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Metal–ligand bonding in some vanadium compounds: a study based onX-ray emission data |
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Dalton Transactions,
Volume 1,
Issue 19,
1975,
Page 1885-1889
J. B. Jones,
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摘要:
1975 Metal-Ligand Bonding in some Vanadium Compounds A Study based on X-Ray Emission Data By J. 6. Jones and D. S. Urch,’ Chemistry Department Queen Mary College London E41 4NS Ku And Kp X-ray emission spectra of V (metal) [V(pd),] (pd = pentane-2,4-dionate) VF, VO, V,05. [ VO] [SO,], [NH,] [VO,] and [NH,] [V,O,] are reported. It is argued that peaks of higher energy than Kpl, are due to transi-tions involving molecular orbitals with V p character rather than V 3d. The relative intensities and emission energies of the Kpl,s Kptt and Kb,, peaks are discussed using m.o. theory. The K,y Kpl,‘ relative intensity is much higher than for comparable main-group compounds; this is probably due to the presence of 3p character in the orbital that gives rise to Kpt; 3p character also enhances the intensity of Kp,,,.Changes in peak positions and relative intensities associated with the oxidation state of vanadium can be qualitatively rationalised by a consideration of 4p character alone. It is suggested that the Kg”) peak observed at higher energies (+15 eV) than Kp,, is formed by relaxation of an electronically excited species (resonance radiation). X-RAY emission spectra arise from electronic transitions to vacancies in inner orbitals. Since atomic selection rules operate and in particular AZ = &l an s vacancy will attract electrons only from @ orbitals etc. Thus when valence-shell electrons are involved X-ray emission spectroscopy becomes a valuable technique for in-vestigating not only the relative energies of the molecular orbitals but also the degree of participation of specific atomic orbitals in bond f0rmation.l The bonding between second-row elements and ligands such as oxygen and fluorine has been investigated in this The purpose of this paper is to attempt to extend the method to a study of the bonding in transition-metal compounds.Vanadium was chosen because the variety of valence states enables the effect of formal charge on X-ray emission frequencies to be determined and also because of the ready availability of a wide range of compounds. The X-ray emission spectra of some vanadium compounds have already been reported. Fischer4 studied a series of oxides borides nitrides and carbides whilst Romand et aL5 made a detailed investigation of vanadium nitrides. Bonding in the vanadate anion and also related oxo-anions of manganese and chromium has been the subject of much theoretical calculation.6-10 In some cases correlations with X-ray emission (e.g.Best ti and Hillier and his co-workers 8 ) or X-ray photo-electron spectra (e.g. Prins lo) have also been attempted. The experimental results presented in this paper will, where possible be compared with theoretical pre-dictions ; in other cases simple molecular-orbital (m.0.) theory will be used to provide a semiqualitative frame-work2p3 for the discussion of the results. The spectra themselves were obtained using secondary excitation (i.e. X-ray fluorescence) since it has been shown that some chemical compounds are decomposed by direct electron bombardment.ll t 1 eV w 1.60 x 10-lB J 1 Torr = (101 325/760) Pa.D. S. Urch Quavt. Rev. 1971 25 343. * D. S. Urch J . Phys. (C) 1970 3 1276. D. S. Urch Adv. X-Ray Agalysis 1971 14 250. D. W. Fischer J . Appl. Phys. 1970 41 3561. M. Romand J . S. Soloman and W. L. Baun Spectrochim. P. E. Best J . Chem. Phys. 1966,44 3248. 1. H. Hillier and V. R . Saunders Proc. Roy. SOC. 1970 A320, Acta 1973 B28 17. 161. NOMENCLATURE A vacancy in the vanadium 1s orbital gives rise to Ku1,* (2p-+ls) Kgl,= (3p+ls) and higher-energy K emission features variously designated Kb (4$+ls) and Kpp (3d+ls) at energies of 4 949 5 426 and 5 462 eV 7 and relative intensities approximately in the ratio Ka,, K,g,, Kp,, = 3 000 400 1.12 That the high-energy peak should be designated Kp‘ corresponding to a dipole-forbidden tran-sition was first suggested by Idei l3 since the 4p orbitals would be empty in an isolated vanadium atom.However, the X-ray emission properties of isolated vanadium atoms have yet to be studied and whether from vanadium metal or from a vanadium compound the X-ray emission spectra that have been