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The X-ray photoelectron spectra of inorganic molecules—VIII[1, 2]
J. inorg, nucl. Chem., 1974, Vol. 36, pp. 1771- 1775. Pergamon Press. Printed in Great Britain.
THE X-RAY PHOTOELECTRON
SPECTRA OF INORGANIC
MOLECULES--VIII[I, 2] COMPOUNDS OF SCANDIUM(Ill), SCANDIUM(Ill) OXIDE AND COMPLEXES WITH ORGANIC OXYGEN DONOR MOLECULES A. D. HAMER, D. G. T1SLEY and R. A. WALTON* Department of Chemistry, Purdue University, West Lafayette, Indiana/47907
(First received 23 August 1973; in rerised form 11 October 1973) Abstract The X-ray photoelectron spectra of ten compounds of scandium(III) are reported. Spectra were recorded with the samples exposed to a 'zero" volt electron flux to reduce to a minimum any surface charging effects. Where appropriate scandium 2p, oxygen Is and carbon ls binding energies were measured. The scandium 2p binding energies were found to be relatively insensitive to the environment about the central scandium atoms. Oxygen Is and carbon ls binding energies are discussed in terms of the known or presumed structure of the compounds. In the case of Sc20 3 and Sc2(C204) 3 . 6HzO, "satellite" peaks were observed on the high binding energy side of the scandium 2p doublets. The possible origin of these satellites is discussed.
INTRODUCTION
EXPERIMENTAL
IN THIS paper we present the results of our studies on the X-ray photoelectron spectra of a series of derivatives of scandium(III). This work was undertaken for several reasons. First, in a recent study on manganese complexes we observed[l] broad, asymmetric manganese 2p peaks (F.W.H.M. ~ 3-0-3.5 eV), spectral features which can be attributed to the occurrence of multiplet splittings[3, 4] due to the coupling of the hole in the metal 2p core with the unfilled manganese 3d valence shell. Multiplet splittings will not be observed with 3d ° s c a n d i u m ( l i l t species, and we thus have the ideal opportunity to observe well-defined binding energies which should be representative of those to be expected for diamagnetic species of the first transition series. Furthermore, under these conditions, we hoped to locate satellite peaks in the X-ray photoelectron spectra and relate this data to that already in the literature for other metal ions of the first transition series. Second, this technique is of potential use in affording structural information on d ° and other diamagnetic species when more traditional methods (via ligand-field theory) are not appropriate. Third, since the compounds studied in the present work were expected to have properties representative of nonconductors in the solid-state with the associated problems of electrostatic charging, we have investigated the effect upon the measured binding energies of bathing the samples in fluxes of "zero" energy electrons.
Scandiumllll) oxide (99.9 per cent purity) was supplied by Research Organic/Inorganic Chemical Corporation and was used as received. Samples of the acetylacetonate complex Sc(acac)3 and the tropolonato[5] complexes ScT3, HScT~ and CsScT4, where T represents the tropolonato ion (CTHsOf-), were kindly provided by Professor Gordon A. Melson of Michigan State University. The oxalato complexes Sc2(C204) 3 . 6H20 and NaSc(C20~) 2 .4H20 were prepared by the method outlined in[6]. Treatment of NaSc(CaO4) 2 . 4H20 with 1 : 10-phenanthroline in aqueous solution afforded a white precipitate of Sc2(C20,~)3 . 2phen [61 (Found: C, 49.8: H, 2.2°i.: Calcd for CIsH806N4Sc: C, 50.5; H, 2.7",,). The identity of this derivative as an anhydrous mixed oxalato phenanthroline complex was confirmed by i.r. spectral measurements {4000 600 cm ~1. Two new complexes of pyridine-2,6-dicarboxylic acid (dipicolinic acid), Sc(dipic)(dipicHl. 7H20 and NaSc (dipic)2.4H20, where dipic and dipicH represent the dianionic and monoanionic forms of pyridine-2.6-dicarboxylic acid respectively[7], were prepared during the present investigation. Sc(dipic) (dipicH). 7H20 ScCI 3 . 6H20 (0.2 g) and dipicH 2 (0.3 gl were dissolved in 120 ml of water, the clear solution evaporated to - 3 0 ml on a hot-plate and then cooled to room temperature. Crystals of the complex were filtered off, washed with watcr and acetone and dried. (Found: C, 34.3; H, 4.2; N, 5.6: So, 9.0. Calcd for C14H21015N2Sc: C, 33"5: H. 4.2: N, 5.6: Sc. 9.0"~;I. NaSc{dipic)2.4H_,O.
*Address correspondence to this author.
Passage of an aqueous solution of the above complex 1771
1772
A. D. HAMER, D. G. TISLEYand R. A. WALTON Table 1. Binding energies for complexes of scandium(III) *
1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
Compound-~
Sc2pl/2
Sc2p3/2
O ls
N ls
Sc203 ' 5c2(C204) 3 . 6H20 NaSc(C204)2 . 4H20 5 c 2 ( C 2 0 4 ) 3 . 2phen
405.5 406.5 406.4 406.2 405.6 405.6 405.3 405.3 406.7 406.2
41)1.1 402.1 401.7 401-6 401-1 401.0 400.7 400.8 402.2 401.6
529.2 530.9 530.7 530.7 530.4 530.3 530.0 529.9 531.0 530.4
---
Sc(acac)a ScTa HScT4 CsScT4 Sc(dipic) (dipicH). 7H20 NaSc(dipic)2.4H20
398.3 ---398.8 398.4
C ls -287.6 287-4 287.3 ;284.1 285.7 ;283.7 285.5 ;283.9:~ 285.0 ;283-6~ 285.9 ;284.2:~ -287.5 ;284.5
Na ls --1070.6 ----1070.9
* Binding energies are quoted relative to a value of 284.0 eV for the carbon ls binding energy of graphite. t Ligand abbreviations are as follows: phen = 1:10-phenanthroline; acac = acetylacetonato; T = tropolonato; dipic = dianion of pyridine-2,6-dicarboxylic acid; dipicH = monoanion of pyridine-2,6dicarboxylic acid. :~ Resolution of the carbon ls energy distribution curve into two components was accomplished using a DuPont 310 Curve Resolver. down a sodium cation exchange column afforded a solution, which when evaporated to dryness, yielded white crystalline NaSc(dipic) 2 . 4H20. (Found: C, 36.5; H, 3.0: N, 5.4. Calcd for C14H14012N2NaSc: C, 35.7; H, 3.0; N, 5.9.) The i.r. spectrum (4000-600 cm- 1) of Sc(dipic) (dipicH). 7H20 closely resembled the corresponding spectra which we had previously observed for the lanthanide(lll) complexes Ln(dipic) (dipicH). 6H20[7].
