Coordination of silicon tetrafluoride with alicyclic ethers and dimethyl ether
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Departmetzt of Chemistry, McGill University, Montreal, Quebec Received October 2, 1967 Tensimetric titrations at -78' of silicon tetrafluoride with ethylene oxide, trimethylene oxide, tetrahydrofuran, tetrahydropyran, and dimethyl ether prove the formation of only 1 :2 complexes, SiF4.2(ether). All are unstable at 25' and either dissociate completely, as do SiF4.2(CH,),0, SiF4.2(CH,),0, and SiF4.2(CH3),0, or decompose into SiF, and a polymethylene oxide polymer, as do SiF4.2(CH2),0 and SiF4-2(CH2)30.Silicon tetrafluoride does not coordinate with 1,4-dioxane in the range -94 to 25" and less than 1 atm pressure. Condensed phase heats of dissociation of SiF4.2(ether) complexes follow the order ( C H Z ) ~ O ?. (CH2)40 > ( C H 2 ) 5 0 2 (CH2),0 > (CH3)20, which suggests that this is the relative order of basicities towards S1F4. Canadian Journal of Chemistry, 46, 987 (1968)
Introduction Few coordination compounds of silicon tetrafluoride and oxygen electron-pair donor molecules have been prepared and characterized. Muetterties (1) has described SiF4.2(CH3),S0, SiF4.2(CH ,),NCHO, and SiF,.xCH ,COCH,COCH, (where x is indefinite). More recently, Isslieb and Reinhold (2) have reported SiF4.2(C6H5),P0, SiF4.2(C6H 1),PO, SiF4.2(CH3),NO, and SF4-2C5H5N0.These 1 :2 complexes illustrate the marked tendency of silicon t o exhibit a coordination number of six, which is especially evident in complexes of silicon tetrafluoride with nitrogen electron-pair donors (1). Although alcohols form 1:4 complexes, SiF4.4ROH in which R is CH,, C2H5, i-C,H,, and i-C5Hll (3, 4), these probably do not involve eight-coordinate silicon. In a recent detailed study of SiF,-4CH30H, we concluded that this complex contains tetracovalent rather than hexacovalent silicon and strong hydrogen bonds between methanol and each of the fluorine atoms (5). The only previous report of an interaction between silicon tetrafluoride and an ether is that by Schmeisser and Elischer (6), who found that ethylene oxide forms a 1 :2 complex which decomposes at 10-15" to give 1,4-dioxane. Surprisingly, 1,4-dioxane does not coordinate with silicon tetrafluoride (6), neither do phenyl ethyl ether and isopropyl phenyl ether (3). As part of our continuing interest in the coordination compounds of silicon tetrafluoride, we have studied its interaction with aliphatic cyclic ethers, 1,4dioxane, and dimethyl ether. Our object has been t o determine the conditions of temperature and pressure required for coordination, the stoichi-
ometry of the complexes, and the variation in stability with ring size of the cyclic ethers. Experimental Reagents Commercial samples of ethylene oxide, trimethylene oxide, dimethyl ether, and silicon tetrafluoride were purified by vacuum distillation at low temperatures. Tetrahydrofuran, tetrahydropyran, and 1,4-dioxane were refluxed over freshly cut sodium for 1 h and then fractionally distilled until their boiling points agreed with literature values. The purities of all gaseous conlpounds were checked by measurements of their vapor pressures, nlolecular weights, and infrared spectra. Apparatus Volatile materials were lnanipulated in a conventional glass high-vacuum apparatus which had stopcocks lubricated with Kel-F grease and ground-glass joints equipped with Viton rubber O-rings. Infrared spectra in the range 4000-650 cm-I were recorded on Perkin-Elmer model 21 or Infracord spectrophotometers. Dissociation pressures were measured with a standard tensimeter (7), the temperature of which was kept constant to k 0.2' at any chosen temperature in the range -94 to -114". Pressures were measured to k0.1 mm with a cathetometer.
