DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1998, Pages 2199–2204 2199 X-Ray absorption fine structure study of the bound state electronic transitions at the vanadium K and L edges in low symmetry, molecular, vanadium-(IV) and -(V) complexes with oxyoxime and oxyoximate ligands ‡ David Collison,*,a C. David Garner,*,a Julian Grigg,a Catherine M. McGrath,b J. Frederick W. Mosselmans,c Elna Pidcock,a Mark D. Roper,c Jon M. W. Seddon,b Ekk Sinn,b Peter A. Tasker,d,e GeoV Thornton,f John F.Walsh f and Nigel A. Young *,†,b a Department of Chemistry, The University of Manchester, Oxford Road, Manchester, UK M13 9PL b Department of Chemistry, The University of Hull, Hull, UK HU6 7RX c CCLRC Daresbury Laboratory, Warrington, Cheshire, UK WA4 4AD d Zeneca Specialties Research Centre, Blackley, Manchester, UK M9 3DA e Department of Chemistry, The University of Edinburgh, Kings Building, West Mains Road, Edinburgh, UK EH9 3JJ f IRC in Surface Science, Department of Chemistry, The University of Manchester, Oxford Road, Manchester, UK M13 9PL A combination of vanadium K- and L-edge XAFS has been used to characterise a series of monomeric oxovanadium(IV), monomeric dioxovanadium(V) and dimeric oxovanadium(V) complexes with oxyoxime and oxyoximate ligands.The K- and L-edge spectra confirm the presence of VV in the dimeric species and the L-edge spectra have been used to discriminate between six-co-ordinate V]N]O]V bridged oxovanadium(V) dimers and seven-co-ordinate phenolate bridged oxovanadium(V) dimers containing h2-N]O groups.The bound state electronic transitions in the X-ray absorption fine structure (XAFS) spectra of 3d transition elements at their K- and L-edges have very diVerent intensities because of the dipole selection rules.1 Thus, at the K-edge, the 1s æÆ 3d transitions are dipole-forbidden and therefore in general are only seen as weak ‘pre-edge’ features prior to the onset of the absorption edge and the subsequent extended X-ray absorption fine structure (EXAFS) which can be used to give structural information.1 In the case of vanadium the intensity of the 1s æÆ 3d transitions can be significantly enhanced depending upon: (i) the extent of 3d–4p mixing at the vanadium; (ii) the overlap of the vanadium 3d orbitals with ligand 2p orbitals (especially in compounds containing V]] O moieties); and (iii) deviations from high symmetry geometries.The position, shape and relative intensity of these pre-edge features are dependent on oxidation state and co-ordination environment, and hence they have been used to investigate vanadoenzymes and synthetic analogues of their active sites,2,3 as well as an extensive range of vanadium compounds and complexes.4,5 Although useful information has been obtained using this approach, Weidemann et al.3 have advised exercising caution against over interpretation of the data.In contrast to the K-edge, the L-edge spectra are dominated by intense, well structured transitions which arise from dipoleallowed 2p63dn æÆ 2p53dn11 transitions which have about a quarter of the natural linewidths of the corresponding K-edge features;6 whilst 2p æÆ 4s transitions are allowed, their transition probabilities are much smaller.7 The L3 (2p3/2) and L2 (2p1/2) components are split by spin–orbit coupling of the core–hole, and as there is a strong interaction between the 2p core–hole and the valence 3d orbitals, the position of, and structure on, † E-Mail: n.a.young@chem.hull.ac.uk ‡ Non-SI units employed: eV ª 1.60 × 10219 J, bar = 105 Pa.the L2,3 absorptions are very sensitive to oxidation state, spin state, ligand field and local environment, as well as being element specific.7 This can be especially powerful for d0 electronic con- figurations, and where d–d transitions are obscured by (intense) charge transfer bands in the optical spectra. The technique has found use in the study of minerals, metalloproteins and coordination compounds.8 The application of L-edge spectroscopy to solid state physics and chemistry has also been reviewed.9 Abbate et al.have shown that vanadium L-edge XAFS is suYciently sensitive to distinguish between V2O5, VO2 and V2O3,10 and that there are observable diVerences in the spectrum of VO2 above and below its first order phase transition at 67 8C.11 However, in the case of LiVO2 there was very little change in the spectra above and below the phase transition. 