Spectral measurements X-ray photoelectron spectra were recorded using a Hewlett-Packard Model 5950A ESCA spectrometer. Monochromatic aluminium K~I, 2 radiation (1486-6 eV) was used as the X-ray excitation source and the powdered samples were dispersed on a gold-plated copper surface. Under these conditions, the carbon ls binding energy of graphite was close to 284-0 eV. As before[8,9], we used this binding energy to monitor the performance of the instrument. Over a period of several months during which we recorded the spectra of the scandium compounds, this binding energy had a F.W.H.M. value of 0.85 ___0.15 eV. The spectrometer could be operated with the sample bathed in a flux of "zero" volt electrons, a procedure which has been found to eliminate, or at least reduce to a minimum, surface charging effects. With the "Floodgun" * in operation, the sample surface receives ~ 10-lo A of "zero" volt electrons. In practice, the mean energy of these electrons was adjusted such that the electron binding energies of a conductor (gold), which had been deposited on an insulator, had the same values as those of the free metal. When necessary, a DuPont 310 Curve Resolver was used for peak deconvolutions using a Gaussian shape fit.
RESULTS
Preliminary binding energy measurements which we carried out on scandium(Ill) oxide, scandium(Ill) oxalate S c 2 ( C 2 0 4 ) 3 . 6 H 2 0 and the tropolonate com*Supplied by the Hewlett Packard Company
plex CsScT4, showed that surface charging effects were sufficiently severe that the scandium 2p and oxygen Is binding energies were shifted by several electron-volts to higher energies and were rather broad and ill-defined. Accordingly, we chose to measure the binding energy spectra of the scandium c o m p o u n d s described in the present report, under such conditions that the samples were bathed in a flux of " z e r o " volt electrons (see Experimental Section). This served to eliminate or reduce to a m i n i m u m these charging effects and was found to be preferable to the procedure we have previously used[l, 2, 8, 9] of diluting the samples with graphite, for the following reasons. First, the electron " F l o o d g u n " affords a method of uniformly neutralizing the surface charge, whereas mixing the sample with graphite may sometimes not entirely eliminate "islands of charge" within the mixture. Indeed, we have observed in a separate study[10] that sodium, potassium and caesium per-rhenates undergo severe charging under our experimental conditions and that this charging is affected very little by graphite dilution. Second, diluting with any material clearly reduces the content of species under study and results in a decrease in intensity of the binding energy peaks. Use of the " F l o o d g u n " does not result in this undesirable feature. Scandium 2pl/2, 3/2, oxygen ls, nitrogen ls and carbon ls binding energies of these scandium complexes are given in Table 1. As described above, these data refer to measurements made with the sample bathed in a flux of " z e r o " volt electrons. In addition, we found that for certain of the complexes (i.e. Sc(acac)3, ScT 3, HScT4, and N a S c ( d i p i c ) 2 . 4 H 2 0 ) measurement of the spectra without this ~lectron flux gave binding energies which were shifted by less than an electron volt from the values listed in Table 1. Clearly for these particular complexes, surface charging is much less than that we encountered for 8 c 2 0 3 , 5 c 2 ( C 2 0 4 . ) 3 , 6 H 2 0 and CsScT,~. The binding energies were usually located with a
X-ray photoelectron spectra of inorganic molecules--VIII precision of + 0.1 eV and are considered accurate to + 0.2 eV relative to a carbon ls binding energy of graphite of 284.0 eV. Full width at half maximum (F.WHM.t values for the binding energies in Table 1 are as follows: Sc 2p3/2 1.3 + 0.1 eV;O ls 1.4 + 0.2 eV; N ls 1.2 4- 0-1 eV; C ls 1.5 +_ 0.3 eV. The broadest oxygen ls binding energies were those for the hydrates Sc2(C204) 3 . 6H20, NaSc(C204)2 . 4H20, Sc(dipic) (dipicH).7HzO and NaSc(dipic)z.4H20, a feature which is most likely due to the overlap of the binding energies of the oxygen atoms associated with the organic ligand molecules and water. A check on the reliability of the binding energy data in Table 1 is provided by a comparison with certain data already available in the literature. The oxygen Is binding energy of the vanadium oxide V103 (529.6 eV)[11], recorded on a similar instrument to that used herein, is very close to the value observed in the present work (Table 1) for Sc203 (529.2 eV). Also, for the acetylacetonate complex V(acac)3, the oxygen ls binding energy (measured on an AEI ES100 instrument relative to a platinum 4)'7/2 value for platinum foil of 71.0 eV)[12] is 529.6 eV, fairly similar to the value of 530.4 eV observed for the oxygen ls binding energy of Sc(acac)3. This latter binding energy also agrees well with a value of 530.8 eV (corrected for charging) for Mniacac)3[1], measured relative to a carbon ls value for graphite of 284.0 eV. Finally, our measured scandium 2p binding energies can be compared with the values of 402 (2P3/2) and 407 (2Pl/2) eV which are given in the compilation of electron binding energies in Siegbahn's text El3], and are taken to be characteristic of scandium 2p binding energies for this oxidation state. Since our work described in this report was completed we have become aware of the detailed study by JCrgensen and Berthou[14] on the X-ray photoelectron spectra of over 600 inorganic compounds. * Included in their report are the scandium 2pu2,3/2 binding energies of Sc2(SOj3, (NH4)3ScF6 and Sc20 3. For SczO 3, the scandium 2pu 2 and 2P3/2 binding energy values (relative to a value of 290 eV for carbon ls of scotch tape) which are quoted, are significantly greater than the values we give in Table 1. However, the value given by JC~rgensen and Berthou[14] for the spin-orbit separation A(Sc2p112 Sc2p3/2) is identical with the value we observe and since we find that other binding energies they have tabulated are greater than the values ourselves and others have found in separate independent studies (e.g. for the rhenium 417/2 binding energy of K2ReCI6)[15 t71, these disparities are presumably attributable to differences in sampling and calibration techniques. To illustrate the dramatic effect of the "zero" volt electron flux upon the measured binding energies of a charged insulator such as Sc203, the scandium 2p
*We are grateful to Professor C. K. J0rgensen for drawing our attention to this reference,
1773
E
L C~ L_
b
J
J
420
415
~
i
I
405
400
eV Fig. 1. Scandium 2p binding energy spectrum of Sc20 s la} with and (b) without exposure of the sample to a "'zero" volt electron flux.
binding energy spectrum of this oxide is shown in Fig. 1. Spectrum (b) is that of a fairly thick layer of this oxide pressed onto a gold-plated copper surface without exposure to the electron flux. Spectrum (a) is that for this same sample measured under identical conditions (count time = 1015 sec)except that the sample is now exposed to the "zero" volt electron flux. Not only are the binding energies under the latter conditions close to those to be expected for this oxidation state[13], but the peaks are much narrower; the F.W.H.M values for the scandium 2p3/2 peaks in spectra (b) and (a) are 2.4 and 1-2 eV respectively. This is a clear indication that charging has now been reduced to a minimum.