obtained have all been from materials with a complex m.0. bond structure in which 4p orbitals might play a part. Ligand-field theory l4 certainly assumes that 4p orbitals are involved in first-row transition-metal atom-ligand interactions. Whilst most authors have followed Idei’s proposal some have been more cautious; they refer t o the high-energy peak in transition-metal K emission spectra as Kp,, (e.g. ref. 6). In elements with atomic numbers greater than 31 (gallium) it is possible to observe Kp and K@ as distinct features.Invariably Kbl is very much weaker than an authentic Kb peak (intensity ratio Kp Kp ca. 1 20).12 This would seem to suggest that for elements of atomic number ca. 30 the transition moment Jt,blsP+ap dr is very much greater than Jt,blsP$3p d r (where P is the transition operator). If there is no drastic change in the ratio of these two integrals on going to lower atomic numbers (i.e. 30-22) then it is most reasonable to assume that the high-energy peaks in the K emission spectrum arise from p character in occupied m.0.s and that the intensity of peaks from the corresponding presence of 3d character in occupied m.0.s will be too weak to be observed. Since, however the point cannot be unequivocally resolved by argument the most prudent designation would seem to be KbI,&.Whatever their atomic origin however these X-ray * J . A. Connor I. H. Hillier V. R. Saunders M. H. Wood and 9 D. W. Clack J.C.S. Faraday II 1972 1672. lo R. Prins J . Chem. Phys. 1974 01 2580. l1 D. W. Fischer and W. L. Baun Norelco Reportev 1967 14, 92; Analyt. Chem. 1968 37 902. l2 E. W. White and G. G. Johnson jun. ‘ X-Ray Emission and Absorption Wavelengths and Two-theta Tables,’ 2nd edn. Amer. SOC. for Testing and Materials Philadelphia 1970 ASTM data series DS 37A. l3 S. Idei Nature 1929 123 643. l4 C. J. Ballhausen ‘ Ligand Field Theory,’ McGraw-Hill, H. Barber Mol. Phys. 1972 24 497. New York 1962 1886 emission peaks of higher energy than vanadium 1<B1,3 result from electronic transitions from valence-shell m.0.s and for this reason i t is the K B ~ ~ region which is investigated in detail in this paper.EXPERIMENTAL Apart from ammonium trivanadate [NHJ[V,O,] which was prepared by boiling [SH,][VO,] with 10% acetic acid the compounds used were purchased from Koch-Light Ltd. and used without further purification. The finely powdered samples were pressed with terephthalic acid t o form discs (40 mm diameter) which could be inserted into the spectrometer . X-Ray emission spectra were obtained using a Philips PW 1410 XRF spectrometer equipped with a tungsten-anode X-ray tube operated a t 40 mA (60 kV). The spectrometer was evacuated to ca. 10-3 Torr. In order to obtain good resolution of the characteristic X-rays emitted by a sample i t is necessary to work at a high angle of incidence (0) to the diffracting crystal [the differential of Bragg's law is (d0ldE) = -E-ltanO where E is the X-ray energy].In the experiments reported here fourth-order diffraction from an ammonium dihydrogenphosphate (ADP 101 2d = 1 062.8 pm) crystal was used which gave values of 8 in the range 57-60' for vanadium Kg X-rays. The X-rays were detected using a proportional counter (1 pm Mylar window) and amplified and counted using Harwell 2000 series electronics. In all cases the spectra were obtained as a ratemeter output on a chart-recorder trace. The slowest possible scanning speed (0.25' min-l) was used in order to obtain the best counting statistics. The peak width a t half height which can be achieved with this type of spectrometer operated as described above is limited by the quality of the diffracting crystal and by the degree of collimation brought about by the Soller slits.In t h i s work fine collimation was used i.e. 150 pm spacing between the blades of a collimator giving an angular divergence of cn. f0.1". This gave a peak width a t half 5440 5460 5480 5500 FIGURE 1 Typical spectra from the Iis region (a) [NH4]-[VO,] ; ( b ) V (metal) (- - -) estimated background height for monocliroinatic radiation of 5 450 eV and with a perfect diffracting crystal of ca. 6.5 eV which limits the resolution that can be obtained. J.C.S. Dalton RESULTS The peaks KB2,s and A'B~? etc. were found on the high-energy side of the intense h'pI, peak.