Results Preparation of SiF4.2(CH,) ,O In a typical experiment, SiF, (8.12 mmoles) was condensed in a vacuum over (CH,),O (6.07 mmoles) and the temperature of the mixture was increased slowly from -195 to -78". After the mixture had been kept at -78" for 30 min, the excess of SiF, (5.06 mmoles) was removed by distillation at - 129", and a white solid remained. These results proved that SiF, and (CH,),O had combined in a mole ratio of 1 :1.99, corresponding to the formula SiF4.2(CH2),O. This compound and all other 1:2 SiF,-ether complexes are formulated as molecular addition compounds
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CANADIAN JOURNAL OF
on the basis of their physical properties described later. The infrared spectrum of the gas phase at 25" in equilibrium with the solid complex at -65" consisted of bands due to SiF, superimposed on those of (CH,),O. The molecular weight of this gaseous mixture was 87.2, which compares favorably with 85.1, calculated on the assumption that complete dissociation had occurred and that the liberated SiF, was completely in the gas phase while (CH,),O exerted its equilibrium vapor pressure of 10.0 mm at -65". It was not possible to correct for any lowering of the vapor pressure of (CH,),O by dissolved complex; such a correction would raise the calculated value and thus bring it into closer agreement with the experimental one. When the temperature of the 1:2 complex was raised to 25", all of the remaining SiF, (3.04 mmoles) was recovered and a colorless, nonvolatile, and viscous liquid resulted, free from ethylene oxide and 1,4-dioxane, the most likely contaminants. The infrared spectrum of the viscous liquid resembled that of 1,4-dioxane except that absorption bands were broader, especially the band in the 1125-1045 cm-' range stretchwhich is characteristic of the C-0-C ing vibration. A similar broad band has been observed in the spectra of polyethyleneglycol polymers and in ethylene oxide - acrylonitrile copolymers (8). The broadening of the C-0-C stretching band and the physical properties of the nonvolatile product strongly suggest that it is an ethylene oxide polymer. It was not possible to obtain pure SiF4.2(CH,),O by a direct synthesis using excess (CH,),O initially. Unconsumed (CH,),O could not be distilled from the mixture of (CH,),O and SiF,.2(CH2),O because the complex has an appreciable dissociation pressure at the temperature at which (CH,),O can be distilled. Therefore, the complex was first prepared at -78" by condensing excess SiF, (8.12 mmoles) with (CH,),O (6.07 mmoles); additional (CH,),O (8.32 mmoles) was added, and then the temperature of the mixture was raised to 25" for 1 h. The following fractions were obtained by distillation of the product mixture at temperatures in the range -78 to 25': (i) a mixture of SiF, and (CH,),O which could not be separated by distillation (12.9 mmoles; found: molecular weight (M), 65.0; calcd. for an ideal 1 :2 mole mixture
:HEMISTRY.
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VOL. 46, 1968
of SiF, and (CH,),O: 64.0); (ii) 1,4-dioxane (1.50 mmoles; found: M, 86.0; calcd.: 88.1); (iii) a small amount of a dark violet, nonvolatile, polymeric material. Thus, there had been a 21 % conversion of (CH,),O into 1,4-dioxane and a 9 % yield of polymer. Preliminary experiments indicated that -78" was a suitable temperature for the tensimetric titration of SiF, with (CH,),O. At this temperature the complex is not greatly dissociated and any excess SiF, (b.p. -94.8") should be entirely in the gas phase. Measured amounts of (CH,),O were added in successive small quantities (- 3 mmoles at a time) to a fixed quantity of SiF, (12.2 mmoles). After each addition the mixture was warmed quickly to -78" and then kept at this temperature for 15 min, after which time the pressure became constant. A plot of total pressure against mmoles of (CH,),O present shows that the pressure decreased linearly and approached the vapor pressure of pure ethylene oxide (- 5 mm at -78") at a mole ratio (CH,),O/SiF, equal to 2.03, corresponding to the complex SiF4.2(CH,) ,O. In the tensimetric titration of (CH,),O under the conditions used in the previously described reverse titration, the pressure remained close to 5 mm (the vapor pressure of (CH,),O at -78") until the mole ratio (CH,),O/SiF, decreased to about 2, after which the pressure increased rapidly as more SiF, was added. Extrapolation of the final linear portion of this plot to 5 mm pressure gave a mole ratio of 2.00, confirming the formation of the 1 :2 complex, SF4.2(CH2)20. Preparation of SiF4.2(CH,) ,O Under experimental conditions identical with those used in the preparation of SiF4.2(CH2),O, SiF, (1 1.9 mmoles) and (CH,),O (3.86 mmoles) combined in a 1:2.05 mole ratio as evident from the amount of uncombined SiF, (10.0 mmoles). Infrared and molecular weight measurements showed that the gas phase at 25" in equilibrium with the solid at 0" consisted of only SiF, and (CH,),O. (Found: M, 75.0. Calcd.: M, 73.4, assuming complete dissociation of the complex in the gas phase and that the liberated SiF, and (CH,),O are completely gaseous. The vapor pressure of (CH,),O of 125 mm at 0" was not established due to the large volume of the reaction vessel.)
