- Email: [email protected]
Proton NMR evidence for π-bonding in oxovanadium(IV) complexes
Notes
2273
size of the complex species appears to increase. The values of the transfer coefficient, a, show that the interface represents a barrier for the transfer of zinc. This could be due to chemical reaction, since the zinc complex is probably created immediately next to the interface, as both TOA and its salts are practically insoluble in the aqueous phase[6]. The method for the determination of the interface transfer-coefficients as described is very simple. Its precision depends on the magnitude of the interface resistance; the transfer coefficients are calculated on the basis of the difference of diffusion out of capillaries with and without interface resistance. It is assumed in calculations that the interface is planar, but it is in fact spherical, due to the different surface tensions of the liquids. Consequently, this assumption contributes an additional error in the calculated transfer coefficients. Most extractions are usually carded out by agitating the systems. In such experiments the interfacial area and the extent of convection in both phases are not known, and it is difficult to define the kinetics. The knowledge of transfer processes helps us to understand the extraction rate in some special cases, e.g. extraction in systems at rest and falling drop extraction.
Chemistry Division Nuclear Institute "Jo~ef Stefan'" Ljubljana, Slovenia Yugoslavia
K. J U Z N I C
6. A. A. Mazurova and L. M. Gindin, Zh. neorg. Khim. 10 (11), 2559 (1965).
J. inorg, nucl. Chem., 1968, VoL 30, pp. 2273 to 2277.
Pergamon Press.
Printed in Great Britain
Proton NMR evidence for ~r-bonding in oxovanadium(IV) complexes (Received 10 November 1967) INTRODUCTION EVEa since the publication of the molecular orbital calculation of an energy level scheme for VOSO4.5H20 by Ballhausen and Gray[l], this energy scheme (hereafter termed the BG scheme) has been utilized by some to explain their data and challenged and. altered by others to account for their optical spectral or ESR data[2, 3]. In a primarily qualitative energy level scheme, Selbin, Holmes and McGlynn [4] utilized the b2 molecular orbital of the water molecule to bond with the b2 orbital of VO 2÷ and an energy level scheme quite similar to the BG scheme was deduced. The b2 orbital had been assumed by Ballhausen and Gray to be a pure non-bonding d orbital (d~), in which the lone outer electron of the V(IV) entity resides. Kivelson and Lee[5] examined the ESR spectra of [VO(aca)2] ( a c a = acetylacetonate ion) and [VO(tetraphenylporphyrin)] and carried out a somewhat more inclusive MO calculation, bat in the manner of that of Ballhausen and Gray. From their experiments they deduced that the lone electron is in a b*, orbital which is "almost completely localized on the vanadium in a d~., atomic orbital" and that "in-plane ~r-bonding is slight" in both complexes. Zerner and Gouterman [6] carried out more detailed "extended Hiickel calculations" on VO ~+porphins, including in their calculations all valence orbitals of all the atoms in the porphin system. They deduced that the b2o orbital is - 9 9 per cent an atomic 3d~u vanadium orbital. Hecht and Johnston[7] performed ESR experiments on V 4+ in Na~O-B203 glass systems, and they carried out MO calculations for the VO 2+ system in this oxide ion environment. They concluded that there is some in-plane ~r-bonding and that 1. 2. 3. 4. 5. 6. 7.
C.J. Ballhausen and H. B. Gray, lnorg. Chem. 1, 111 (1962). J. Selbin, Coordn. chem. Rev. 1,293 (1966), and Refs. therein. J. Selbin, G. Maus and D. L. Johnson, J. inorg, nucl. Chem. 29, 1735 (1967). J. Selbin, L. H. Holmes,Jr. and S. P. McGlynn, J. inorg, nucl. Chem. 25, 1359 (1963). D. Kivelson and S-K. Lee, J. chem. Phys. 41, 1896 (1964). M. Zerner and M. Gouterman, lnorg. Chem. 5, 1699 (1966). H. G. Hecht and T. S. Johnston, J. chem. Phys. 46, 23 (1967).
2274
Notes
therefore the lone electron is not entirely localized on the vanadium atom. Thus the square of their bonding coefficient,/32(i.e., (fl2)2), in the MO wave function ~'ta = fl2(3d~y)+/3~(½)(2pu~ + 2p~, -- 2pu:~-- 2p~), for the b2 orbital was in the range 0.83--0.86, rather than 1.00 as had been assumed in the BG scheme and as Kivelson and Lee had concluded. A more rigorous semi-empirical MO calculation carried out by Vanquickenborne and McGlynn [8] the x's and y's refer to specific oxygens, 1,2, 3, and 4, located on the xy axes in the equatorial plane). The specific object of the work reported here was to determine if ~r-bonding to VO 2+ by equatorial oxygens could be experimentally substantiated by proton NMR studies.