-4 typical experi-mental rcsiilt is shown in Figurc 1; a. smooth curve has 5440 5460 5480 5500 5440 5460 5480 5500 been drawn through the noise on the ratemeter trace. Kp,,& And related peaks were unfortunately not completely resolved from KB,,,. I t was thcrefore necessarv to estimate the ' t a i l ' of the intense peak as a curve. This was done for all the spectra of the coinpounds discussed in this paper and the results are shown i n Figure 2. Because of the uncertainties inherent in such a procedure the relative intensities of the peaks in these spectra could not bc measured with great accuracy but even so the general features could bc observed without doubt. These are the main KBp,I peak and for most compounds low-energy 1<fifif and high-energy K,y#r satellites.The origins of these peaks and of the shifts in their energies brought about by changes in the ligands are cliscusscd in detail below. The energies of these peaks and also of Kplt3 and Ka1,* arc summarised in Table 1. DISCUSSION With the exception of vanadium metal the tri-fluoride and ammonium trioxovanadate the vanadium atom in all the compounds studied here is surrounded by six oxygen atoms. I n some cases e.g. vanadium(111) pentane-2,4-dionate,15 the arrangement probably ap-proximates to a regular octahedron in others e.g. vanadium pentaoxide,16 the arrangement is highl 1975 1887 irregular Despite these differences it is convenient to discuss the spectra of this group of compounds together to see if effects due to formal valence or distortion of the ligand environment can be discerned and distinguished.In all these examples the general appearance of the spectra is similar. The most intense peak (KB,,,) is at cn. 5 460 eV. There is a lower-energy satellite peak (Kppt) which is almost as intense as Kp2 at ca. 5 448 eV and a weak high-energy satellite at 5 479 eV ( K p e t p ) . Since the vanadium 1s ionisation energy is 5465 eV,17 K p t g r cannot result from a normal transition from an occupied bound m.0. to a 1s vacancy its origin is discussed in a later section. On the other hand there seems no reason why the two peaks K p z 5 and should not have a conventional origin ; electronic transitions I t can be seen that the ab initio calculations give a rather poor estimate of the KB2,s KB.1 intensity ratio for vanadate but that agreement improves through chromate to permanganate.It is unfortunate that the calcu-lations of Connor do not describe the particular atomic orbitals which contribute to the computed X-ray intensities neither do they estimate the relative intensity of the KB,, peak. This would have permitted the contribution made by 3p orbitals to the intensities of X-ray emissions from valence-shell orbitals to be assessed. The importance of 39 character was proposed by Best whose enigmatic calculations attempted to estimate the KBI,s Kp" intensity ratio for chromate. The result 0.07 1 can be compared with 0.02 1 experimentally observed (Table 2) ; this agreement was TABLE 1 Energies and relative intensities of vanadium X-ray emission peaks Relative intensities a 1<L3?,S m .3 V (metal) 0.0:: 1 [V(Pd),! 0.01 1 VF 0.026 1 vo2 0.032 1 [VOj [S0,]-3O1-I2 0.016 1 V,Oj 0.04 1 ;?JH,J [VO,] 0.038 1 ;NI4,] [V3O9] 0.038 1 0 . 7 2 1 0 . 6 0 1 0 . 8 2 1 0 . 8 9 1 0 . 9 0 1 0 . 9 2 1 c Ka1.2 4 952.2 4 951.9 4 952.0 4 951.8 4 951.9 4 951.8 4 951.8 4 961.8 IQ1.3 5 427.3 5 426.9 5 426.9 5 426.6 5 426.9 5 420.3 5 426.6 5 426.3 5 449.0 5 448.4 5 448.1 5 448.1 5 448.1 5 448.1 C l i P 2 . 5 5 462.0 5 460.5 5 458.9 5 460.5 5 462.0 5 462.9 6 462.0 5 465.2 5 478.1 5 476.3 5 478.7 5 478.1 5 478.7 5 478.7 5 478.7 Estimated from peak heights. pd = Pentane-2,4-dionate.Not observed (see t e s t ) . TABLE 2 Relative intensities of [vo4]3- [CrO,] 2-Expt. 1 a 0.0034" 0.0038a O.6Oa 1 % Ccntral-nictal atomic orbital in various m.0.s c lief. 8 {i; Ref. 9 [CrO,]*-Ref. 10 [MnO,:- 1 X-ray eniission h'BIs3 K p Kp,,s K 8 1 . 3 ' Kp' ' K&5 KBL3 : Expt. ref. 6 1 0.0022 0.019 1 0.0024 0.0016 1 A b initio calc. ref. 8 27 3 + 66 61 3 + 91 3t2 4t 5t +- Gt 3 4 4t2 5t + 6t2 3t2 96.7 0.0 0 -t 0.5 96.7 1.3 0 + 0.8 95.9 0.3 4.9 4.1 + 10.6 0.3 3.6 4.7 + 5.1 0.2 17.1 3.3 + 7.5 69 6 + 86 " This work. Ref. 18. The numbering of m.0.s is taken from ref. 8 ; data from other sources have been altered where neces-sary. from m.0.s with 49 character to a vacancy in a 1s orbital The existence of two peaks a t ca. 12-15 eV apart from compounds where oxygen is a ligand is very similar to that observed when main-group elements are bonded to oxygen.2 Vnnadate and Related Anions.