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GUERTIN AND ONYSZCHUK: COORDIIVATION OF SILICON TETRAFLUORIDE
A tensimetric titration of SiF, with (CH,),O (added in small portions) showed a combining ratio (CH,),O/SiF, = 2.24. The discrepancy from the expected 2.00 is probably due to some polymerization of (CH2),0. During the titration, the temperature of the mixture had to be raised at a rate of lo per min to minimize polymerization. When the tensimetric titration was done in reverse, i.e. by addition of successive small portions of SiF, to a fixed amount of (CH,),O, combining ratios of (CH2),0 to SiF, were always abnormally high, usually greater than 4:l. This shows clearly that in the SiF,-(CH,),O system polymerization is enhanced when more (CH,),O is present than is necessary to produce the 1:2 complex. When (CH,),O (8.00 mmoles) was added to previously prepared SiF,.2(CH2),0 (1.88 mmoles) and the temperature of the mixture was increased to 25" for 12 h, the products were SiF, (1.8 mmoles) and a colorless, viscous, nonvolatile polymer. Thus, SiF, had simply catalyzed the polymerization of (CH ,),O. An infrared spectrum of the crystalline SiF4.2(CH,),O was taken at -195" using a lowtemperature infrared cell. A sample of the complex was sublimed in a vacuum at 3" onto an NaCl plate cooled to -195". It was difficult to obtain a good spectrum because, in spite of repeated attempts, the thickness of the crystalline film was not uniform and much of the infrared beam was scattered by the microcrystalline deposit. Nevertheless, it was possible to identify the following absorption bands: CH, stretching at 2955,2935, and 2870 cm-I; CH, deformation at 1500, 1450, and 1375 cm-I ; C-0 stretching at 1113 cm- ; Si-F octahedral stretching at 720 cm-I. The intensity of the last band decreased by about 75 % when the sample was removed, confirming that the band was due mainly to an absorption by the sample. The remaining absorption was undoubtedly due to SF6'- formed by an interaction of SiF,-2(CH2),0 with the NaCl surface. A similar reaction occurs between gaseous SiF, and NaCl plates (9).
'
Preparation of SiF4-2(CH,) ,O Slow cooling of a mixture of SiF, (4.94 mmoles) and (CH,),O (4.01 rnmoles) from 25 to -78" resulted in excess SiF, (2.84 mmoles) and a white solid which contained SiF, and (CH,),O in a mole ratio of 1 :1.91, corresponding to the complex SiF,-2(CH2),0. In several other ex-
989
periments combining ratios in the range 1:1.91 t o 1 :2.13 were also obtained. On raising the temperature of the complex from -78 to 25", complete dissociation into SiF, and (CH,),O occurred, as shown by infrared and molecular weight measurements on the gas phase in equilibrium with the liquid at 25". (Found : M, 93.9. Calcd. : M, 98.0, assuming complete dissociation of the complex in the gas phase and establishment of the vapor pressure of (CH,),O (125 mm) at 25".) The absence of polyinerization was evident from an infrared spectrum of the liquid phase which showed only the presence of (CH,),O. A tensimetric titration in which (CH,),O was added in successive small portions to SiF, (9.43 mmoles) confirmed the formation of only a 1:2 complex, as did the reverse titration of (CH,),O (20.10 mmoles) by successive additions of SiF,.
Preparation of SiF,.2(CH2) ,0 N o visible reaction occurred when SiF, (9.03 mmoles) and (CH2),0 (2.21 mmoles) were mixed and kept at 25" for 30 min. However, the amount of unreacted SiF, (8.00 mmoles) recovered by distillation at - 129" indicated that an interaction in a mole ratio of 1:2.14 had occurred at this temperature, producing a white solid corresponding to the complex SiF,.2(CH2),O. The large discrepancy between measured and theoretical ratios is undoubtedly due to the fact that the complex is formed only below -54.3", as evident from subsequent dissociation pressure measurements. At this temperature (CH,),O (m.p. -49") is a solid so that the complex probably forms slowly at a gas-solid interface. Infrared and molecular weight measurements on the gas in equilibrium with the liquid at 25" confirmed that the complex was completely dissociated in the gas phase at 25". (Found: M, 95.0. Calcd.: M, 97.5, assuming complete dissociation of the complex in the gas phase and a vapor pressure of (CH,),O of 70 mm at 25".) An infrared spectrum of the liquid phase at 25" showed only the presence of (CH2),0. Erratic pressure-composition plots were obtained in tensimetric titrations of SiF, with (CH2),0 at -95". Pressure readings were always higher than expected on the assumption that a 1 :2 complex, SiF4.2(CH2),0, had formed, and the change in slope of the plots was too gradual to show the combining ratio with any degree of
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TABLE I Temperature-pressure data for the SF,-1,4-dioxane
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Temperature ("C)
SiF, partial pressure (mm)
1,4-Dioxane partial pressurea (mm)
Observed total pressure (mm)
I968
system Calculated total pressureb (mm)
aThe equilibrium vapor pressures of 1,Cdioxane at the temperature quoted. busing Dalton's law of partialpressures.