EXPERIMENTAL The compound VOSO4-xH20 was purchased from the Varlacoid Chemical Company. It was dried in vacuum and analyzed only for hydrogen. Found: 3.40% H. Calculated for VOSO4.4H20: 3.41% H. Others [9, 10] have also found the sulphate salt to be a tetrahydrate. In order to prepare anhydrous solutions of VOSO4 in methanol, the following procedure was employed. Ten grams of VOSO4.4H20 was added to 300 ml of anhydrous methanol. The solution was refluxed until all of the solid had dissolved. Then the solution was heated until about half of the methanol had evaporated. Upon cooling, the solution yielded a blue crystalline compound which proved to be VOSO4"2CHaOH. Calculated: 11.5% C, 3.5% H; Found: 11.2% C, 3-0% H. This product was added to anhydrous methanol to prepare the solutions used in the NMR studies. Tetramethyl silane (TMS) was used here as the internal standard. A stock solution of 2 per cent (CHa)Si(CH2)2SO3Na in water was prepared and used to make up the aqueous solutions of vanadyl sulfate for N M R investigations. The trimethyl silane salt was chosen as an internal standard because of its solubility in aqueous solution and because its resonance methyl peaks coincide with those of TMS. A Varian H A60 was used to obtain the N MR spectra of the aqueous solutions and a Varian HA100 was used to obtain the N M R spectra of the methanol solutions.
THEORY A general theory of the NMR of paramagnetic complexes in solutions is given by Drago [ 11 ] and by Benson and Phillips[12]. The theory and its application to the present work is given by Wayland and Rice [ 13]. In order to determine the proton chemical shift of the coordinated molecules, Equation (I) is applied to the chemical shift in the vanadyl sulfate solutions of the water protons and of the -OH and -CH3 protons of methanol, shifted from pure water and pure methanol, respectively.
Av~omo.Xfcomp. = Avobs..
(1)
Av~omp~is the chemical shift of the protons of the solvent molecules which are complexed to the VO 2÷ entity,fcomp, is the fraction of the total solvent molecules complexed, and AVobs.is the chemical shift of the solution protons, i.e., shifted from the pure solvent protons. When paramagnetic ions are placed in a solution of ligands which can co-ordinate to the ions, two types of contact shifts, the pseudo contact and the Fermi contact shift, can result[11]. The small anisotropy of the g factor for VO 2+ compounds makes it unlikely that the pseudo contact shift will 8. 9. 10. 11. 12.
L. G. Vanquickenborne and S. P, McGlynn, To be published. M. B. Palma-Vittorelli, M. U. Palma, D. Palumbo, and F. Sgarlata, Nuovo Cim. 3, 718 ( 1956). W. B. Lewis, lnorg. Chem. 6, 1737 (1967). R.S. Drago, Physical Methods in Inorganic Chemistry. Reinhold, New York (1965). R. E. Benson and W. D. Phillips, inAdvances in Magnetic Spectroscopy, Vol. 1, pp. 103, 148. Academic Press, New York (1965). 13. B. B. Wayland and W. L. Rice, lnorg. Chem. 5, 54 (1966).
Notes
2275
play alarge part in the overall contact shift. The Fermi contact shift, which occurs when some unpaired spin density is transferred from the ligand to the metal or from the metal to the ligand atoms through either the o- or zr bonds formed between the metal and ligand atoms, makes up the greatest part of the chemical shift. It is this effect that is to be analyzed in this paper. The amount of spin density transferred to or from an atom to which the resonate proton is attached is related to the electron spin-nuclear spin coupling constant, .4. The latter constant, in turn, is related to the contact shift by Equation (2): Av . . . . . . . A ~'e g/3S (S + l ) (2) v TN 3kT ' where v is the resonating frequency (here 60 or 100 Me), g is the Lande splitting factor, 13 is the Bohr magneton, S is the electron spin of the ion, Ye/TN is the gyromagnetic ratio for the electron and the nucleus, and k and T have their usual meaning. These contact shifts thus can be used to infer certain types of bonding between the paramagnetic ion and its bonded ligands. RESULTS The average proton chemical shift for the water protons of the aqueous solution of VOSO4 was found to be 8.54 ppm (513 counts/sec) downfield from the unco-ordinated water as shown in Table 1. F r o m Equation (2), (`4/h) was determined to be 3-0 × 105, and since the shift was downfield `4 must therefore be positive. The proton chemical shift for the - O H proton in the methanolic solution of VOSO4 was determined to be 10 ppm ( 1000 counts/sec) downfield from the unco-ordinated methanol (Table 1). The value for A / h was calculated as 3.45 × 105. N o shift was found for the -CH3 peak.