'S
x5
H
I
420
I
I
410 eV
I
I
I
400
Fig. 2. Scandium 2p binding energy spectrum of Sc203 showing "satellite" peaks.
1774
A. D. HA~IER, D . G . T1 I. b. .L.E. .¥. . .a l l L~I R. A. WALTON
With the improvement in spectral quality which resulted from the use of the electron "Floodgun", we were able to observe "satellite" peaks on the high binding energy side of the scandium 2p doublets of Sc203 and Sc2(C20,~.)3.6H20. These satellites were fairly sharp and well defined in the spectrum of Sc203 (Fig. 2), being at ~11.3 and ~15.0 eV above the scandium 2p3/2 peak. However, in the corresponding spectrum of Sc2(C204)3 • 6H20, a single broad satellite peak ,was observed at ~ 11.5 eV above the scandium 2p3/2 peak; a marked asymmetry on the high binding energy side of this satellite indicates the presence of a further unresolved component at ~ 15 eV. Our failure to observe satellites accompanying the scandium 2p binding energies of the other compounds listed in Table 1 reflects the much lower scandium content of these species since in the spectra of the oxide and oxalate phases the satellites are only about one-tenth as intense as the main scandium 2p3/2 component. In the case of Sc(acac)3, data acquisition was restricted by the slow volatility of the complex in the spectrometer. Consequently, measurement of the scandium 2p binding energies was not carried out for a sufficiently long period for any satellites to acquire the desired intensity.
DISCUSSION
The measured scandium 2p3/2 binding energies of the ten compounds in Table 1 occur in the range 400-7-402.2 eV, characteristic of scandium(III)[13]. These values do not show any clear dependence upon coordination number, for example, eight in Sc2(C204)3 • 6H20[18] and six in Sc(acac)3[19], and a measurement of scandium 2p binding energies is therefore generally unlikely to provide information on the molecular structures of scandium(III) complexes. Although the Sc-O bond lengths are expected to increase as the coordination number increases from six, as in Sc(acac)3 (r(Sc-O) = 2.06-2.08,~,)[19], to eight in Sc2(C204)3. 6H20 (r(Sc O) = 2.18 2.26 A)[18], and this might be expected to lead to an increase in the scandium 2,o binding energies, this effect is presumably compensated for by the increase in the number of metal-ligand bonds. The net result is that the charge at scandium is not very dependent upon the stereochemistry within this range of coordination complexes. In previous papers[l, 2, 20] we have used differences between the metal (Mn 2p or Re 4./) and oxygen ls binding energies of metal complexes containing oxygen ligand molecules as a probe to the nature of the metaloxygen bonds and a description of the formal oxidation state at the metal center. With the exception of Sc203, the energy differences ,~(Ols, Sc2p3/2) for the compounds listed in Table 1, fall in the range 129.1 _+ 0.3 eV. The narrow range over which this energy difference occurs for these derivatives of scandium(III) is gratifying, and is support for our contention[l, 2, 20] that in certain systems it may be used to distinguish different
metal oxidation states. The smaller value of 6(Ols, Sc2p3/2 ) observed for Sc203 (128.1 eV) is consistent with the expected increase in ionic character of the Sc-O bonds in such an oxide phase relative to the other compounds in Table 1, since as the bond polarity increases in the sense Sc~+-O ~-, so the scandium 2p binding energies should increase while the corresponding oxygen ls energies decrease. The consistently narrow widths of the scandium 2p peaks is in keeping with the idea that within the transition series broadening effects arise in paramagnetic systems principally from multiplet splittings[3,4]. Unfortunately, for much of the available published data on compounds of the first transition series, peak widths of the metal 2p binding energies have not been documented, but it is clear that the scandium 2p peaks we have measured are among the narrowest yet reported for the series Sc-Cu. One of the features of current interest in the X-ray photoelectron spectra of inorganic compounds is the appearance of satellites on the high binding energy side of core levels. For the first transition series such satellites, within 12 eV of the metal 2,o binding energies, have usually been attributed to 3d---, 4s transitions accompanying the primary photoionization[21,22]. Support for this assignment comes from the observation that apparently neither d 1° systems, such as ZnFz and CuCI[21, 22] nor d ° systems, such as TiO 2 and V205 [21], exhibit satellites in this same energy range. It was also indicated in the paper by Rosencwaig et aI.E211, that scandium(III) has no such satellites, although it was not made clear whether a specific scandium(III) compound was in fact studied. In the present work, we find that satellites are located in this energy region for Sc203 and Sc2(C204)3.6H20 (see Results Section and Fig. 2), so that a partially occupied 3d shell need not necessarily be a prerequisite for satellite formation. Our results suggest that an investigation of other d° systems would be desirable, and in particular, a careful reexamination should be made for satellite peaks in the spectra of TiO2 and V20 s. For d° systems, these satellites cannot arise from 3d ~ 4s transitions, so that an alternative assignment is clearly required. Since 3p --, 4s excitation is not feasible[21], the most attractive possibility is in terms ofa monopole charge transfer transition (ligand---, metal 3d), an assignment which has been proposed by Kim[23], as an alternative to 3d --, 4s excitation, for the 5 10 eV satellites in the 2p electron spectra of the 3d transition metal ions. The carbon ls and oxygen ls binding energies listed in Table 1 for complexes (2) (10), may be interpreted in terms of the known or anticipated structures of these species. A comparison of the carbon ls energies of S c 2 ( C 2 0 4 ) 3 . 6H20, NaSc(C204)2.4H20 and Sc 2 (C204) a . 2phen confirms that the 287-3 and 284.1 eV peaks of the latter complex, having an intensity ratio of 1:4, are assigned to the carbon atoms of oxalate and 1 : 10-phenanthroline, respectively. Although the structure of Sc2(C204) 3 . 6H20 is known to involve eightcoordinate scandium[18], we are not able in the case