-The vanadate ion has attracted the attention of other workers usually in comparison with the isoelectronic chromate and per-manganate ions and attempts have been made to identify the origins of the KBPIJ and K p # r peaks.Relative intensities as recorded by Best (whose resolution was better than that reported in this paper) and as calculated by Connor et using ab initio methods are given in Table 2 together with the atomic composition of relevant orbitals by Clack (CNDO) and Prins (INDO).l0 l5 €3. Morosin and H. Montgomery Acta Cryst. 1969 B25, A . Bystrom I<->\. Wilhelmi and 0. Rrotzen Acta Chem.1354. Scand. 1950 4 1119. said to be ' acceptable.' In these calculations 49 orbitals were ignored since it was felt that the Cr 4p-0 2s ionisation-energy difference (i.e.d.) would be too large for effective overlap and that the integral $i,btPt,b4p dr would be too small. However the Cr 49-0 2s i.e.d. of 20 eV is very similar to that of Cr 3p-0 2s (18.3 eV) and estimates of the transition integral involving 49 orbitals suggest that it is between 10 and 15?( of the corresponding integral with 39 orbitals. These estimates are taken from heavier elements where Kp2 and KB,, can be compared directly. It therefore seems unreasonable to exclude 49 orbitals as proposed by Best; they are included in the other calculations discussed in this paper. In the orbital approximation the relative intensities l7 J .A. Bearden and A. F. Burr Rev. Mod. Phys. 1967,39,125. A. T. Shoovaev and G. M. Koolyani Izvest. Akad. Nuuk, U.S.S.R. (Sev. F i z ) 1963 27 322 1888 J.C.S. Dalton of X-ray emission peaks should be directly related to the relative amounts of atomic orbitals present in the various m.o.s.2 Thus that part of the KB,, and KBJ~ peaks which is to be ascribed to 39 character in these orbitals can be directly estimated from Table 1. In all cases it is between 0.5 and 1.2% relative to KB,,, i.e. appreciable but insufficient to explain completely the observed relative intensities of these peaks. Perhaps of greater importance than a correct intensity estimate is the fact that for chromate and permanganate the observed Kpp KB2, ratio is the reverse of that which would be predicted from 3p character alone.If it is assumed that [#lsP$4p d.r is ca. 10% of [t,hhPt,h3p d r as suggested above it is possible to estimate the ' 4$' contribution to the relative intensity of the KB,, and KBa# peaks. For vanadate the 3p and 49 contributions are comparable for KB#f and 1 3 for KB,,~. The in-tensity relative to KBl,s of the latter peak is well estimated but low (1.1 instead of 2.2%) for KB". The experimental data presented here would seem to be in somewhat better agreement with calculations than that of Best. Fair agreement is also found for chromate and permanganate. It is interesting to note that Clack's CNDO results for chromate predict (Cr 3p was ignored as suggested by Carlson19) 4p contributions to 4t2 and 5t2 +.6t2 orbitals opposite to those calculated by the ab znztzo method and which would if 49 orbitals alone determined ITB,, and intensities also predict the wrong ratio for these peaks (i.e.K B J ~ > KBJ. The INDO calculations and the ab initio calculations for permanganate both seem in fair agreement with each other and with the experimental result of Kb,, and Krt peaks of comparable intensity with the former a little more intense than the latter. It therefore seems reasonable to conclude that both ab initio and INDO calculations are capable of calcu-lating fairly accurately relative X-ray emission in-tensities. Further it has been shown that the K emission features associated with valence-band orbitals derive their intensity not only from 49 character but also from the participation of 3p orbitals.As would intuitively be expected 3p character is greater in the more tightly bound 4t2 orbitals than in 5t2 or 6t2 orbitals. This provides a simple explanation for the most remark-able feature of these transition-metal spectra (Figure 2) when they are compared with main-group compounds (e.g. silicate or sulphate) i.e. the very high relative intensity of the lower-energy satellite peak KBrt to the main valence-band peak K,,, (comparable peaks in second-row main-group compounds 2 are Kp*t and with a typical intensity ratio of 1 5 ) . Vanadium Oxidation States.