Table 1. Nuclear magnetic resonance contact shift data for our aqueous solutions of VOSO4-4HzO* and our methanolic solution of VOSO4.2CH3OHT
Moles VOSO4
Total moles of H20* or C H 3 O H t
Av in counts/sec:~ at 43°C or 40°Ct
(A)§ ,~
1 2 3
0.00120 0.00139 0.00181
0.275 0.274 0.277
4
8-28 × 10-5
0-0822
524 501 514 Average=513 lOOffI
---3.0 × 105 3"45 × 105
*These data are for runs l, 2 and 3. "tThese data are for run 4. ~Resonance frequency = 60 Mc for aqueous solutions and 100 Mc for methanolic solution. §See the note under Table 2. Iq000 _+ 25 counts/sec.
DISCUSSION Electronic spin resonance measurements[5, 14] place the unpaired electron of V O 2+ in the dxu orbital, in agreement with the BG scheme. If in-plane 7r-bonding should be occurring, it should be primarily between the dxu orbital of the vanadium and the p orbitals of proper zr symmetry on the oxygen atoms of the equatorial water molecules in VO(H20)52+. The results of Wayland and Rice [ 13] shown in Table 2 demonstrate the existence of r - b o n d i n g between coordinated water molecules and the metal ions Co s÷, F e 2÷, Mn 2+, and Fe z+. A n M O energy level scheme of the water molecule[15] (increasing energy: al < b2 < al < bl < b~* < a~*) reveals that both bl and bz orbitals have the 14. K. D e Armond, B. B. Garrett and H. S. Gutowsky, J. chem. Phys. 42, 1019 (1965). 15. D. G. Carrol, A. T. Armstrong and S. P. McGlynn, J. chem. Phys. 44, 1865 (1966).
2276
Notes
appropriate symmetry for zr-bonding between a metal ion and the water molecules. Indeed, the paramagnetic shifts observed by Wayland and Rice are interpreted using both bl and b2 water MO's bonded to the t~uorbitals of the metal ion. It has been shown by Radford [16] that spin density transfer through the b~ orbital would cause a shift upfield whereas spin density transfer through the b2 orbital would cause a shift to be observed downfield. Since the observed shift for the metal ions was downfield, Wayland and Rice concluded that although the b~ orbital must play the major role in bonding, the b~ orbital, which is more efficient in transmitting spin density, is responsible for the observed shift[17]. In any case, delocalization of unpaired ~r-electron density into the water molecules is present. The observed shift for the VO 2÷ ion in water is found to be downfield, but of a much smaller magnitude than those shifts observed by Wayland and Rice for transition metal ions with more d (i.e., tzy) electrons. However, it can be concluded on the basis of the foregoing results and discussion that there is indeed experimental proton N M R support for ~r-bonding between the VO 2+ and water molecules. The observed shift cannot be interpreted as arising from the tumbling of the VO ~+ ions in solution because a dipolar shift of this type would result in an upfield shift, whereas the observed shift is actually downfield. Therefore, as in the case demonstrated by Wayland and Rice, the shift is most likely due to bonding between the b2 MO of VO 2+ and the bl and b2 MO's of H20. The coupling constant, (A/h), was observed to 3-0 x 105 whereas the same for the other transition metal ions were about 14 × 105. The observed shift cannot be due to o- transfer alone since the magnitude of this shift would result in an (A/h) value of about 1.5 × 105. (See Table 2 results for Cu 2+ and Ni 2+) . Furthermore, the unpaired electron is unambiguously placed in VO 2+ in a primarily dxu orbital anyway, which is not suitable for tr bonding. Table 2. Nuclear magnetic resonance contact shift data of paramagnetic ions in aqueous solution [ 13]*
Cu 2÷ Ni Co 2. Fe 2÷ Mn 2+ Fe 3÷
1/2 1 3/2 2 5/2 5/2
1-5 1.1 4.2 5-0 5.9 7.7
0 0 1/2 1 3/2 3/2
--18.7 14.0 12.7 16.5
* (A/h) is the coupling constant resulting from the total shift; (A/h),, (or the 7r-only contribution) was determined by "stripping out" the o- contribution, which can be taken from the Ni 2. value for (A/h), from the (A/h) values of Co 2÷, Fe 2+ , Mn2+ and Fe 3+. Other recent 170 NMR work supports the conclusion of ~r-bonding presented here. Wiithrich and Connick [18] examined the 170 NMR spectrum of VO z÷ in water and found that the (A/h) value was 3.8 × 106 cps. Furthermore, they found that this value was small compared to the analogous values found earlier [ 19] for Mn 2+, Fe 2÷, Co 2÷ and Ni 2÷ also using 170 N M R data. They therefore also suggest the existence of zr-bonding from their data. 16. H. E. Radford, Phys. Rev. 126, 1035 (1962). 17. The reason that the b2 orbital is more efficient for transferring spin density is because it is made up of both oxygen 2p and hydrogen Is orbitals and thus any spin density transferred through this orbital goes directly into a hydrogen orbital. The bl orbital, however, is less efficient because hyperconjugation (or spin polarization) must be involved (a much less efficient process) to transfer spin density to a hydrogen orbital. 18. K. Wiithrich and R. E. Connick, lnorg. Chem. 6, 683 (1967). 19. T.J. Swift and R. E. Connick, J. chem. Phys. 37, 307 (1962).