X-ray photoelectron spectra of inorganic molecules of 8 c 2 ( C 2 0 4 ) 3 . 2phen to distinguish between a related polymeric eight-coordinate species (i.e. six carboxylate oxygens and two phenanthroline nitrogens) and the sixcoordinate dimeric structure p-oxalatobis (oxalato)bis (1 : 10-phenanthroline)discandium(III). The carbon ls binding energy spectrum of Sc(acac) 3 showed two peaks at 285.7 and 283.7 eV, the one at higher energy being assigned to the ketonic carbon atoms and the broader asymmetric peak at 283.7 eV 1o a composite of the methyl and methine carbon atoms. The 283.7 eV peak could in fact be readily deconvoluted into two components, with maxima at 284-1 and 283.5 eV, in the intensity ratio of 1:2. Thus from our results, the order of binding e n e r g i e s ~ C - O >~2: C H > - C H 3 which is anticipated on the basis of electronegativity differences, is in fact observed. Introduction of substituents into a fi-diketonate moiety may of course profoundly affect the C ls binding energy order, as shown by measurements on C r ( C F 3 C O C H C O CF3}31241, where the order is - - C F 3 > [ ~ C ' - ' O > ,,~- C H. The complexes of tropolone and pyridine-2, 6-dicarboxylic acid (6) (10) also show two-component carbon lx binding energy spectra. For the three complexes of the former ligand, the peaks at ~285.5 and ~ 284 eV, are in the intensity ratio 1:2.5 (+0.3), close to the expected ratio of 1 : 2.5 for this seven-membered carbon ring system. The carbon ls spectra of Sc(dipic) (dipicH). 7 H 2 0 and NaSc(dipic) 2. 4 H 2 0 are typical of those expected for pyridine-carboxylate complexes[7]. Of particular interest are the oxygen ls binding energy spectra of CsScT4 and HScT4, which show little difference either in the observed binding energies or peak widths (F.W.H.M. = 1.1 1.5 eV). This would be consistent with the existence of discrete eight-coordinate ScT,~ anions in both salts or with HScT,~ being eight-coordinate [ScT3(TH)] and possessing one neutral tropolone and three tropolonate ions bound in a bidentate fashion to scandium. It has already been shown that the related sodium salt NaScT,~ is isom o r p h o u s with its eight-coordinate rare earth ana1ogs[5,25]. There is no evidence for HScT 4 being formulated as six-coordinate ScT 3 with an extra molecule of uncomplexed tropolone present in the crystal lattice: such a possibility would have been supported by the observation of more than one oxygen Is binding energy in the spectrum of HScT~. Acknowledgements This work was supported by the National Science Foundation (Grant No. GP-19422 and MRL Program GH-33574) to whom we are most grateful. We also thank the National Science Foundation for providing funds for the purchase of the ESCA spectrometer.
Vlll
1775
Professor G. A. Melson very kindly provided samples of the acetylacetonato and tropolonato complexes.
REFERENCES
1. J, Sheets, D. G. Tisley and R. A. Walton, J. inorg, hue~. Chem, 35, 3541 (1973) [Part VII 1. 2. D.G. Tisley and R. A. Walton, J. Molec. Struel. 17,401 (1973). !Part VII, 3. R. L. Cohen, G. K. Wertheim, A. Rosencwaig and H. J. Guggenheim, Phys. Rer. B5, 1037 (1972). 4. C. S. Fadley and D. A. Shirley, Phv,~. Rer. A2. 1109 (1970). 5. E. L. Muetterties and C. M. Wright, J. Am. chem. So~. 87, 4706 (1965). 6. B. N. Ivanov-Emin, R. K. Gridasova, B. E. Zaitsev, G. Val'karsel and A. 1. Ezhov, Russ. J. inm7~. Chem. 15, 347 (1970). 7. D. L. Hoof, D. G. Tisley and R. A. Wahon, J. chem. Soc. Dalton 200 (1973). 8. A. D, Hamer, D. G. Tisley and R. A. Walton, J. chem. Soc. Dalton 116 (1973). 9. D, P. Murtha and R. A. Walton, hmrg. Chem, 12, 368 (1973). 10. D. G. Tisley and R. A. Walton, Unpublished work. 11. J. M. Honig, L. L. Van Zandt, R. D. Board and H. E. Weaver, Phys. Rev. B6, 1323 (1972). 12. R. Larsson, B. Folkesson and G. Sch6n, ('hemica Seripta 3, 88 (1973). 13. K. Siegbahn, C. Nordling, A. Fahhnan, R. Nordberg, K. Hamrin, J. Hedman, G. Johanssom Y. Bergmark, S. E. Karlsson, I. Lindgren and B. Lindberg, ESCA: Atonlie Molecular and Solid Slate Structures Studied hi' Means o f Electron Speetroseopy, p. 225. Almquist and Wiksells, Uppsala (1967). 14. C. K. Jorgensen and H. Berthou, Mac. /q's. Me&h" Danske Vidensk. Selskab 38, N o. 15 (1972). 15. D, G. Tisley and R. A, Walton, J. diem. So~. (Dalton) 1039 (1973). 16. G. J, Leigh and W. Bremser, J. chem. Soe. (Dalton) 1216 (1972). 17. L. E. Cox and D. M, Hercules, J. Electron. Spectro~<. I, 193(197273). 18. E. Hansson, Acta ehem, seand. 26, 1337 (1972). 19. T. J. Anderson, M. A. Neuman and G. A. Melson, hTorg. Che~n. 12, 927 (1973). 20. D. L. Hoof, D. G. Tisley and R. A. Wahon, hmrg. m,/. Chem. Lett. 9, 571 (1973). 21. A. Rosencwaig, G. K. Wertheim and H. J. Guggenheim, Phys. Rer. Left. 27, 479 (1971). 22. D. G. Frost, A. lshitani and C. A. McDowell, hlolee. Phv.s. 24, 861 (1972). 23. K. S. Kim, Phys, Rer. To be published. 24. D. T. Clark and D. B. Adams, Chem. PIzvs. Lett. 10, 121 (1971). 25. G. A. Melson and R. W. Stotz, ('oord. chem. Rec. 7, 133 (1971).