-Small changes in KB emission energies are observed (Table 1) when the oxidation state of vanadium is altered even when the first co-ordination sphere remains wholly composed of oxygen atoms.But because the resolution between the KB" and KB*, peaks is poor it is not possible to deter-mine very accurately either the energies or the relative intensities of these peaks. Even so qualitative trends, as a function of the valency of the vanadium can be discerned. The overall shift in the KB2, peak of 2.5 eV in going from V1I1 to Vv is probably greater than can be explained by the deficiencies of resolution; the corre-sponding shift of Kp-t is more difficult to determine. Estimates of relative intensities are particularly difficult because of the underlying ' tail ' from K,,, (Figure 1) and the figures given in Table 1 are probably subject to an error of a t least 5.20%.Despite these difficulties it does seem reasonable that on increasing the formal oxidation state of vanadium from 111 to v in an octa-hedral environment of oxygen atoms (i) KBp,S shifts to higher energies by ca. 2.5 eV; (ii) KB2,' KB,, relative intensity increases by a factor of ca. 2; and (iii) KB" KB,, relative intensity increases from ca. 2 3 to ca. 1 1. {It is interesting to note that comparable effects [(i) and (iii)] are observed l8 for Cr20 and [Cr0,I2- KB,, shifts to higher energies by ca. 4 eV and KB" KB,, increases from ca. 0.25 1 to ca. 0.6 1.) In the absence of more sophisticated calculations these trends can be rationalised by means of simple m.0. theory.2 In an octahedral ( O h ) arrangement of ligand atoms the valence-shell orbitals of vanadium will belong to the irreducible representations 3d (e and tzg) 4s (al,), and 49 (tlu).Ligand orbitals orientated along the V-0 bonds will transform as alg -i- tl + e,. Only K emission spectra are discussed in this paper and so only the tl, orbitals need be considered in detail. When the formal oxidation state of vanadium is increased the ionisation energies in all the atomic orbitals will be increased. Assuming the vanadium 4fi ionisation energy to be less than that of oxygen 2p the interactions between oxygen 2s and 29 orbitals and the vanadium 49 orbitals can be treated in an analogous manner to those between main-group atom p and ligand s and p orbitals. Moving to the right in the Periodic Table has the same effect as increasing the formal valence ; valence-shell j5 orbitals become more tightly bound.Thus it is to be expected that an increase in the formal oxidation state of vanadium will result in an increase in the (small) amount of 4p character in t l ( G ) m.0.s that are mostly 29 ligand in nature. This provides an explanation for the increase in relative intensity of the KB2,s peak [(ii) above]; the intensity of the KBr, peak can be used as a reference since the 39 electron population will not be affected by chemical changes. Although the amount of 4p character will increase in these m.o.s their ionisation energy will be largely determined by the excess of 29 ligand character. This ionisation energy is rather indifferent to changes in the central-atom p-orbital ionisation energy at least over a limited range.2 Thus on oxidation it is to be expected that the valence tl,) (V 49-0 2p 0 ) orbitals will have an almost unchanged ionisation energy whilst that of the vanadium 1s orbital will increase.In this way the observed increase in KP2, emission energy with oxidation of the vanadium may be understood [(z) above]. M.o. calculations were also made to determine the way in which central-atom character entered 19 K. D. Carlson a n d C. hloser J . Chem. Phys. 1966 44 3250 1975 orbitals that were mostly ligand 2p and 2s as a function of + ionisation energy. As might be expected the p contribution to the ' 2s ' orbitals increased relative to the contribution to ' 2 p ' orbitals thus providing a rationalisation of (iii) the increase in the K, Kp2,0 intensity ratio energy from VII1 to Vv.Ammonium Trioxovanadate [NH,] [VO,] .-The features of the X-ray emission spectra of this compound are very similar to those of ammonium trivanadate, [NH,][V,O,]. The bonding commitments of V 49 orbitals in four- and six-co-ordinate situations must therefore be quite similar. Certainly there would not seem to be any aspects of these spectra that would enable the different co-ordination numbers to be distinguished. Vanadium Trijuoride.-The vanadium atom is here octahedrally co-ordinated but by fluorine and not by oxygen atoms. The most dramatic effect is the dis-appearance of the Kbtr peak from the spectrum. By analogy with second-row main-group elements the I(,## peak associated with fluorine ligands is anticipated to have an energy ca.20 eV less than Kp2.