Notes
2277
Hausser and Laukien[20] measured the temperature dependence of the relaxation times T1 and T~ of VO 2+ in aqueous solution by ESR. If these data are used to calculate the A/h value, one obtains •4/h = 1-65 × l0 s. This value is much larger than the value we derived directly from the chemical shift method but, again, the evidence strongly favors the presence of ~--bonding in the VO2+-H20 complex. Reuben and Fiat [21 ] also observed similar evidence for 7r-bonding through the 1tO spectrum of an aqueous solution of VOSO4. Finally, Lewis[22] obtained an ESR spectrum of VOSO4'4H20 doped in zinc Tutton salt. He observed a hyperfine splitting which he interprets as arising from electron spin-proton spin coupling. However, these results are somewhat in doubt due to the large spin coupling constant (,4 = 2.6 gauss) found by him. In an effort to add new evidence to that cited above, we obtained the N M R spectrum of VOSO4.2CH3OH in methanol. It was assumed in this case that the MO's on the methanol oxygen atom would bond to VO 2+ in a way similar to those of the water oxygen atom. Thus, observing the methanol - O H proton peak, one should expect it to behave similarly to the water proton peak in aqueous VOSO4. Indeed the water proton and the methanol - O H proton do have similar coupling constants (See Table 1) and thus it appears that 7r-bonding from methanol to VO 2 is also occurring.
A c k n o w l e d g e m e n t - T h i s research was supported by the National Science Foundation Grant No. GP-4938, for which we express our appreciation. Coates Chemical Laboratories Louisiana State University Baton Rouge, Louisiana 70803
G. V I G E E J. SELBIN
20. Hausser and Laukien, Z. Physik. 153, 294 (1959). 21. J. Reuben and D. Fiat, lnorg. Chem. 6, 579 (1967). 22. W. B. Lewis, lnorg. Chem. 6, 1737 (1967).
J. inorg, nucl. Chem., 1968, Vol. 30, pp. 2277 to 2279.
Pergamon Press.
Printed in Great Britain
Hydrofluorination of curium dioxide* (Received 11 September 1967) INTRODUCTION CualvM trifluoride has been prepared by precipitation from hydrochloric acid solution by the addition of an excess of hydrofluoric acid[l], yielding a hydrated fluoride. The trifluoride has also been prepared by an NH4F-HF fusion of the oxide and of the hydrated trifluoride[2]. No details of either method were given. The reaction of curium dioxide with hydrogen fluoride gas seemed a more direct method of preparation. This study was undertaken to determine temperature conditions necessary for a rapid, complete reaction between CmOz~,) and HF~u). This reaction has been examined at three temperatures (435°, 310% 185°C) using thermogravimetric analysis techniques. EXPERIMENTAL All work was performed in glovebox enclosures designed for work with plutonium. These enclosures were kept at a slight negative pressure to prevent the spread of this highly radioactive element. Hand contact with the sample and its immediate container was minimized to prevent un*This work was supported by the University of California, Lawrence Radiation Laboratory, Livermore, California, and by U S A E C Contract AT(29-1)-1106 with The Dow Chemical Company. 1. B. B. Cunningham and J. C. Wallman, J. inorg, nucl. Chem. 26, 271, 1964. 2. J. L. Burnett, Trans. Am. nucl. Soc. 8, 335, November, 1965.