12 Polarisation dependent, single crystal vanadium L-edge experiments have been used to study the 2p æÆ 3d transitions in greater detail in V2O5,13–15 VO2,13 V2O3 13 and V6O13.13,14 Chen et al.16 have demonstrated a shift in the vanadium L-edge features of vanadium oxide model compounds of 0.68 ± 0.03 eV per oxidation state, and have used this to identify the oxidation state of vanadium carbide on a vanadium (110) surface. A comparison of the experimental and theoretical spectrum of VF3 has been reported,17 and vanadium L-edge XAFS has also been used for oxidation state identification and quantification in a range of vanadium-containing minerals.18,19 We have recently developed the synthesis and structural characterisation of a series of oxovanadium-(IV) and -(V) complexes containing oxyoxime and oxyoximate ligands.20–22 Fig. 1(a) shows the formulae of these proligands, which bond to vanadium either as mononegative oxyoxime units by deprotonation of the phenol function or as dinegative oxyoximate ligands when the N-hydroxy group is additionally deprotonated. Fig. 1(b)–(e) shows the various structural types which we have identified using single crystal X-ray diVraction on2200 J. Chem. Soc., Dalton Trans., 1998, Pages 2199–2204 selected complexes. The monomeric oxovanadium(IV) complexes [Fig. 1(b)] show a typical hydrogen-bonded uninegative form of the ligand in a bis complex. They are stable over a period of months in the solid state, but in chlorinated or hydrocarbon solvents the solutions rapidly darken as the complexes undergo oxidation to dimeric species.The particular structural type formed depends on the substituent in the hydroxyoxime proligand and also the starting material since some of the dimers can be formed without oxidation using [NH4][VO3] as the vanadium precursor. In these proposed higher oxidation state dimers the oxovanadium(V) centres are bound by both mononegative oxyoxime and dinegative oxyoximate ligands.The V]N]O]V bridged structure [Fig. 1(c)] has two six-co-ordinate pseudo-octahedral metal centres, whereas the phenolate-bridged structure [Fig. 1(d)] has pseudopentagonal bipyramidal seven-co-ordination at each metal, which includes the h2-N]O bonding mode of the oxyoximate ligands. Reaction of the oxovanadium(IV) monomers with nitriles (MeCN, PhCN and 4-O2NC6H4CN) led to oxidation to dioxovanadium(V) monomers [Fig. 1(e)] with the formation of a novel organic ligand.23 The assignment of (formal) metal oxidation state in the dimeric compounds can be non-trivial as the magnetic distinction between vanadium-(IV) and -(V) is no longer present if there is strong antiferromagnetic coupling between the oxovanadium( IV) centres. The identification of the dimeric species [Fig. 1(c) and (d)] as oxovanadium(V) followed from a consideration of the bond lengths in the structurally characterised compounds20,21 in conjunction with Bond Valence Sum Analysis,24 together with the observation and position of 51V NMR resonances.21 Whilst the EPR signal characteristic of monomeric vanadium(IV) complexes was absent from the Fig. 1 Representation of the structures of: (a) proligands (R1 = H, R2 = H, H2LH; R1 = H, R2 = Me, H2LMe; R1 = H, R2 = Et, H2LEt; R1 = Octt, R2 = H, H2LOctt; R1 = But, R2 = H, H2LBut; R1 = Cl, R2 = H, H2LCl; (b) five-co-ordinate monomeric bis(oxyoxime)oxovanadium( IV) complex: (c) six-co-ordinate oxyoximate-bridged oxovanadium( V) dimer containing m-N]O units; (d) seven-co-ordinate, phenolate-bridged, oxovanadium(V) dimer, containing h2-N]O units; (e) dioxovanadium(V) monomer, [VO2{C6H4(O)C(R)]] NOC(R9)]] NH}] (R = Me, R9 = Ph; R = H, R9 = Me, Ph or 4-O2NC6H4) spectra of the dimeric complexes, this could be due to the presence of vanadium(V) or the spin pairing of the single unpaired electron of two vanadium(IV) centres in the dimers.Therefore, the present study explored the use of vanadium K- and L-edge XAFS spectroscopy to probe the oxidation state of the vanadium centres in several monomeric and dimeric oxyoxime and oxyoximate complexes of vanadium-(IV) and -(V). This work also assesses the applicability of bound state transitions at these edges to the discrimination of co-ordination environment. Experimental Sample preparation Vanadium(V) oxide and [NH4][VO3] were obtained from Aldrich and used as received.The complexes [VO(acac)2],21 [VO(acac)2(py)],25 [VO(salen)],21 K3[V O(O2)2(C2O4)] 26 and [VO2{C6H4(O)C(R)]] NOC(R9)]] NH}] 23 (R = Me, R9 = Ph; R = H, R9 = Me, Ph or 4-O2NC6H4) [Fig. 1(e)] were synthesized using previously published methods.