THE X-RAY PHOTOELECTRON
SPECTRA OF INORGANIC
MOLECULES--VIII[I, 2] COMPOUNDS OF SCANDIUM(Ill), SCANDIUM(Ill) OXIDE AND COMPLEXES WITH ORGANIC OXYGEN DONOR MOLECULES A. D. HAMER, D. G. T1SLEY and R. A. WALTON* Department of Chemistry, Purdue University, West Lafayette, Indiana/47907
(First received 23 August 1973; in rerised form 11 October 1973) Abstract The X-ray photoelectron spectra of ten compounds of scandium(III) are reported. Spectra were recorded with the samples exposed to a 'zero" volt electron flux to reduce to a minimum any surface charging effects. Where appropriate scandium 2p, oxygen Is and carbon ls binding energies were measured. The scandium 2p binding energies were found to be relatively insensitive to the environment about the central scandium atoms. Oxygen Is and carbon ls binding energies are discussed in terms of the known or presumed structure of the compounds. In the case of Sc20 3 and Sc2(C204) 3 . 6HzO, "satellite" peaks were observed on the high binding energy side of the scandium 2p doublets. The possible origin of these satellites is discussed.
INTRODUCTION
EXPERIMENTAL
IN THIS paper we present the results of our studies on the X-ray photoelectron spectra of a series of derivatives of scandium(III). This work was undertaken for several reasons. First, in a recent study on manganese complexes we observed[l] broad, asymmetric manganese 2p peaks (F.W.H.M. ~ 3-0-3.5 eV), spectral features which can be attributed to the occurrence of multiplet splittings[3, 4] due to the coupling of the hole in the metal 2p core with the unfilled manganese 3d valence shell. Multiplet splittings will not be observed with 3d ° s c a n d i u m ( l i l t species, and we thus have the ideal opportunity to observe well-defined binding energies which should be representative of those to be expected for diamagnetic species of the first transition series. Furthermore, under these conditions, we hoped to locate satellite peaks in the X-ray photoelectron spectra and relate this data to that already in the literature for other metal ions of the first transition series. Second, this technique is of potential use in affording structural information on d ° and other diamagnetic species when more traditional methods (via ligand-field theory) are not appropriate. Third, since the compounds studied in the present work were expected to have properties representative of nonconductors in the solid-state with the associated problems of electrostatic charging, we have investigated the effect upon the measured binding energies of bathing the samples in fluxes of "zero" energy electrons.
Scandiumllll) oxide (99.9 per cent purity) was supplied by Research Organic/Inorganic Chemical Corporation and was used as received. Samples of the acetylacetonate complex Sc(acac)3 and the tropolonato[5] complexes ScT3, HScT~ and CsScT4, where T represents the tropolonato ion (CTHsOf-), were kindly provided by Professor Gordon A. Melson of Michigan State University. The oxalato complexes Sc2(C204) 3 . 6H20 and NaSc(C20~) 2 .4H20 were prepared by the method outlined in[6]. Treatment of NaSc(CaO4) 2 . 4H20 with 1 : 10-phenanthroline in aqueous solution afforded a white precipitate of Sc2(C20,~)3 . 2phen [61 (Found: C, 49.8: H, 2.2°i.: Calcd for CIsH806N4Sc: C, 50.5; H, 2.7",,). The identity of this derivative as an anhydrous mixed oxalato phenanthroline complex was confirmed by i.r. spectral measurements {4000 600 cm ~1. Two new complexes of pyridine-2,6-dicarboxylic acid (dipicolinic acid), Sc(dipic)(dipicHl. 7H20 and NaSc (dipic)2.4H20, where dipic and dipicH represent the dianionic and monoanionic forms of pyridine-2.6-dicarboxylic acid respectively[7], were prepared during the present investigation. Sc(dipic) (dipicH). 7H20 ScCI 3 . 6H20 (0.2 g) and dipicH 2 (0.3 gl were dissolved in 120 ml of water, the clear solution evaporated to - 3 0 ml on a hot-plate and then cooled to room temperature. Crystals of the complex were filtered off, washed with watcr and acetone and dried. (Found: C, 34.3; H, 4.2; N, 5.6: So, 9.0. Calcd for C14H21015N2Sc: C, 33"5: H. 4.2: N, 5.6: Sc. 9.0"~;I. NaSc{dipic)2.4H_,O.
*Address correspondence to this author.
Passage of an aqueous solution of the above complex 1771
1772
A. D. HAMER, D. G. TISLEYand R. A. WALTON Table 1. Binding energies for complexes of scandium(III) *
1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
Compound-~
Sc2pl/2
Sc2p3/2
O ls
N ls
Sc203 ' 5c2(C204) 3 . 6H20 NaSc(C204)2 . 4H20 5 c 2 ( C 2 0 4 ) 3 . 2phen
405.5 406.5 406.4 406.2 405.6 405.6 405.3 405.3 406.7 406.2
41)1.1 402.1 401.7 401-6 401-1 401.0 400.7 400.8 402.2 401.6
529.2 530.9 530.7 530.7 530.4 530.3 530.0 529.9 531.0 530.4
---
Sc(acac)a ScTa HScT4 CsScT4 Sc(dipic) (dipicH). 7H20 NaSc(dipic)2.4H20
398.3 ---398.8 398.4
C ls -287.6 287-4 287.3 ;284.1 285.7 ;283.7 285.5 ;283.9:~ 285.0 ;283-6~ 285.9 ;284.2:~ -287.5 ;284.5
Na ls --1070.6 ----1070.9
* Binding energies are quoted relative to a value of 284.0 eV for the carbon ls binding energy of graphite. t Ligand abbreviations are as follows: phen = 1:10-phenanthroline; acac = acetylacetonato; T = tropolonato; dipic = dianion of pyridine-2,6-dicarboxylic acid; dipicH = monoanion of pyridine-2,6dicarboxylic acid. :~ Resolution of the carbon ls energy distribution curve into two components was accomplished using a DuPont 310 Curve Resolver. down a sodium cation exchange column afforded a solution, which when evaporated to dryness, yielded white crystalline NaSc(dipic) 2 . 4H20. (Found: C, 36.5; H, 3.0: N, 5.4. Calcd for C14H14012N2NaSc: C, 35.7; H, 3.0; N, 5.9.) The i.r. spectrum (4000-600 cm- 1) of Sc(dipic) (dipicH). 7H20 closely resembled the corresponding spectra which we had previously observed for the lanthanide(lll) complexes Ln(dipic) (dipicH). 6H20[7].