2273
size of the complex species appears to increase. The values of the transfer coefficient, a, show that the interface represents a barrier for the transfer of zinc. This could be due to chemical reaction, since the zinc complex is probably created immediately next to the interface, as both TOA and its salts are practically insoluble in the aqueous phase[6]. The method for the determination of the interface transfer-coefficients as described is very simple. Its precision depends on the magnitude of the interface resistance; the transfer coefficients are calculated on the basis of the difference of diffusion out of capillaries with and without interface resistance. It is assumed in calculations that the interface is planar, but it is in fact spherical, due to the different surface tensions of the liquids. Consequently, this assumption contributes an additional error in the calculated transfer coefficients. Most extractions are usually carded out by agitating the systems. In such experiments the interfacial area and the extent of convection in both phases are not known, and it is difficult to define the kinetics. The knowledge of transfer processes helps us to understand the extraction rate in some special cases, e.g. extraction in systems at rest and falling drop extraction.
Chemistry Division Nuclear Institute "Jo~ef Stefan'" Ljubljana, Slovenia Yugoslavia
K. J U Z N I C
6. A. A. Mazurova and L. M. Gindin, Zh. neorg. Khim. 10 (11), 2559 (1965).
J. inorg, nucl. Chem., 1968, VoL 30, pp. 2273 to 2277.
Pergamon Press.
Printed in Great Britain
Proton NMR evidence for ~r-bonding in oxovanadium(IV) complexes (Received 10 November 1967) INTRODUCTION EVEa since the publication of the molecular orbital calculation of an energy level scheme for VOSO4.5H20 by Ballhausen and Gray[l], this energy scheme (hereafter termed the BG scheme) has been utilized by some to explain their data and challenged and. altered by others to account for their optical spectral or ESR data[2, 3]. In a primarily qualitative energy level scheme, Selbin, Holmes and McGlynn [4] utilized the b2 molecular orbital of the water molecule to bond with the b2 orbital of VO 2÷ and an energy level scheme quite similar to the BG scheme was deduced. The b2 orbital had been assumed by Ballhausen and Gray to be a pure non-bonding d orbital (d~), in which the lone outer electron of the V(IV) entity resides. Kivelson and Lee[5] examined the ESR spectra of [VO(aca)2] ( a c a = acetylacetonate ion) and [VO(tetraphenylporphyrin)] and carried out a somewhat more inclusive MO calculation, bat in the manner of that of Ballhausen and Gray. From their experiments they deduced that the lone electron is in a b*, orbital which is "almost completely localized on the vanadium in a d~., atomic orbital" and that "in-plane ~r-bonding is slight" in both complexes. Zerner and Gouterman [6] carried out more detailed "extended Hiickel calculations" on VO ~+porphins, including in their calculations all valence orbitals of all the atoms in the porphin system. They deduced that the b2o orbital is - 9 9 per cent an atomic 3d~u vanadium orbital. Hecht and Johnston[7] performed ESR experiments on V 4+ in Na~O-B203 glass systems, and they carried out MO calculations for the VO 2+ system in this oxide ion environment. They concluded that there is some in-plane ~r-bonding and that 1. 2. 3. 4. 5. 6. 7.
C.J. Ballhausen and H. B. Gray, lnorg. Chem. 1, 111 (1962). J. Selbin, Coordn. chem. Rev. 1,293 (1966), and Refs. therein. J. Selbin, G. Maus and D. L. Johnson, J. inorg, nucl. Chem. 29, 1735 (1967). J. Selbin, L. H. Holmes,Jr. and S. P. McGlynn, J. inorg, nucl. Chem. 25, 1359 (1963). D. Kivelson and S-K. Lee, J. chem. Phys. 41, 1896 (1964). M. Zerner and M. Gouterman, lnorg. Chem. 5, 1699 (1966). H. G. Hecht and T. S. Johnston, J. chem. Phys. 46, 23 (1967).
2274
Notes
therefore the lone electron is not entirely localized on the vanadium atom. Thus the square of their bonding coefficient,/32(i.e., (fl2)2), in the MO wave function ~'ta = fl2(3d~y)+/3~(½)(2pu~ + 2p~, -- 2pu:~-- 2p~), for the b2 orbital was in the range 0.83--0.86, rather than 1.00 as had been assumed in the BG scheme and as Kivelson and Lee had concluded. A more rigorous semi-empirical MO calculation carried out by Vanquickenborne and McGlynn [8] the x's and y's refer to specific oxygens, 1,2, 3, and 4, located on the xy axes in the equatorial plane). The specific object of the work reported here was to determine if ~r-bonding to VO 2+ by equatorial oxygens could be experimentally substantiated by proton NMR studies.