§ The synthesis and characterisation of the hydroxyoxime proligands [Fig. 1(a)] will be published elsewhere.20,21,27 The synthesis of the oxovanadium( IV) oxyoxime complexes [VO(HLR)2] (R = H, Me, Et, But or Cl) [Fig. 1(b)] from vanadyl sulfate and the appropriate proligand, and the oxovanadium(V) oxyoxime oxyoximate complexes [{VO(HLR)LR}2] [R = H, Me, Et or Octt (Octt = Me3CCH2CMe2)] [Fig. 1(c) and (d)] from either oxidation of the vanadium(IV) complex in chlorocarbon or hydrocarbon solvents or the addition of the proligand to [NH4][VO3], will be published in detail elsewhere, together with the spectroscopic and crystallographic data.20,21,27 The numbering scheme for the compounds is given in Table 1.K-edge X-ray absorption spectroscopy The vanadium K-edge X-ray absorption data were collected on station 4.2 28 at the Daresbury Laboratory Synchrotron Radiation Source (SRS) operating at 2 GeV with circulating currents of 220–240 mA. The samples were investigated as a slurry in hexane (reagent grade), evaporated onto vanadiumfree stainless steel holders and mounted in an ultra-high vacuum chamber. Total electron yield detection was used for all experiments and the spectra were calibrated using the maximum intensity position of the V2O5 1s æÆ 3d pre-edge transition at 5470.60 eV.5 The accuracy of the measurement of the absolute energy is estimated to be ±0.5 eV, and that of the relative energy to be ±0.25 eV.L-edge X-ray absorption spectroscopy The vanadium L-edge data were obtained on station 5U.1 29 of the undulator beamline at the SRS operating at 2 GeV with circulating currents of 150–250 mA. The general features of the experimental set-up employed have been described in detail elsewhere.8 The samples were prepared either as slurries of graphite (99.999%; Goodfellows, Cambridge) with CH2Cl2 or EtOH and left to evaporate on aluminium plates or as powders on conductive epoxy resin on aluminium plates.The aluminium plates were mounted in an ultra-high vacuum chamber with a base pressure of ca. 1027 mbar. Data were collected as total electron yield using drain current methods which gives an estimated sampling depth of ca. 30–50 Å. The spectra were normalised to the intensity of the L3 feature and were calibrated using oxygen K-edge features in the I0 channel, these having been referenced to the vanadium L3 peak maximum of V2O5 at 516.9 eV, based on the reported XPS 2p3/2 binding energy of V2O5 at 516.9 eV.30 The oxygen K-edge features starting at ca. 528 eV have been removed from the spectra for clarity. Whilst § Hacac = Acetylacetone = pentane-2,4-dione; H2salen = N,N9-bis(salicylidene) ethane-1,2-diamine.J.Chem. Soc., Dalton Trans., 1998, Pages 2199–2204 2201 the accuracy of the absolute energy measurement is estimated to be ±0.2 eV, the relative energies are considered to be accurate to at least ±0.1 eV due to the use of in situ calibration. Results and Discussion Vanadium K-edge spectra Vanadium K-edge spectra of [VO(salen)] 3 and V2O5 4, with pre-edge features of moderate intensity about 10 eV below the absorption edge, are shown in Fig. 2, and these were essentially identical to those observed previously.2–5 The pre-edge feature in the spectrum of [VO(salen)] was at 5469.0 eV, compared to a literature value of 5469.1 eV,¶,2 and previous workers have reported values for the position of the pre-edge features for molecular oxovanadium(IV) complexes of 5469.1 to 5469.8,2 5469.05 to 5469.8,3 and 5469.7 to 5469.9 eV.4 For V2O5 the preedge feature in the spectrum was observed at 5470.6 eV, ca. 1.5 eV higher in energy than the above values, as to be expected for vanadium-(V) vs.