2p excitation spectra of transition metal compounds as a probe of local electronic structure: A theoretical determination

Chemical Physics North-Holland

168 ( 1992) 5 l-60

2p excitation spectra of transition metal compounds as a probe of local electronic structure: a theoretical determination P. Decleva,

G. Fronzoni

and A. Lisini

Dipartmento dl Sclenze Chmrche, Via A. Valerio 38, 34127 Tneste, Italy Received

18 May 1992

Excitation energies and oscillator strengths for Ni, Fe, Cr 2p and Pd 3p excnations in carbonyl, nitrosyl and bis-ally1 complexes have been calculated at 1h- 1p CI level, providing for an accurate choice of the basis set and an adequate treatment of relaxation. The spectra are found to be strongly sensitive to the nature of the ligands and the character of the metal-ligand bonding, and less dependent on the molecular symmetry and the d count of the metal atoms. An interpretation of the spectral pattern in terms of appropriate MO energy level diagrams is proposed that allows to correlate spectral data to the extent of the backbonding in x ligand complexes.

1. Introduction Spectroscopy of the inner shell by soft X-ray photon absorption (XAS) or high energy electron energy loss spectroscopy (EELS) has received a strong impetus in the recent years because of instrumental advances, notably the use of synchrotron radiation sources [ 11, and increased sensitivity and resolution in the EELS techniques [ 2 1, providing a vast amount of important information, which complements that already obtainable by fixed photon energy photoelectron spectroscopy. The main advantage of core excitations lies in the strongly localized nature of the transitions involved, which allows probing in detail the local environment of a particular species, even when it is embedded in a very complex situation. Examples range from metal centres in large biomolecules to species adsorbed on metal or semiconductor surfaces, to active sites in chemical reactions or catalytic process. While the continuum features in the high energy region above the ionization edge (EXAFS) give essentially a geometrical information on the short range spatial structure [ 3 ] the details of both the preedge quasibound resonances and the near edge shape resonances (NEXAFS region) contain the Correspondence to: G. Fronzoni, Chtmtche,

Dipartimento Via. A. Valerio 38, 34127 Trieste, Italy.

di Scienze

0301-0104/92/% 05.00 0 1992 Elsevier Science Publishers

full detail of the local electronic structure [ 41. From this point of view the available information parallels that obtainable by other techniques which probe virtual levels, like valence absorption, inverse photoemission or electron transmission spectroscopy, with the obvious difference that in core excitations relaxation associated with the core vacancy may signilicantly affect the structure of the unoccupied states, but with the great advantage over the latter techniques given by the strictly local character of such excitations already stressed. Recently the first data have been obtained by EELS relative to the 2p edge in transition metal compounds [ 5 1. This is a potentially very interesting region, as it probes directly the metal centers, which occur mostly isolated even in large systems. Moreover the excitations are expected to be dominated by the 2p+ 3d transition moment, much larger than excitations to higher quantum number orbitals, thus directly mapping the d content of the different unoccupied levels. In this respect this edge appears much richer than the deep core 1s excitation, which is being studied mainly aiming at structural information, whose main transition moment is expected towards metal 4p and ligand final states [ 6 1. Moreover the much narrower natural width of the lowest energy 2p,,, (L,) core hole, which is about 0.2-0.4 eV for the first transition row should allow the resolution of

B.V. All rights reserved.

52

P. Decleva et al. /Chemical Phpcs

many more details than is possible from deeper hole excitations. Several questions have been raised in the former study [ 51, concerning the influence on the absorption pattern of local symmetry and ligand field splitting of the d manifold, versus the individual nature of the metal and ligand species and the metal-ligand bonding involved. The relatively weak involvement of d orbitals in covalent bonding would suggest an interpretation based on ligand field splitting of the 3d shell mainly determined by local symmetry and field (or bonding) strength. A different picture has however emerged from the gas-phase experimental data on metal carbonyls and cyclopentadienyls [ 5 1. A strong sensitivity to the nature of the ligands has been observed, with all spectra of carbonyl compounds quite close to each other, and markedly different from those of the cyclopentadienyls, again very similar to each other. This implies on the contrary little dependence on the nature of the metal species involved, which reflects mainly in the decrease of the oscillator strength associated with the filling of the d shell. Moreover almost no influence of the different structure and symmetry is detected within the same class of compounds. This had led to the tentative suggestion that 2p excitation spectra will reflect essentially the nature of the ligand, and the chemical bonding involved, with total oscillator strength as a measure of the density of unoccupied d states. However a still different picture is suggested by the results of extended Htickel calculations performed on different carbonyls, and different geometries of the Co, (CO ) B species, which indicated also a strong variance within members in the same class, and a similar dependence on the geometry of the compound [ 5 1. This underlines again the very sensitive information potentially contained in the 2p absorption spectra, indicating at the same time the need of a detailed and accurate parallel theoretical investigation to assign and understand the origin of the spectral features. In fact although the EHT scheme previously employed has been of significant value as an aid in the interpretation of the spectra, the approach is too crude to strive for accurate and dependable detailed information. At the same time, since experimental information is still at an early stage, and is limited by instrumental resolution and rather low signal to background ratio, the more detailed information which is obtainable from