EXPERIMENTAL The compound VOSO4-xH20 was purchased from the Varlacoid Chemical Company. It was dried in vacuum and analyzed only for hydrogen. Found: 3.40% H. Calculated for VOSO4.4H20: 3.41% H. Others [9, 10] have also found the sulphate salt to be a tetrahydrate. In order to prepare anhydrous solutions of VOSO4 in methanol, the following procedure was employed. Ten grams of VOSO4.4H20 was added to 300 ml of anhydrous methanol. The solution was refluxed until all of the solid had dissolved. Then the solution was heated until about half of the methanol had evaporated. Upon cooling, the solution yielded a blue crystalline compound which proved to be VOSO4"2CHaOH. Calculated: 11.5% C, 3.5% H; Found: 11.2% C, 3-0% H. This product was added to anhydrous methanol to prepare the solutions used in the NMR studies. Tetramethyl silane (TMS) was used here as the internal standard. A stock solution of 2 per cent (CHa)Si(CH2)2SO3Na in water was prepared and used to make up the aqueous solutions of vanadyl sulfate for N M R investigations. The trimethyl silane salt was chosen as an internal standard because of its solubility in aqueous solution and because its resonance methyl peaks coincide with those of TMS. A Varian H A60 was used to obtain the N MR spectra of the aqueous solutions and a Varian HA100 was used to obtain the N M R spectra of the methanol solutions.
THEORY A general theory of the NMR of paramagnetic complexes in solutions is given by Drago [ 11 ] and by Benson and Phillips[12]. The theory and its application to the present work is given by Wayland and Rice [ 13]. In order to determine the proton chemical shift of the coordinated molecules, Equation (I) is applied to the chemical shift in the vanadyl sulfate solutions of the water protons and of the -OH and -CH3 protons of methanol, shifted from pure water and pure methanol, respectively.
Av~omo.Xfcomp. = Avobs..
(1)
Av~omp~is the chemical shift of the protons of the solvent molecules which are complexed to the VO 2÷ entity,fcomp, is the fraction of the total solvent molecules complexed, and AVobs.is the chemical shift of the solution protons, i.e., shifted from the pure solvent protons. When paramagnetic ions are placed in a solution of ligands which can co-ordinate to the ions, two types of contact shifts, the pseudo contact and the Fermi contact shift, can result[11]. The small anisotropy of the g factor for VO 2+ compounds makes it unlikely that the pseudo contact shift will 8. 9. 10. 11. 12.
L. G. Vanquickenborne and S. P, McGlynn, To be published. M. B. Palma-Vittorelli, M. U. Palma, D. Palumbo, and F. Sgarlata, Nuovo Cim. 3, 718 ( 1956). W. B. Lewis, lnorg. Chem. 6, 1737 (1967). R.S. Drago, Physical Methods in Inorganic Chemistry. Reinhold, New York (1965). R. E. Benson and W. D. Phillips, inAdvances in Magnetic Spectroscopy, Vol. 1, pp. 103, 148. Academic Press, New York (1965). 13. B. B. Wayland and W. L. Rice, lnorg. Chem. 5, 54 (1966).
Notes
2275
play alarge part in the overall contact shift. The Fermi contact shift, which occurs when some unpaired spin density is transferred from the ligand to the metal or from the metal to the ligand atoms through either the o- or zr bonds formed between the metal and ligand atoms, makes up the greatest part of the chemical shift. It is this effect that is to be analyzed in this paper. The amount of spin density transferred to or from an atom to which the resonate proton is attached is related to the electron spin-nuclear spin coupling constant, .4. The latter constant, in turn, is related to the contact shift by Equation (2): Av . . . . . . . A ~'e g/3S (S + l ) (2) v TN 3kT ' where v is the resonating frequency (here 60 or 100 Me), g is the Lande splitting factor, 13 is the Bohr magneton, S is the electron spin of the ion, Ye/TN is the gyromagnetic ratio for the electron and the nucleus, and k and T have their usual meaning. These contact shifts thus can be used to infer certain types of bonding between the paramagnetic ion and its bonded ligands. RESULTS The average proton chemical shift for the water protons of the aqueous solution of VOSO4 was found to be 8.54 ppm (513 counts/sec) downfield from the unco-ordinated water as shown in Table 1. F r o m Equation (2), (`4/h) was determined to be 3-0 × 105, and since the shift was downfield `4 must therefore be positive. The proton chemical shift for the - O H proton in the methanolic solution of VOSO4 was determined to be 10 ppm ( 1000 counts/sec) downfield from the unco-ordinated methanol (Table 1). The value for A / h was calculated as 3.45 × 105. N o shift was found for the -CH3 peak.