Spectral measurements X-ray photoelectron spectra were recorded using a Hewlett-Packard Model 5950A ESCA spectrometer. Monochromatic aluminium K~I, 2 radiation (1486-6 eV) was used as the X-ray excitation source and the powdered samples were dispersed on a gold-plated copper surface. Under these conditions, the carbon ls binding energy of graphite was close to 284-0 eV. As before[8,9], we used this binding energy to monitor the performance of the instrument. Over a period of several months during which we recorded the spectra of the scandium compounds, this binding energy had a F.W.H.M. value of 0.85 ___0.15 eV. The spectrometer could be operated with the sample bathed in a flux of "zero" volt electrons, a procedure which has been found to eliminate, or at least reduce to a minimum, surface charging effects. With the "Floodgun" * in operation, the sample surface receives ~ 10-lo A of "zero" volt electrons. In practice, the mean energy of these electrons was adjusted such that the electron binding energies of a conductor (gold), which had been deposited on an insulator, had the same values as those of the free metal. When necessary, a DuPont 310 Curve Resolver was used for peak deconvolutions using a Gaussian shape fit.
RESULTS
Preliminary binding energy measurements which we carried out on scandium(Ill) oxide, scandium(Ill) oxalate S c 2 ( C 2 0 4 ) 3 . 6 H 2 0 and the tropolonate com*Supplied by the Hewlett Packard Company
plex CsScT4, showed that surface charging effects were sufficiently severe that the scandium 2p and oxygen Is binding energies were shifted by several electron-volts to higher energies and were rather broad and ill-defined. Accordingly, we chose to measure the binding energy spectra of the scandium c o m p o u n d s described in the present report, under such conditions that the samples were bathed in a flux of " z e r o " volt electrons (see Experimental Section). This served to eliminate or reduce to a m i n i m u m these charging effects and was found to be preferable to the procedure we have previously used[l, 2, 8, 9] of diluting the samples with graphite, for the following reasons. First, the electron " F l o o d g u n " affords a method of uniformly neutralizing the surface charge, whereas mixing the sample with graphite may sometimes not entirely eliminate "islands of charge" within the mixture. Indeed, we have observed in a separate study[10] that sodium, potassium and caesium per-rhenates undergo severe charging under our experimental conditions and that this charging is affected very little by graphite dilution. Second, diluting with any material clearly reduces the content of species under study and results in a decrease in intensity of the binding energy peaks. Use of the " F l o o d g u n " does not result in this undesirable feature. Scandium 2pl/2, 3/2, oxygen ls, nitrogen ls and carbon ls binding energies of these scandium complexes are given in Table 1. As described above, these data refer to measurements made with the sample bathed in a flux of " z e r o " volt electrons. In addition, we found that for certain of the complexes (i.e. Sc(acac)3, ScT 3, HScT4, and N a S c ( d i p i c ) 2 . 4 H 2 0 ) measurement of the spectra without this ~lectron flux gave binding energies which were shifted by less than an electron volt from the values listed in Table 1. Clearly for these particular complexes, surface charging is much less than that we encountered for 8 c 2 0 3 , 5 c 2 ( C 2 0 4 . ) 3 , 6 H 2 0 and CsScT,~. The binding energies were usually located with a
X-ray photoelectron spectra of inorganic molecules--VIII precision of + 0.1 eV and are considered accurate to + 0.2 eV relative to a carbon ls binding energy of graphite of 284.0 eV. Full width at half maximum (F.WHM.t values for the binding energies in Table 1 are as follows: Sc 2p3/2 1.3 + 0.1 eV;O ls 1.4 + 0.2 eV; N ls 1.2 4- 0-1 eV; C ls 1.5 +_ 0.3 eV. The broadest oxygen ls binding energies were those for the hydrates Sc2(C204) 3 . 6H20, NaSc(C204)2 . 4H20, Sc(dipic) (dipicH).7HzO and NaSc(dipic)z.4H20, a feature which is most likely due to the overlap of the binding energies of the oxygen atoms associated with the organic ligand molecules and water. A check on the reliability of the binding energy data in Table 1 is provided by a comparison with certain data already available in the literature. The oxygen Is binding energy of the vanadium oxide V103 (529.6 eV)[11], recorded on a similar instrument to that used herein, is very close to the value observed in the present work (Table 1) for Sc203 (529.2 eV). Also, for the acetylacetonate complex V(acac)3, the oxygen ls binding energy (measured on an AEI ES100 instrument relative to a platinum 4)'7/2 value for platinum foil of 71.0 eV)[12] is 529.6 eV, fairly similar to the value of 530.4 eV observed for the oxygen ls binding energy of Sc(acac)3. This latter binding energy also agrees well with a value of 530.8 eV (corrected for charging) for Mniacac)3[1], measured relative to a carbon ls value for graphite of 284.0 eV. Finally, our measured scandium 2p binding energies can be compared with the values of 402 (2P3/2) and 407 (2Pl/2) eV which are given in the compilation of electron binding energies in Siegbahn's text El3], and are taken to be characteristic of scandium 2p binding energies for this oxidation state. Since our work described in this report was completed we have become aware of the detailed study by JCrgensen and Berthou[14] on the X-ray photoelectron spectra of over 600 inorganic compounds. * Included in their report are the scandium 2pu2,3/2 binding energies of Sc2(SOj3, (NH4)3ScF6 and Sc20 3. For SczO 3, the scandium 2pu 2 and 2P3/2 binding energy values (relative to a value of 290 eV for carbon ls of scotch tape) which are quoted, are significantly greater than the values we give in Table 1. However, the value given by JC~rgensen and Berthou[14] for the spin-orbit separation A(Sc2p112 Sc2p3/2) is identical with the value we observe and since we find that other binding energies they have tabulated are greater than the values ourselves and others have found in separate independent studies (e.g. for the rhenium 417/2 binding energy of K2ReCI6)[15 t71, these disparities are presumably attributable to differences in sampling and calibration techniques. To illustrate the dramatic effect of the "zero" volt electron flux upon the measured binding energies of a charged insulator such as Sc203, the scandium 2p
*We are grateful to Professor C. K. J0rgensen for drawing our attention to this reference,
1773
E
L C~ L_
b
J
J
420
415
~
i
I
405
400
eV Fig. 1. Scandium 2p binding energy spectrum of Sc20 s la} with and (b) without exposure of the sample to a "'zero" volt electron flux.
binding energy spectrum of this oxide is shown in Fig. 1. Spectrum (b) is that of a fairly thick layer of this oxide pressed onto a gold-plated copper surface without exposure to the electron flux. Spectrum (a) is that for this same sample measured under identical conditions (count time = 1015 sec)except that the sample is now exposed to the "zero" volt electron flux. Not only are the binding energies under the latter conditions close to those to be expected for this oxidation state[13], but the peaks are much narrower; the F.W.H.M values for the scandium 2p3/2 peaks in spectra (b) and (a) are 2.4 and 1-2 eV respectively. This is a clear indication that charging has now been reduced to a minimum.