-(IV). Previously reported values of the positions of the pre-edge features for molecular oxovanadium( V) complexes are in the range 5470.1 to 5470.7,2 5469.9 to 5471.1 and 5470.2 to 5470.8 eV.4 Therefore, the position of the pre-edge peaks in the vanadium K-edge spectra will be able to identify with reasonable certainty the oxidation state in the oxyoxime and oxyoximate complexes. The vanadium K-edge spectra (Fig. 2, Table 1) of the oxo- Fig. 2 Vanadium K-edge spectra of [VO(salen)] 3, [VO(HLH)2] 7, [VO(HLMe)2] 8, [VO(HLEt)2] 9, V2O5 4, [{VO(HLMe)LMe}2] 15, [{VO- (HLEt)LEt}2] 16, [{VO(HLH)LH}2] 17 and [{VO(HLEt)LEt}2] 18 ¶ This value has been amended from the published value of 5459.1 eV, since the original work2 used a value of 5456.0 eV for the pre-edge feature in vanadium foil, whereas a value of 5466.0 eV (see ref. 5) is more compatible with the calibration used herein. The other data taken from ref. 2 have been treated in a similar manner.vanadium(IV) complexes containing oxyoxime ligands (7–9) have 1s æÆ 3d transitions in the range 5469.3 to 5469.8 eV, consistent with the presence of VIV. In contrast, the position of the pre-edge features in the vanadium K-edge spectra (Fig. 2, Table 1) of the complexes formulated as dimeric oxovanadium( V) species (15–18) are in the range 5470.6 to 5470.8 eV, indicative of the presence of VV. Therefore, vanadium K-edge XAFS confirms the presence of VV in the dimeric species. The vanadium K-edge spectra of 15 and 16 are virtually identical, confirming the same type of structural motif (seven-coordinate, phenolate bridged dimer containing an h2-N–O group [see Fig. 1(d)] is present in both complexes. The spectrum of 17 is diVerent to those of 15 and 16, indicating the presence of a diVerent structural type (six-co-ordinate, V]N]O]V bridged dimers [see Fig. 1(c)]. The spectrum of 18 is broadly similar to that of 17 and very diVerent from those of 15 or 16, indicating that the same structural type is probably present in 17 and 18.Whilst the spectra of the dimeric complexes have indicated the sensitivity of vanadium K-edge XAFS to changes in structural type at the metal centre, the diVerences are fairly small and subtle. Therefore, we decided to investigate the applicability of vanadium L-edge XAFS to the identification of the structural form present in the oxovanadium(V) dimers, as this technique is very sensitive to changes in the electronic and geometric structure at the metal centre.Vanadium L-edge spectra Compounds for calibration and comparison. Fig. 3 shows the vanadium L-edge spectra obtained for several oxocompounds of VIV and VV. These spectra are split into the L3 (2p3/2) and L2 (2p1/2) components by spin–orbit coupling (ca. 7 eV) of the core–hole, and the structure on each of these components is related to the ligand field at, and site symmetry of, the vanadium.The structure on the higher energy L2 feature is less well resolved than that at the L3 edge due to interaction with the L3 continuum states.7 The maxima of the L3 components for [VO(acac)2] 1, [VO(acac)2(py)] 2 and [VO(salen)] 3 all occur at 515.7 eV, with shoulders at 512.7 (512.8 for 1) and 513.4 eV in each case. The maxima of the broad, slightly asymmetric L2 Fig. 3 Vanadium L-edge XAFS spectra of [VO(acac)2] 1, [VO(acac)2- (py)] 2, [VO(salen)] 3, V2O5 4, [NH4][VO3] 5, K3[VO(O2)2(C2O4)] 62202 J.Chem. Soc., Dalton Trans., 1998, Pages 2199–2204 Table 1 Vanadium K- and L-edge XAFS data (eV) for some vanadium-(IV) and -(V) compounds Compound 1 [VO(acac)2] 2 [VO(acac)2(py)] 3 [VO(salen)] 4 V2O5 5 [NH4][VO3] 6 K3[VO(O2)2(C2O4)] 7 [VO(HLH)2] 8 [VO(HLMe)2] 9 [VO(HLEt)2] 10 [VO(HL(Cl))2] 11 [VO2{C6H4(O)CH]] NOC(Me)]] NH}] 12 [VO2{C6H4(O)CH]] NOC(Ph)]] NH}] 13 [VO2{C6H4(O)C(Me)]] NOC(Ph)]] NH}] 14 [VO2{C6H4(O)CH]] NOC(4-O2NC6H4)]] NH}] 15 [{VO(HLMe)LMe}2] (h2-N]O)i 16 [{VO(HLEt)LEt}2] (h2-N]O)i 17 [{VO(HLH)LH}2] (m-N]O)i 18 [{VO(HLEt)LEt}2] (m-N]O)i 19 [{VO(HLoct)Loct}2] K edge 1s æÆ 3d transition a 5469.05;c 5469.4 d f 5469.0;e 5469.1 d 5470.6 e 5470.6;g 5470.67;c 5470.3 d 5469.8;g 5470.1 d 5470.61 c 5469.8 e 5469.7 e 5469.3 e fffff 5470.7 e 5470.6 e 5470.6 e 5470.8 e f L3 edge b 512.8, 513.4, 515.7 e 512.7, 513.4, 515.7 e 512.7, 513.4, 515.7 e 513.9, 515.3, (516.2), 516.9 e 513.5, 514.7, 515.5, 516.7 e 513.6, 515.3, 517.0 e h 512.8, 513.6, 515.7 e 512.8, 513.6, 515.7 e 512.7, 513.6, 515.6 e 515.3, 516.9 e 515.0, 516.6 e 514.9, 516.8 e 515.0, 516.7 e 514.9, 515.8, 516.8 e 515.0, 515.8, 516.8 e 514.0, 515.1, 517.1 e (513.9), 514.9, (516.3), 517.1 e 515.1, 517.1 e L2 edge b 522.6 e 522.6 e 522.4 e 523.7 e 523.5 e 522.4, 523.9 e h 522.6 e 522.6 e 522.4 e 523.5 e 523.3 e 523.4 e 523.3 e (522.6), 523.4 e 523.3 e 523.7 e (522.2), 523.7 e (522.7), 523.7 e a Peak maxima ± 0.25 eV, referenced to V2O5 1s æÆ 3d transition at 5470.6 eV.b Peak maxima ± 0.1 eV referenced to L3 maximum of V2O5 at 516.9 eV; figures in parentheses are poorly defined shoulders. c Ref. 3. d Ref. 2 [value amended from that given in the original work which used a value for the vanadium foil pre-edge feature of 5456.0 eV; a value of 5466.0 eV was used herein (see ref. 5)]. e This work. f Vanadium K-edge data not recorded. g Ref. 5. h Reliable vanadium L-edge data could not be obtained. i The bonding mode of the deprotonated oxime moiety (see Fig. 1). features are observed at 522.6 eV for [VO(acac)2] and [VO(acac)2(py)] and 522.4 eV for [VO(salen)]. The spectra for all three complexes are very similar, implying that vanadium L-edge XAFS is not suYciently sensitive to discriminate between the small changes in the ligand field experienced by the vanadium in these compounds. (Single crystal electronic absorption measurements have shown that the 3dz2 orbitals is at least 4000 cm21 higher in energy in six-co-ordinate VO21 complexes than in their five-co-ordinate counterparts.31) Therefore, we believe that the profile observed could be useful as a spectroscopic signature for VO21 bound to O,N-donor ligands.For V2O5 4 the L3 maximum was observed at 516.9 eV, with well defined features at 513.9 and 515.3 eV and a weak shoulder at 516.2 eV. In the case of [NH4][VO3] 5 the L3 maximum was at 516.7 eV together with shoulders at 513.5, 514.7 and 515.5 eV.For K3[VO(O2)2(C2O4)] 6 the L3 maximum at 517.0 eV was accompanied by a well defined shoulder at 515.3 eV and a weaker feature at 513.6 eV. The L2 features for V2O5, [NH4][VO3] and K3[VO(O2)2(C2O4)] were observed at 523.7, 523.5 and 523.9 eV, respectively. For the last of these features there was a shoulder at 522.4 eV, whereas in the other two spectra the L2 band was asymmetric. The shift of ca. 1 eV in the maxima of both the L3 and L2 peaks on going from VIV to VV clearly indicates the sensitivity of vanadium L-edge XAFS spectra to changes in oxidation state.Chen at al.16 have previously observed a shift of 0.68 eV per oxidation state for vanadium oxocomplexes, but the resolution obtained was very poor (e.g. the spectrum of V2O5 contained only a broad feature at both the L3 and L2 edges) casting some doubt on the accuracy of this value. Whilst the L-edge spectra of the oxovanadium( IV) compounds appear to be very similar, there are significant diVerences between the spectra of the vanadium(V) systems, presumably related to the diVerences in the local environment of the vanadium which aVect the relative energies of the 3d orbitals.Previously, it has been noted that the simulation of L-edge spectra for d0 configurations is complicated because of the large number of individual dipole-allowed 2p æÆ 3d transitions.7 Although polarisation-dependent, single crystal measurements on V2O5 have been used to identify the spectroscopic features arising from the diVerent 3d orbitals,15 we have been unable to collect polarisation-dependent data on molecular complexes due to the loss of crystallinity, presumably related to the loss of solvent from the top (ca. 30–50 Å) layer of the compounds under the ultra-high vacuum conditions required for the experiment. The complexity of the L-edge spectra observed for the vanadium(V) systems reported herein also calls into question the approach previously used for identification of oxidation state in vanadium-containing minerals.18,19 This assumed that any VV present would be represented by a single peak in the L3-edge region of the spectrum.Thus, whilst we have observed one major peak at the L3 edge, lower energy shoulders associated with the vanadium(V) spectra may overlap with, or could be misassigned to, features from lower oxidation state species. Oxovanadium oxyoxime and oxyoximate complexes. The vanadium L-edge spectra obtained from the oxovanadium-(IV) and -(V) complexes containing oxyoxime and oxyoximate ligands are shown in Fig. 4. Essentially identical spectra were observed for all of the oxovanadium(IV) oxyoxime complexes studied (8–10) and representative spectra of [VO(HLMe)2] 8 and [VO(HLEt)2] 9 are given in Fig. 4. The L3 maxima were observed at 515.7 eV, together with lower energy features at 512.8 and 513.6 eV, whilst the L2 absorption band was observed at 522.6 eV for 8 and 9 and at 522.4 eV for 10.These spectra and values are very similar to those observed for [VO(acac)2], [VO(acac)2(py)] and [VO(salen)], consistent with the presence of VIV in these complexes, further supporting the idea that this spectroscopic motif is