Table 1. Nuclear magnetic resonance contact shift data for our aqueous solutions of VOSO4-4HzO* and our methanolic solution of VOSO4.2CH3OHT
Moles VOSO4
Total moles of H20* or C H 3 O H t
Av in counts/sec:~ at 43°C or 40°Ct
(A)§ ,~
1 2 3
0.00120 0.00139 0.00181
0.275 0.274 0.277
4
8-28 × 10-5
0-0822
524 501 514 Average=513 lOOffI
---3.0 × 105 3"45 × 105
*These data are for runs l, 2 and 3. "tThese data are for run 4. ~Resonance frequency = 60 Mc for aqueous solutions and 100 Mc for methanolic solution. §See the note under Table 2. Iq000 _+ 25 counts/sec.
DISCUSSION Electronic spin resonance measurements[5, 14] place the unpaired electron of V O 2+ in the dxu orbital, in agreement with the BG scheme. If in-plane 7r-bonding should be occurring, it should be primarily between the dxu orbital of the vanadium and the p orbitals of proper zr symmetry on the oxygen atoms of the equatorial water molecules in VO(H20)52+. The results of Wayland and Rice [ 13] shown in Table 2 demonstrate the existence of r - b o n d i n g between coordinated water molecules and the metal ions Co s÷, F e 2÷, Mn 2+, and Fe z+. A n M O energy level scheme of the water molecule[15] (increasing energy: al < b2 < al < bl < b~* < a~*) reveals that both bl and bz orbitals have the 14. K. D e Armond, B. B. Garrett and H. S. Gutowsky, J. chem. Phys. 42, 1019 (1965). 15. D. G. Carrol, A. T. Armstrong and S. P. McGlynn, J. chem. Phys. 44, 1865 (1966).
2276
Notes
appropriate symmetry for zr-bonding between a metal ion and the water molecules. Indeed, the paramagnetic shifts observed by Wayland and Rice are interpreted using both bl and b2 water MO's bonded to the t~uorbitals of the metal ion. It has been shown by Radford [16] that spin density transfer through the b~ orbital would cause a shift upfield whereas spin density transfer through the b2 orbital would cause a shift to be observed downfield. Since the observed shift for the metal ions was downfield, Wayland and Rice concluded that although the b~ orbital must play the major role in bonding, the b~ orbital, which is more efficient in transmitting spin density, is responsible for the observed shift[17]. In any case, delocalization of unpaired ~r-electron density into the water molecules is present. The observed shift for the VO 2÷ ion in water is found to be downfield, but of a much smaller magnitude than those shifts observed by Wayland and Rice for transition metal ions with more d (i.e., tzy) electrons. However, it can be concluded on the basis of the foregoing results and discussion that there is indeed experimental proton N M R support for ~r-bonding between the VO 2+ and water molecules. The observed shift cannot be interpreted as arising from the tumbling of the VO ~+ ions in solution because a dipolar shift of this type would result in an upfield shift, whereas the observed shift is actually downfield. Therefore, as in the case demonstrated by Wayland and Rice, the shift is most likely due to bonding between the b2 MO of VO 2+ and the bl and b2 MO's of H20. The coupling constant, (A/h), was observed to 3-0 x 105 whereas the same for the other transition metal ions were about 14 × 105. The observed shift cannot be due to o- transfer alone since the magnitude of this shift would result in an (A/h) value of about 1.5 × 105. (See Table 2 results for Cu 2+ and Ni 2+) . Furthermore, the unpaired electron is unambiguously placed in VO 2+ in a primarily dxu orbital anyway, which is not suitable for tr bonding. Table 2. Nuclear magnetic resonance contact shift data of paramagnetic ions in aqueous solution [ 13]*
Cu 2÷ Ni Co 2. Fe 2÷ Mn 2+ Fe 3÷
1/2 1 3/2 2 5/2 5/2
1-5 1.1 4.2 5-0 5.9 7.7
0 0 1/2 1 3/2 3/2
--18.7 14.0 12.7 16.5
* (A/h) is the coupling constant resulting from the total shift; (A/h),, (or the 7r-only contribution) was determined by "stripping out" the o- contribution, which can be taken from the Ni 2. value for (A/h), from the (A/h) values of Co 2÷, Fe 2+ , Mn2+ and Fe 3+. Other recent 170 NMR work supports the conclusion of ~r-bonding presented here. Wiithrich and Connick [18] examined the 170 NMR spectrum of VO z÷ in water and found that the (A/h) value was 3.8 × 106 cps. Furthermore, they found that this value was small compared to the analogous values found earlier [ 19] for Mn 2+, Fe 2÷, Co 2÷ and Ni 2÷ also using 170 N M R data. They therefore also suggest the existence of zr-bonding from their data. 16. H. E. Radford, Phys. Rev. 126, 1035 (1962). 17. The reason that the b2 orbital is more efficient for transferring spin density is because it is made up of both oxygen 2p and hydrogen Is orbitals and thus any spin density transferred through this orbital goes directly into a hydrogen orbital. The bl orbital, however, is less efficient because hyperconjugation (or spin polarization) must be involved (a much less efficient process) to transfer spin density to a hydrogen orbital. 18. K. Wiithrich and R. E. Connick, lnorg. Chem. 6, 683 (1967). 19. T.J. Swift and R. E. Connick, J. chem. Phys. 37, 307 (1962).