accuracy. These difficulties are attributed, as before, to the occurrence of a slow gas-solid surface reaction between SiF, and (CH,),O at temperatures below -49", the melting point of (CH,),O, above which the complex has a high dissociation pressure. Preparation of SiF4.2(CH3) ,O A mixture of SiF, (3.83 moles) and (CH,),O (3.09 mmoles), prepared at 25" and cooled slowly to - 129", gave unreacted SiF, (2.29 mmoles) and a white solid in which the combining ratio of SiF, to (CH,),O was 1 :2.01, corresponding to the formula SiF4.2(CH3),0. This complex dissociated completely into SiF, and (CH,),O at 25", as shown by infrared and molecular weight measurements on the gaseous mixture at this temperature. (Found: M, 65.4. Calcd.: M, 65.4 for a 1:2 mole mixture of SiF, to (CH,),O.) The formation of only a 1:2 complex at -78" was also confirmed by tensimetric titrations. Attempted Reaction of SiF, with 1,4-Dioxaize Silicon tetrafluoride (1.96 mmoles) and 1,4dioxane (10.80 mmoles) were condensed together and pressures were measured in the range -94 to 25" and compared with the total pressure calculated assuming no interaction and ideal gaseous behavior according to Dalton's law of partial pressures. The results (Table I) prove that no interaction had occurred in the range -94 to 25". An infrared spectrum of the gas phase in equilibrium with the liquid at 25" showed absorption bands of SiF, superimposed on those of 1,4-dioxane, while a spectrum of the liquid phase contained absorption bands only of 1,4-dioxane, proving that no SiF, was dissolved. Heats of Dissociation Solid SiF4.2(ether) complexes were prepared at low temperatures as previously described and
their temperatures were increased slowly until a pressure of about 0.5 mm was observed; thereafter pressure measurements were made at intervals of about 5 to 10". To be certain that an equilibrium existed at each temperature, a sample of the gas above the solid complex was pumped off and a constant pressure wasallowed to reestablish. True equilibrium at a given temperature existed if the pressure before and after the pumping off procedure were identical. Pressure-temperature measurements were made until the complex decomposed irreversibly, as evident by an abnormally large pressure increase and failure to observe identical pressures before and after the pumping off procedure. Equilibrium pressures, approached from above as well as from below a given temperature, were identical in the temperature ranges given in Table 11. The heats of dissociation summarized in Table I1 were calculated from the slopes of the linear log p,,, against T-' plots using the integrated form of the ~lausius-clapeyronequation. Slopes were determined by the method of least squares and error limits are expressed in terms of standard deviations.