characteristic of VO21 bound to O,Ndonor ligands. For the vanadium(V) complexes 11–19 a much wider variation in the nature of the L-edge spectra was observed. For the monomeric, five-co-ordinate, dioxovanadium(V) complexes 11– 14 [see Fig. 1(e)] the spectra were similar, with the L3 maxima at 516.6–516.9 eV with low energy shoulders at ca. 515.0 eV and the L2 maxima at ca. 523.4 eV; Fig. 4 shows the spectrum obtained for 14. The position of the L3 maxima at ca. 516.7 eV suggests that this feature is indicative of VV in such systems which have been shown unambiguously to be VV from 51V NMR spectroscopy, consistent with the lack of an EPR signal.23 For the oxovanadium(V) dimeric species 15–18, two diVerent structural forms have been identified using X-ray crystallography.Red-brown, [{VO(HLMe)LMe}2] 15 has a seven-co-J. Chem. Soc., Dalton Trans., 1998, Pages 2199–2204 2203 ordinate, dimeric structure [Fig. 1(d)] with bridging phenolate ligands from two monodeprotonated hydroxyoxime ligands and the other two oximes co-ordinated via h2-N]O units.21 The vanadium L-edge spectrum (Fig. 4) of this material consists of three features (514.9, 515.8 and 516.8 eV) at the L3 edge and a broad, asymmetric peak at 523.4 eV at the L2 edge.The lowest and highest energy components at the L3 edge, and the shape and position of the L2 band, correlate very well with those observed for the monomeric dioxovanadium(V) complexes, confirming the presence of VV in these dimeric complexes. We do not believe that the central component of the L3 band at 515.8 eV is a genuine part of the spectral profile of the oxovanadium( V) dimer spectrum as its intensity was found to depend upon the length of time the sample was exposed to the soft X-ray beam (the intensity increased with successive scans) and whether epoxy- or graphite-mounted samples were used, even though the same batch of compound was used in both cases.The energy of this band (515.8 eV) is very similar to that observed for the vanadium(IV) complexes and therefore it is most likely to be due to the presence of some VIV in the sample. Previously, we have observed soft X-ray induced spin state transitions and photochemistry in some iron complexes,8 soft X-ray induced photoreduction and photoisomerisation in nickel complexes,32 and diVerent rates of photoreduction in a number of vanadium(V) compounds.33 Therefore, the 515.8 eV feature in the spectra of some of the oxovanadium(V) oxyoxime oxyoximate complexes is probably due to VIV caused by some photoreduction of the sample on exposure to soft X-rays.As 3d L-edge XAFS is essentially a surface sensitive technique (with a penetration depth of ca. 30–50 Å) the vanadium(IV) component in the spectra is most likely to be located in the Fig. 4 Vanadium L-edge XAFS spectra of [VO(HLMe)2] 8, [VO- (HLEt)2] 9, [VO2{C6H4(O)CH]] NOC(4-O2NC6H4)]] NH}] 14, [{VO- (HLMe)LMe}2] 15, [{VO(HLEt)LEt}2] 16, [{VO(HLH)LH}2] 17, [{VO- (HLEt)LEt}2] 18, [{VO(HLoct)Loct}2] 19 surface layer, rather than as a result of bulk decomposition of the sample. Black, [{VO(HLH)LH}2] 17 has a six-co-ordinate, dimeric structure [Fig. 1(c)] with V]N]O]V bridging.20 The vanadium L-edge spectrum of this compound has an L3 band with features at 514.0, 515.1 and 517.1 eV, and a broad asymmetric L2 band with a maximum at 523.7 eV.There is no evidence of a feature at 515.8 eV, indicating the absence of any significant photoreduction. Of interest is the eVect of changes in molecular geometry on the rate of soft X-ray photoreduction in these two oxovanadium(V) complexes. The L3 maximum at 517.1 eV for 17 is 0.3 eV higher in energy than that for 15 and the L2 band is also ca. 0.3 eV higher in energy. These shifts were consistently observed for diVerent samples and are considered to be indicative of changes in the 3d orbital energies due to changes in bonding and site symmetry at the vanadium between the two structural forms. Deconvolution of the spectrum of the sevenco- ordinate species 15 using a vanadium(IV) spectrum 9 and a spectrum of the six-co-ordinate species 17 showed that this shift of ca. 0.3 eV cannot be accounted for by the presence of the 515.8 eV band in the spectrum of the seven-co-ordinate species.Therefore, we believe that the position of the L3 maximum can be used to diVerentiate between the two types of dimeric structures. In respect of the synthesis of complexes 