Notes
2277
Hausser and Laukien[20] measured the temperature dependence of the relaxation times T1 and T~ of VO 2+ in aqueous solution by ESR. If these data are used to calculate the A/h value, one obtains •4/h = 1-65 × l0 s. This value is much larger than the value we derived directly from the chemical shift method but, again, the evidence strongly favors the presence of ~--bonding in the VO2+-H20 complex. Reuben and Fiat [21 ] also observed similar evidence for 7r-bonding through the 1tO spectrum of an aqueous solution of VOSO4. Finally, Lewis[22] obtained an ESR spectrum of VOSO4'4H20 doped in zinc Tutton salt. He observed a hyperfine splitting which he interprets as arising from electron spin-proton spin coupling. However, these results are somewhat in doubt due to the large spin coupling constant (,4 = 2.6 gauss) found by him. In an effort to add new evidence to that cited above, we obtained the N M R spectrum of VOSO4.2CH3OH in methanol. It was assumed in this case that the MO's on the methanol oxygen atom would bond to VO 2+ in a way similar to those of the water oxygen atom. Thus, observing the methanol - O H proton peak, one should expect it to behave similarly to the water proton peak in aqueous VOSO4. Indeed the water proton and the methanol - O H proton do have similar coupling constants (See Table 1) and thus it appears that 7r-bonding from methanol to VO 2 is also occurring.
A c k n o w l e d g e m e n t - T h i s research was supported by the National Science Foundation Grant No. GP-4938, for which we express our appreciation. Coates Chemical Laboratories Louisiana State University Baton Rouge, Louisiana 70803
G. V I G E E J. SELBIN
20. Hausser and Laukien, Z. Physik. 153, 294 (1959). 21. J. Reuben and D. Fiat, lnorg. Chem. 6, 579 (1967). 22. W. B. Lewis, lnorg. Chem. 6, 1737 (1967).
J. inorg, nucl. Chem., 1968, Vol. 30, pp. 2277 to 2279.
Pergamon Press.
Printed in Great Britain
Hydrofluorination of curium dioxide* (Received 11 September 1967) INTRODUCTION CualvM trifluoride has been prepared by precipitation from hydrochloric acid solution by the addition of an excess of hydrofluoric acid[l], yielding a hydrated fluoride. The trifluoride has also been prepared by an NH4F-HF fusion of the oxide and of the hydrated trifluoride[2]. No details of either method were given. The reaction of curium dioxide with hydrogen fluoride gas seemed a more direct method of preparation. This study was undertaken to determine temperature conditions necessary for a rapid, complete reaction between CmOz~,) and HF~u). This reaction has been examined at three temperatures (435°, 310% 185°C) using thermogravimetric analysis techniques. EXPERIMENTAL All work was performed in glovebox enclosures designed for work with plutonium. These enclosures were kept at a slight negative pressure to prevent the spread of this highly radioactive element. Hand contact with the sample and its immediate container was minimized to prevent un*This work was supported by the University of California, Lawrence Radiation Laboratory, Livermore, California, and by U S A E C Contract AT(29-1)-1106 with The Dow Chemical Company. 1. B. B. Cunningham and J. C. Wallman, J. inorg, nucl. Chem. 26, 271, 1964. 2. J. L. Burnett, Trans. Am. nucl. Soc. 8, 335, November, 1965.