the acid H[ReOF(H20)(CN)3] was pKa = 3-3 at 30°C at an ionic strength of 0.5. The pKa value of hydrofluoric acid determined by the same method and under the same experimental conditions was 3.2. The acid is thus almost equal in strength to hydrofluoric acid. The pKa value of H[ReO(OH)(H20)(CN)3] was 3"5 at 29°C. The acid is very weakly paramagnetic. The molecular susceptibility corrected for diamagnetism of other groups was 68.6 × 10-6 c.g.s, units at 30°C (/zeff = 0.40 B.M.). This agrees with the weak paramagnetism or diamagnetism of other quinquevalent rhenium complexes [ 15]. The/zeff value of the analogous H[ReO(OH)(H20)(CN)3] was 0.25 B.M. at 27°C. It is interesting to note that the replacement of one hydroxide group by a fluoride has not altered the paramagnetism of the complex to any significant extent. Acknowledgements-The authors express their heartiest thanks to Dr. P. Bandyopadhyay for his keen interest and kind cooperation. Thanks are also due to the Council of Scientific and Industrial Research for financing the scheme.