15 and 17, the same product was obtained from either oxidation of the corresponding vanadium(IV) complex or the addition of the proligand to a vanadium(V) precursor such as [NH4][VO3].20,21 However, in the case of [{VO(HLEt)LEt}2], the nature of the product depended on the synthetic route chosen.Oxidation of [VO(HLEt)2] in CHCl3 led to the production of black crystals of [{VO(HLEt)- LEt}2] 18 which have been shown by X-ray crystallography 21 to possess a structure of the type shown in Fig. 1(c). The vanadium L-edge spectrum of 18 is similar to that of 17, with L3 components at 514.9 and 517.1 eV, together with weak shoulders at 513.9 and 516.3 eV, and a broad asymmetric L2 peak at 523.7 eV.The positions of the L3 and L2 maxima and the general form of the spectral profile are very similar to those observed for 17, as expected since both 18 and 17 have the same structural form. The origin of the subtle change in relative intensity and slight energy shift of the shoulders at the L3 peak is unclear, and it should be noted that there were also small diVerences between the vanadium K-edge spectra of these two complexes. Crystallographic studies have revealed that whilst both complexes have the structure type of Fig. 1(c), there are statistically significant diVerences in the angles subtended at the vanadium in the two compounds.20,21 Thus, the experimental data indicate the sensitivity of L-edge spectroscopy to small changes in the local environment of d0 systems. Reaction of proligand H2LEt with [NH4][VO3] led to the synthesis of brown [{VO(HLEt)LEt}2] 16. Whilst we have not been able to determine the structure of 16 by X-ray crystallography, the spectroscopic data (IR, NMR, UV/VIS) indicate a structure of the type Fig. 1(d).21 The vanadium L-edge spectrum of 16 was essentially identical to that of 15, with L3 features at 515.0, 515.8 and 516.8 eV and a broad L2 peak at 523.3 eV, consistent with the structure of 16 being very similar to that of 15, as indicated by other spectroscopic data. Given the above results which show that vanadium L-edge XAFS is able to discriminate between the two structural forms observed for the oxovanadium(V) dimers, we employed this approach to investigate the nature of the vanadium centre present in a material that was only available as an amorphous solid but has industrial importance given the use of the proligand in metal extraction.Attempts to synthesize an oxovanadium(IV) complex using a toctyl substituted hydroxyoxime proligand (R1 = Octt, R2 = H in Fig. 1(a)] invariably led to a rapid darkening of the reaction solution and the formation of an almost black amorphous product formulated as [{VO(HLoct)Loct}2] 19 on the basis of microanalytical and IR data.21 The vanadium2204 J.Chem. Soc., Dalton Trans., 1998, Pages 2199–2204 L-edge spectrum obtained for this product is shown in Fig. 4 with the L3 maximum at 517.1 eV and a low energy shoulder at 515.1 eV, together with a broad asymmetric L2 peak with a maximum at 523.7 eV. This spectrum is very similar to that obtained from the six-co-ordinate V]N]O]V bridged [{VO- (HLEt)LEt}2] dimer 18 at both the L3 and L2 edges and this, together with the observed colour, strongly implies that the structure of this compound is based on a six-co-ordinate vanadium(V) dimer with V]N]O]V bridging.This is in good agreement with the solid state IR spectroscopic experiments which also indicated a structure based on a six-co-ordinate dimeric structure.21 Conclusion The position of the pre-edge features in the vanadium K-edge XAFS spectra gives a clear indication of the oxidation state in the molecular compounds.Subtle changes in the edge structure give a probable indication of the structural type present in the dimeric species, but this is not suYciently diagnostic to be used as a fingerprint of co-ordination environment. In contrast, vanadium L-edge XAFS has been shown to be a powerful fingerprint of both the oxidation state and the local geometry of the vanadium in a range of oxyoxime and oxyoximate complexes. In particular vanadium L-edge XAFS spectra have shown that the dimeric vanadium oxime oxyoximate complexes should be formulated as vanadium(V) systems with the L3 maximum being diagnostic of the presence of either a V]N]O]V bridged structure with two six-co-ordinate pseudo-octahedral metal centres, or a phenolate-bridged structure with h2-N]O