*ym Un-fortunately at this energy (5440 eV) Kp2,s has con-siderable intensity which is rapidly increasing with decreasing energy. Any satellite peak which might be present cannot therefore be observed. The actual energy of Ks,, is of interest since it is the lowest observed in this suite of compounds. Since fluorine 2$ orbitals are more tightly bound that their oxygen counterparts, this is perhaps not unexpected. However fluorine is more electronegative than oxygen and so V 1s might well be more tightly bound in vanadium trifluoride than in other VTrl compounds [e.g. vanadium(II1) pentane-2,4-dionate]. This would counteract the tendency for KB,,~ to have a much lower energy in fluoro- than in oxo-figand compounds.This effect has been quanti-tatively studied for silica and the hexafluorosilicate anion.21 The increase in the Si Is ionisation energy, estimated from Si-Kp X-ray fluorescence and Si 2p X-ray photoelectron experiments was almost exactly equal to the increase in binding energy of the valence-shell orbitals so that Si-KBI,3 is almost identical for the two compounds. I t therefore seems reasonable to propose a similar explanation for the very slight increase in the KB,,s energy in going from the pentane-2,4-dionate to the fluoride. The K r ~ j Peak-This peak has a higher energy than can be accounted for by a conventional transition from an occupied orbital to an inner vacancy. Two general mechanisms suggest themselves one that this is a transition in a doubly ionised atom; and the other that 2O D.F. Lawrence and D. S. Urch Spectrochim. Acta 1970 B25, 2 1 D. S. Urch X-Rny Spectromcti~~ 1973 2 3. 305. it represents the relaxation of an electronically excited atom (atom here is used to describe vanadium in its original chemical state). Interesting features of K,.,. are that it is only present as a clearly defined peak in chemical compounds and that the I<pz-Kp-' energy difference is remarkably constant at ca. 15 eV. If this peak is due to transitions in a doubly ionised atom there would seem to be no good reason why it should not also be observed for metallic vanadium. A feature of the peak shapes of X-ray emission spectra however is that they are broad and distorted for metals but quite sharp and sym-metrical for compounds.l Peak shape reflects the band structure of the solid and absorption spectra should show the same distinction because the nature of the underlying orbital structure is similar for occupied and unoccupied m.0.s.A weak diffuse peak from an electronically excited atom in the metal might not be detected but a narrower sharp peak from a compound would be. I t therefore seems possible that the Kptr feature is due t o relaxation of a vanadium atom in which an electron has not been ejected by the incident X -radiation but rather excited to a specific antibonding m.0. which would be in the continuum. I t is hoped to investigate the absorption spectra of these compounds to provide corroboration for this idea. (Similar mechan-isms have recently been proposed to explain high-energy emission features in both sulphur hexafluoride 22 and nitrogen.23) Conclusions.-An analysis of m.0.calculations in the light of vanadium K emission spectra shows that 3p and 49 character in valence-shell m.0.s must be con-sidered for a satisfactory explanation of the relative intensities of the Kp-e and Kp,,' peaks. However, for a qualitative rationalisation of changes in the KBl, Kpep KB,, relative intensities and of changes in the Kp2,6 emission energy with the formal oxidation state of vanadium it is sufficient to consider only the variations in 4p character as described by simple m.0. theory. The participation of 4p orbitals increases from VrTT to Vv i.e. the bonds become more covalent as might be expected. The contribution made by 311 character in valence-shell m.0.s to the intensities of K emission peaks seems to be negligible it is therefore better to refer to them as Kpl than Kpz,k or KB,. We thank the Royal Society the Central Research Fund of London University and Queen Mary College for support for the purchase of the spectrometer and Standard Tele-comniunication Laboratories Ltd. and the S.R.C. for the award of a C.A.P.S. research studentship (to J. B. J.). [3/1837 Received 3vd Septembev 19731 a2 R. E. LaVilla J . Chem. Phys. 1972 57 899. 23 R . E. LaVilla J . Chem. Phys. 1972,56 2346
ISSN:1477-9226
DOI:10.1039/DT9750001885
出版商:RSC
年代:1975
数据来源: RSC
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