'S
x5
H
I
420
I
I
410 eV
I
I
I
400
Fig. 2. Scandium 2p binding energy spectrum of Sc203 showing "satellite" peaks.
1774
A. D. HA~IER, D . G . T1 I. b. .L.E. .¥. . .a l l L~I R. A. WALTON
With the improvement in spectral quality which resulted from the use of the electron "Floodgun", we were able to observe "satellite" peaks on the high binding energy side of the scandium 2p doublets of Sc203 and Sc2(C20,~.)3.6H20. These satellites were fairly sharp and well defined in the spectrum of Sc203 (Fig. 2), being at ~11.3 and ~15.0 eV above the scandium 2p3/2 peak. However, in the corresponding spectrum of Sc2(C204)3 • 6H20, a single broad satellite peak ,was observed at ~ 11.5 eV above the scandium 2p3/2 peak; a marked asymmetry on the high binding energy side of this satellite indicates the presence of a further unresolved component at ~ 15 eV. Our failure to observe satellites accompanying the scandium 2p binding energies of the other compounds listed in Table 1 reflects the much lower scandium content of these species since in the spectra of the oxide and oxalate phases the satellites are only about one-tenth as intense as the main scandium 2p3/2 component. In the case of Sc(acac)3, data acquisition was restricted by the slow volatility of the complex in the spectrometer. Consequently, measurement of the scandium 2p binding energies was not carried out for a sufficiently long period for any satellites to acquire the desired intensity.
DISCUSSION
The measured scandium 2p3/2 binding energies of the ten compounds in Table 1 occur in the range 400-7-402.2 eV, characteristic of scandium(III)[13]. These values do not show any clear dependence upon coordination number, for example, eight in Sc2(C204)3 • 6H20[18] and six in Sc(acac)3[19], and a measurement of scandium 2p binding energies is therefore generally unlikely to provide information on the molecular structures of scandium(III) complexes. Although the Sc-O bond lengths are expected to increase as the coordination number increases from six, as in Sc(acac)3 (r(Sc-O) = 2.06-2.08,~,)[19], to eight in Sc2(C204)3. 6H20 (r(Sc O) = 2.18 2.26 A)[18], and this might be expected to lead to an increase in the scandium 2,o binding energies, this effect is presumably compensated for by the increase in the number of metal-ligand bonds. The net result is that the charge at scandium is not very dependent upon the stereochemistry within this range of coordination complexes. In previous papers[l, 2, 20] we have used differences between the metal (Mn 2p or Re 4./) and oxygen ls binding energies of metal complexes containing oxygen ligand molecules as a probe to the nature of the metaloxygen bonds and a description of the formal oxidation state at the metal center. With the exception of Sc203, the energy differences ,~(Ols, Sc2p3/2) for the compounds listed in Table 1, fall in the range 129.1 _+ 0.3 eV. The narrow range over which this energy difference occurs for these derivatives of scandium(III) is gratifying, and is support for our contention[l, 2, 20] that in certain systems it may be used to distinguish different
metal oxidation states. The smaller value of 6(Ols, Sc2p3/2 ) observed for Sc203 (128.1 eV) is consistent with the expected increase in ionic character of the Sc-O bonds in such an oxide phase relative to the other compounds in Table 1, since as the bond polarity increases in the sense Sc~+-O ~-, so the scandium 2p binding energies should increase while the corresponding oxygen ls energies decrease. The consistently narrow widths of the scandium 2p peaks is in keeping with the idea that within the transition series broadening effects arise in paramagnetic systems principally from multiplet splittings[3,4]. Unfortunately, for much of the available published data on compounds of the first transition series, peak widths of the metal 2p binding energies have not been documented, but it is clear that the scandium 2p peaks we have measured are among the narrowest yet reported for the series Sc-Cu. One of the features of current interest in the X-ray photoelectron spectra of inorganic compounds is the appearance of satellites on the high binding energy side of core levels. For the first transition series such satellites, within 12 eV of the metal 2,o binding energies, have usually been attributed to 3d---, 4s transitions accompanying the primary photoionization[21,22]. Support for this assignment comes from the observation that apparently neither d 1° systems, such as ZnFz and CuCI[21, 22] nor d ° systems, such as TiO 2 and V205 [21], exhibit satellites in this same energy range. It was also indicated in the paper by Rosencwaig et aI.E211, that scandium(III) has no such satellites, although it was not made clear whether a specific scandium(III) compound was in fact studied. In the present work, we find that satellites are located in this energy region for Sc203 and Sc2(C204)3.6H20 (see Results Section and Fig. 2), so that a partially occupied 3d shell need not necessarily be a prerequisite for satellite formation. Our results suggest that an investigation of other d° systems would be desirable, and in particular, a careful reexamination should be made for satellite peaks in the spectra of TiO2 and V20 s. For d° systems, these satellites cannot arise from 3d ~ 4s transitions, so that an alternative assignment is clearly required. Since 3p --, 4s excitation is not feasible[21], the most attractive possibility is in terms ofa monopole charge transfer transition (ligand---, metal 3d), an assignment which has been proposed by Kim[23], as an alternative to 3d --, 4s excitation, for the 5 10 eV satellites in the 2p electron spectra of the 3d transition metal ions. The carbon ls and oxygen ls binding energies listed in Table 1 for complexes (2) (10), may be interpreted in terms of the known or anticipated structures of these species. A comparison of the carbon ls energies of S c 2 ( C 2 0 4 ) 3 . 6H20, NaSc(C204)2.4H20 and Sc 2 (C204) a . 2phen confirms that the 287-3 and 284.1 eV peaks of the latter complex, having an intensity ratio of 1:4, are assigned to the carbon atoms of oxalate and 1 : 10-phenanthroline, respectively. Although the structure of Sc2(C204) 3 . 6H20 is known to involve eightcoordinate scandium[18], we are not able in the case