bonding mode of the oxyoximate ligands and pseudo-pentagonal bipyramidal seven-co-ordination at each metal.Acknowledgements We thank the SERC/EPSRC for financial support including an Advanced Fellowship (N.A. Y.), equipment and SRS beamtime (GR/J 34200), post-doctoral provision (C. M. McG., GR/K 64662), and Ph.D. studentships (J. G., CASE with Zeneca Specialties; J. F. W., CASE with ICI Chemicals & Polymers; J. M. W. S.). The Royal Society is thanked for a University Research Fellowship (D. C.) and The University of Manchester for a Rona Robinson Scholarship (E. P.). The Director of the Synchrotron Radiation Department at the SRS is thanked for access to experimental and computational facilities.References 1 B. K. Teo, EXAFS Basic Principles and Data Analysis, Springer, Berlin, 1986; X-Ray Absorption: Principles, Applications and Techniques of EXAFS, SEXAFS and XANES, eds. D. C. Koningsberger and R. Prins, Wiley, New York, 1988. 2 J. M. Arber, E. De Boer, C. D. Garner, S. S. Hasnain and R. Wever, Biochemistry, 1989, 28, 7968. 3 C. Weidemann, D. Rehder, U. Kuetgens, J. Hormes and H.Vilter, Chem. Phys., 1989, 136, 405. 4 K. H. Hallmeier, R. Szargan, G. Werner, R. Meier and M. A. Sheromov, Spectrochim. Acta, Part A, 1986, 42, 841. 5 J. Wong, F. W. Lytle, R. P. Messmer and D. H. Maylotte, Phys. Rev. B, 1984, 30, 5596. 6 M. O. Krause and J. H. Oliver, J. Phys. Chem. Ref. Data, 1979, 8, 329. 7 G. van der Laan and I. W. Kirkman, J. Phys.: Condens. Matter, 1992, 4, 4189. 8 D. Collison, C. D. Garner, C. M. McGrath, J. F. W. Mosselmans, M. D. Roper, J. M.W. Seddon, E. Sinn and N. A. Young, J. Chem. Soc., Dalton Trans., 1997, 4371 and refs. therein. 9 F. M. F. de Groot, J. Electron Spectrosc. Relat. Phenom., 1994, 67, 529. 10 M. Abbate, H. Pen, M. T. Czyzyk, F. M. F. de Groot, J. C. Fuggle, Y. J. Ma, C.-T. Chen, F. Sette, A. Fujimori, Y. Ueda and K. Kosuge, J. Electron Spectrosc. Relat. Phenom., 1993, 62, 185. 11 M. Abbate, F. M. F. de Groot, J. C. Fuggle, Y. J. Ma, C.-T. Chen, F. Sette, A. Fujimori, Y. Ueda and K. Kosuge, Phys.Rev. B, 1991, 43, 7263. 12 H. F. Pen, L. H. Tjeng, E. Pellergrin, F. M. F. de Groot, G. A. Sawatzky, M. A. van Veenendaal and C.-T. Chen, Phys. Rev. B, 1997, 55, 15 500. 13 E. Goering, O. Müller, M. L. denBoer and S. Horn, Physica B, 1994, 194, 1217. 14 E. Goering, O. Müller, M. Klemm, J. P. Urbach, H. Petersen, C. Jung, M. L. denBoer and S. Horn, Physica B, 1995, 208 and 209, 300. 15 E. Goering, O. Müller, M. Klemm, M. L. denBoer and S. Horn, Phil. Mag. B, 1997, 75, 229. 16 J. G. Chen, C. M. Kim, B. Fruhberger, B. D. de Vries and M. S. Touvelle, Surf. Sci., 1994, 321, 145. 17 F. M. F. de Groot, J. C. Fuggle, B. T. Thole and G. A. Sawatzky, Phys. Rev. B, 1990, 42, 5459. 18 G. Cressey, C. M. B. Henderson and G. van der Laan, Phys. Chem. Min., 1993, 20, 111. 19 P. F. Schofield, C. M. B. Henderson, G. Cressey and G. van der Laan, J. Synchrotron Rad., 1995, 2, 93. 20 J. M. Thorpe, Ph.D. Thesis, The University of Manchester, 1992. 21 J. Grigg, Ph.D. Thesis, The University of Manchester, 1995. 22 J. Grigg, D. Collison, C. D. Garner, M. Helliwell and P. A. Tasker, J. Inorg. Biochem., 1993, 51, 172. 23 J. Grigg, D. Collison, C. D. Garner, M. Helliwell, P. A. Tasker and J. M. Thorpe, J. Chem. Soc., Chem. Commun., 1993, 1807. 24 C. D. Garner, D. Collison and E. Pidcock, Philos. Trans. R. Soc. London, Ser. A, 1996, 354, 325. 25 A. Rosenheim and H. Y. Mong, Z. Anorg. Allg. Chem., 1925, 148, 25. 26 N. J. Campbell, M. V. Capparelli, W. P. GriYth and A. C. Skapski, Inorg. Chim. Acta, 1983, 77, L215. 27 R. L. Beddoes, D. Collison, C. D. Garner, J. Grigg, M. Helliwell, P. A. Tasker, D. Thorp and J. M. Thorpe, to be published. 28 V. R. Dhanak, A. W. Robinson, G. van der Laan and G. Thornton, Rev. Sci. Instrum., 1992, 63, 1342. 29 C. S. Mythen, G. van der Laan and H. A. Padmore, Rev. Sci. Instrum., 1992, 63, 1313. 30 G. A. Sawatzky and D. Post, Phys. Rev. B, 1979, 20, 1546. 31 D. Collison, B. Gahan, C. D. Garner and F. E. Mabbs, J. Chem. Soc., Dalton Trans., 1980, 667. 32 D. Collison, C. D. Garner, C. M. McGrath, J. F. W. Mosselmans, E. Pidcock, M. D. Roper, B. G. Searle, J. M. W. Seddon, E. Sinn and N. A. Young, to be published. 33 D. Collison, C. D. Garner, C. M. McGrath, J. F. W. Mosselmans, M. D. Roper, J. M. W. Seddon, E. Sinn and N. A. Young, unpublished work. Received 21st January 1998; Paper 8/00568K