Discussion Preparation of SiF4.2(etlzer) Con7plexes The results show that SiF, reacts with alicyclic ethers and (CH,),O to form white, solid 1:2 complexes which are stable at -78". These complexes are not sufficiently stable to be obtained by simply bubbling SiF, through solutions of ethers at 25", as had been attempted unsuccessfully by Muetterties (I), although he was able to prepare SiF,-oxygen donor complexes such as SiF,.2(CH3),SO, SF4-2(CH3),NCH0, and SiF4.xCH3COCH2COCH3 (where x is uncertain) using this technique. In the preparation
GUERTlN AND ONYSZCHUK: COORDlNATlON OF SlLlCON TETRAFLUORlDE
TABLE I1 Heats of dissociation of SiF4.2(ether) complexes
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Complex
Temperature-pressure data W , mm)
Heat of dissociation AHI0(kcal mole-')
of SiF4.2(CH ,) ,O and SiF4.2(CH,) ,O it was GeF,.2(CH2),O decomposes above -78" (10). necessary to warm a SF4-ether mixture, con- Complexes of BF, with (CH3),0, (CH,),O, and taining a small excess of SiF,, slowly from - 195" (CH2),0 are stable liquids at 25", and BF,.(thus providing a good "heat sink" for the heat (CH,),O and BF,.(CH,),O boil without deof reaction) to a temperature at which reaction composition (11). Although SiF, did not react would just occur and then prevent any further with 1,4-dioxane even at -94", GeF, forms a increase in temperature in order to minimize 1:1 complex which sublimes at 12j0, and BF, polymerization of the ethers. The excess of SiF, forms 1:l and 2:l complexes both of which are was removed at a temperature at which the dis- stable at 25", the former being a white solid and sociation pressure of the complex is negligible. the latter a colorless liquid. Complexes of (CH,),O with SiF,, GeF,, and For the preparation of the other SiF,-2(ether) complexes, the ether molecules of which do not BF, are all similar in their thermal decomposition into 1,4-dioxane and a nonvolatile polymer. readily polymerize, it was best to cool the SiF,ether mixture containing a small excess of SiF, These complexes also react with an excess of slowly from 25" to a temperature sufficiently low (CH,),O to produce 1,4-dioxane and a polymer, to stabilize the complex, and then remove the indicating that they catalyze the dimerization excess of SiF, by distillation at this temperature. and polymerization of (CH,),O. The mechanism It is not possible to obtain pure SiF4.2(ether) of these reactions is probably similar to that complexes by a direct synthesis using an excess proposed for the BC1,-(CH2),0 system (12). In of ether initially, because the complexes dissoci- the thermal decomposition of SiF,.2(CH2),O, ate at temperatures below which the excess ether SiF, was liberated quantitatively and a nonvolatile, colorless polymer remained. However, can be distilled. SiF4.2(CH2),O and SiF,.2(CH2),0 dissociated Ge~zeralProperties of SiF4.2(ether) Complexes into their component parts without polymerizaAll of the SiF4.2(ether) complexes prepared tion of the ethers. The ease of sublimation and dissociation of the are white solids stable at low temperatures. They did not appear to have a stable liquid state and SiF4.2(ether) complexes, as well as the presence were completely dissociated in the gas phase at of an Si-F octahedral stretching vibration band 25". Only SiF,.2(CH2),O could be sublimed in the infrared spectrum of SiF4-2(CH,),O at into colorless crystals which were stable only - 19j0, strongly suggests that the complexes are below 13.6". By contrast, ether complexes of octahedral molecular adducts in the solid state, GeF, (10) and BF, (11) are more stable than although the possibility that they are ionic those of SiF,. Thus, GeF4.2(ether) complexes, in cannot be ruled out. The spectrum of SiF4.2which the ether is (CH,),O, (CH,),O, and (CH,),O was not sufficiently resolved to indicate (CH,),O, are stable solids at 25", although whether ether molecules were in either cis or
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TABLE 111 Comparisons of heats of dissociation Dissociation process
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+
SiF4.2py(,) = SiF4(,, ~PY,,, SiF4.2iq(,) = SiF4(,) -k 2iq(,,,,,, ,,I,) SiF4.2NH3,,) = SiF4(,, 2NH3(,, BF3.(CH3)20(s) BF3(g) (CH3)zO(s) B F ~ . ( C H ~ ) L O=( ~BF3(,1 ) (CHz)aO(s)
+
+ +
trans octahedral positions. This uncertainty can be resolved only by a detailed X-ray crystallographic study.
Heat of dissociation (kcal mole-')
Ref.