X-ray photoelectron spectra of inorganic molecules of 8 c 2 ( C 2 0 4 ) 3 . 2phen to distinguish between a related polymeric eight-coordinate species (i.e. six carboxylate oxygens and two phenanthroline nitrogens) and the sixcoordinate dimeric structure p-oxalatobis (oxalato)bis (1 : 10-phenanthroline)discandium(III). The carbon ls binding energy spectrum of Sc(acac) 3 showed two peaks at 285.7 and 283.7 eV, the one at higher energy being assigned to the ketonic carbon atoms and the broader asymmetric peak at 283.7 eV 1o a composite of the methyl and methine carbon atoms. The 283.7 eV peak could in fact be readily deconvoluted into two components, with maxima at 284-1 and 283.5 eV, in the intensity ratio of 1:2. Thus from our results, the order of binding e n e r g i e s ~ C - O >~2: C H > - C H 3 which is anticipated on the basis of electronegativity differences, is in fact observed. Introduction of substituents into a fi-diketonate moiety may of course profoundly affect the C ls binding energy order, as shown by measurements on C r ( C F 3 C O C H C O CF3}31241, where the order is - - C F 3 > [ ~ C ' - ' O > ,,~- C H. The complexes of tropolone and pyridine-2, 6-dicarboxylic acid (6) (10) also show two-component carbon lx binding energy spectra. For the three complexes of the former ligand, the peaks at ~285.5 and ~ 284 eV, are in the intensity ratio 1:2.5 (+0.3), close to the expected ratio of 1 : 2.5 for this seven-membered carbon ring system. The carbon ls spectra of Sc(dipic) (dipicH). 7 H 2 0 and NaSc(dipic) 2. 4 H 2 0 are typical of those expected for pyridine-carboxylate complexes[7]. Of particular interest are the oxygen ls binding energy spectra of CsScT4 and HScT4, which show little difference either in the observed binding energies or peak widths (F.W.H.M. = 1.1 1.5 eV). This would be consistent with the existence of discrete eight-coordinate ScT,~ anions in both salts or with HScT,~ being eight-coordinate [ScT3(TH)] and possessing one neutral tropolone and three tropolonate ions bound in a bidentate fashion to scandium. It has already been shown that the related sodium salt NaScT,~ is isom o r p h o u s with its eight-coordinate rare earth ana1ogs[5,25]. There is no evidence for HScT 4 being formulated as six-coordinate ScT 3 with an extra molecule of uncomplexed tropolone present in the crystal lattice: such a possibility would have been supported by the observation of more than one oxygen Is binding energy in the spectrum of HScT~. Acknowledgements This work was supported by the National Science Foundation (Grant No. GP-19422 and MRL Program GH-33574) to whom we are most grateful. We also thank the National Science Foundation for providing funds for the purchase of the ESCA spectrometer.
Vlll
1775
Professor G. A. Melson very kindly provided samples of the acetylacetonato and tropolonato complexes.
REFERENCES
1. J, Sheets, D. G. Tisley and R. A. Walton, J. inorg, hue~. Chem, 35, 3541 (1973) [Part VII 1. 2. D.G. Tisley and R. A. Walton, J. Molec. Struel. 17,401 (1973). !Part VII, 3. R. L. Cohen, G. K. Wertheim, A. Rosencwaig and H. J. Guggenheim, Phys. Rer. B5, 1037 (1972). 4. C. S. Fadley and D. A. Shirley, Phv,~. Rer. A2. 1109 (1970). 5. E. L. Muetterties and C. M. Wright, J. Am. chem. So~. 87, 4706 (1965). 6. B. N. Ivanov-Emin, R. K. Gridasova, B. E. Zaitsev, G. Val'karsel and A. 1. Ezhov, Russ. J. inm7~. Chem. 15, 347 (1970). 7. D. L. Hoof, D. G. Tisley and R. A. Wahon, J. chem. Soc. Dalton 200 (1973). 8. A. D, Hamer, D. G. Tisley and R. A. Walton, J. chem. Soc. Dalton 116 (1973). 9. D, P. Murtha and R. A. Walton, hmrg. Chem, 12, 368 (1973). 10. D. G. Tisley and R. A. Walton, Unpublished work. 11. J. M. Honig, L. L. Van Zandt, R. D. Board and H. E. Weaver, Phys. Rev. B6, 1323 (1972). 12. R. Larsson, B. Folkesson and G. Sch6n, ('hemica Seripta 3, 88 (1973). 13. K. Siegbahn, C. Nordling, A. Fahhnan, R. Nordberg, K. Hamrin, J. Hedman, G. Johanssom Y. Bergmark, S. E. Karlsson, I. Lindgren and B. Lindberg, ESCA: Atonlie Molecular and Solid Slate Structures Studied hi' Means o f Electron Speetroseopy, p. 225. Almquist and Wiksells, Uppsala (1967). 14. C. K. Jorgensen and H. Berthou, Mac. /q's. Me&h" Danske Vidensk. Selskab 38, N o. 15 (1972). 15. D, G. Tisley and R. A, Walton, J. diem. So~. (Dalton) 1039 (1973). 16. G. J, Leigh and W. Bremser, J. chem. Soe. (Dalton) 1216 (1972). 17. L. E. Cox and D. M, Hercules, J. Electron. Spectro~<. I, 193(197273). 18. E. Hansson, Acta ehem, seand. 26, 1337 (1972). 19. T. J. Anderson, M. A. Neuman and G. A. Melson, hTorg. Che~n. 12, 927 (1973). 20. D. L. Hoof, D. G. Tisley and R. A. Wahon, hmrg. m,/. Chem. Lett. 9, 571 (1973). 21. A. Rosencwaig, G. K. Wertheim and H. J. Guggenheim, Phys. Rer. Left. 27, 479 (1971). 22. D. G. Frost, A. lshitani and C. A. McDowell, hlolee. Phv.s. 24, 861 (1972). 23. K. S. Kim, Phys, Rer. To be published. 24. D. T. Clark and D. B. Adams, Chem. PIzvs. Lett. 10, 121 (1971). 25. G. A. Melson and R. W. Stotz, ('oord. chem. Rec. 7, 133 (1971).