33.1 31.9 54.6 13.3 13.4
16 16 17 18 18
complex and AH,' is the heat of sublimation or vaporization of the ether. Therefore, AH,' AH,' - AH,' + AH,', so that the measured heats of dissociation must be corrected in order Heats of Dissociation of SiF4.2(etlzer) Cotnplexes to obtain the true heats of dissociation. Even In the determination of the measured heats of though values of AH,' are not known, they are dissociation, AH,', reported in Table 11, it was probably similar in as closely related a series of assumed that the complexes were completely compounds as SiF4.2(alicyclic ether). The heats dissociated in the gas phase. In the temperature of vaporization, AH,', are reported (13) only for range that dissociation pressures were measured, (CH,),O and (CH,),O; they are 6.10 and 5.14 the dynamic equilibrium was considered to be kcal mole-' respectively. In the absence of comKP plete data for AH,' and AH,', it is not justifiSiF4.2(ether),,, F), SF,(,, 2(ether)(,,, except able t o attach much significance to the absolute for SiF4.2(CH,),0 where the (CH,),O was a values of the measured heats of dissociation. solid in the temperature range over which Nevertheless, we believe the trend (CH,),O > pressures were measured. T o calculate the equi> (CH2)50 2 (CH2)20 > (CH3)20 librium constant, K,, it was necessary to assume (CH2)40 is real and reflects the relative order of basicithat the complex was involatile and that the ties towards SiF,. Exactly the same order has vapor pressure of the liquid ether was negligible been inferred by others who used a variety of by comparison with the pressure of SiF,. This experimental methods (14, and references cited assumption is justified because even (CH,),O, therein). Previous attempts to rationalize this the most volatile ether used, has a vapor pressure order are examined critically in a separate article about 100 times less than that of SiF, in the in which a new explanation based on electrorange -94 to -56'. Similar assumptions were negativity theory is proposed (15). made in the calculation of thermodynamic data Complexes of SiF, with nitrogen electron-pair of BF,-ether complexes (11). Thus, the disso- donors have much higher, and BF,.ether comciation pressure of the complex is due entirely to plexes only slightly lower, heats of dissociation SiF,, which gives simply K, = p(atm), and the than SiF4.2(ether) complexes, as evident from Clapeyron equation, AH0 = -(2.303R Alogp)/ the data in Table 111. Thus, Si-0 coordinate AT-', was used to calculate AH0, since the plot bond energies appear to be less than half those of log p against T-' is equivalent to log K,, of Si-N and B-0 coordinate bonds. against T-'. The measured heats of dissociation, AH,', Acknowledgment are related t o the true gas phase heats of dissociation, AH,', by the following cycle: We are grateful to the Defence Research Board of Canada for financial assistance (grant number AHI0 9530-20, project D46-95-30-20). SiF4.2(ether)(,) SiF,,,) f 2(ether)(l
-
+
*
71 AH4O
AH,' SiF4.2(ether)(,, F? SF4(,,
in which AH,'
.,,
Tl AH3O
+ 2(ether)(,,,
is the heat of sublimation of the
1. E. L. MUETTERTIES. J. Am. Chern. Soc. 82, 1082 (1960). 2. V. K. ISSLIEB and H. REINHOLD. Z. Anorg. Allgeni. Chem. 314, 113 (1962).
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GUERTIN AND ONYSZCHUK: COORDINATION O F SILICON TETRAFLUORLDE 3. J. A. GIERUT,F. J. SOWA,and J. A. NIEWLAND.J. Am. Chelii. Soc. 58. 786 (1936). 4. A. V. TOPCHIEVa'nd N. F. 'BOGOMOLOVA. Dokl. Akad. Nauk SSSR, 88,487 (1953). 5. J. P. GUERTIN and M. ONYSZCHUK.Can. J. Chem. 41, 1477 (1963). 6. M. SCHMEISSER and S. ELISCHER.Z. Naturforsch. 7b, 583 (1952). 7. R. T. SANDERSON.Vacuuni manipulation of volatile compounds. John Wiley and Sons, Inc., New York. 1958. 8. J. FURNKAWA, T. SAIGUSA, and N. MISE. Makroniol. Chem. 38, 244 (1960). 9. W. R. HESLOP,J. A. A. KETELAAR, and A. BUCHLER. Spectrochirn. Acta, 16, 513 (1960). 10. R. C. AGGARWAL, J. P. GUERTIN, and M. ONYSZCHUK. In Proceedings of the VIIIth international conference on co-ordination chemistry. Eclited by V. Gutmann. Springer-Verlag, Vienna. 1964. p. 198.
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11. N. N. GREENWOOD and R. L. MARTIN. Quart. Rev. London. 8. 1 (1954). 12. J. D. EDWARDS, W. GERRARD, and M. F. LAPPERT. J. Cheni. Soc. 348 (1957). 13. Selected values of chemical thermodynamic properties. Circular 500. National Bureau of Standards. U.S. Government Printing- Office. Washington. , D.C. 1952. p. 599. 14. SISTERM. BRANDON, O.P., M. TAMRES, S. SEARLES, JR. J. Am. Chem. Soc. 82,2129,2134 (1960). 15. J. P. GUERTIN, M. ONYSZCHUK, and M. A. WHITEHEAD. In preparation. 16. J. M. MILLERand M. ONYSZCHUK.J. Chem. Soc. A, 1132 (1967). 17. D. B. MILLERand H. H. SISLER. J. Am. Chem. Soc. 77,4998 (1955); 78, 6412 (1956). 18. H. C. BROWN and R. M. ADAMS. J. Am. Cheni. Soc. 64, 2557 (1942).