168 (1992) 51-60

accurate calculations can be of significant value in assessing the nature and the quality of the information that can be potentially accessible, even in advance of experimental data. We have therefore undertaken a computational investigation on several molecules closely related by the change of the metal and the ligands, in order to examine definite trends associated with the various factors involved. We have considered first the series of carbonyl nitrosyls Ni(C0)4, Fe(C0)2(N0)2 and Cr (NO )_,, which are isoelectronic and isostructural (the iron compound is CZVonly slightly distorted from tetrahedral geometry). This similarity reflects in a close correspondence between the valence photoelectron spectra, despite that the formal d atomic occupation is lowered from d” in Ni to d6 in Cr. So any change in the bonding or metal local electronic structure goes undetected in the valence ionization energies and intensities, a rather puzzling situation [ 7 1. On the contrary large variations in the absorption spectra are predicted by the calculation, distinctly revealing the different involvement of the d orbitals and the changing in the composition of the bonding and virtual orbitals. We have next considered change of ligands from Ni( CO), to bis-ally1 nickel, whose electronic structure and photoelectron spectrum has been intensely investigated [ 8 1. A strong influence of this change is found in the energy position and spectral pattern of the absorption lines, as well as in the total oscillator strength, which show similarities with the change experimentally observed in cyclopentadienyls. Lastly we have examined the substitution of the nickel with the palladium atom in bis-ally1 palladium. While the spectral pattern stays very similar, a very significant drop in oscillator strength is observed, possibly reflecting the expected higher occupation of the d shell in the palladium compound.

2. Computational method Extended basis sets have been employed in ab initio SCF and CI calculations, in order to adequately describe the change in metal orbitals in the presence of the core hole, with diffuse functions to have a good representation of at least the n = 4 metal shell. The Ni, Fe and Cr basis sets are the [ 14~9~1 sets

P. Decleva et al. /Chemrcal Physm 168 (1992) 51-60

of Wachters [ 91 and the [ 6d] sets of Rappe [ lo]. The [ 14~9~1 set has been supplemented with one s function (with exponents (Y,= 0.346 for Ni, 0.298 for Fe and 0.253 for Cr) and three p functions (the two Wachters’ p functions to describe the 4p orbital [ 91 and one p function with exponent (-u,=O.314 for Ni, 0.283 for Fe and 0.244 for Cr); the [ 6d] set has been supplemented with one diffuse d function (Q= 0.05 for Ni, 0.037 for Fe and 0.03 1 for Cr). The final basis set is [ 15sl2p7d] contracted to (8s6p4d). The Pd basis set is the [ 17s 1 lp8d] set of Huzinaga [ 111 enlarged by one s function (a,= 0.233 ), three p functions (the two p functions of Walch [ 12 ] to describe the 5p orbital and one p function with exponent (Y,= 0.249) and two diffuse d functions (the first with exponent of Walch [ 121 a,=0.0838 and the second with cr,=O.O264). The final basis set is [ 18s14plOdl contracted to (lOs8p6d). The first row atoms basis sets are taken from the Huzinaga [ 9s5p] set [ 131 contracted to (3~2~). The general contraction scheme [ 141 has been employed, leaving uncontracted the most diffuse functions. SCF and CI calculations have been performed with the MELD set of programs [ 15 1. Oscillator strengths were computed from the CI vectors employing both the dipole length and velocity operators. Agreement between the two was generally satisfactory and only the latter,

with

is reported. A test of the basis set quality on the Ni 3d lo configuration gives a 2p-’ A SCF value of 858.80 eV, compared with the value of 856.94 eV obtained from numerical calculations [ 16 1. Relaxation of the valence orbitals in response to core hole formation has been taken into account by the use of SCF orbitals relative to the core ionized species. In fact as the excited electron lies outside this almost closed shell core, it is expected that relaxation in the core excited system is well described by this procedure. In fact the simpler Z+ 1 approximation, which is computationally equivalent, has often been employed in describing such states, showing a much improved description over the ground state orbitals

53

[ 171. For the same reason we have limited the CI scheme to single particle excitations from the 2p orbitals in the present survey, as it is not expected that strong correlations are active between the closed valence shell and the excited electron. Nevertheless strong relaxation affecting valence orbitals may give rise to additional 2h-2p states, closely related to the strong satellites appearing in the core photoelectron spectra of transition metal compounds. In fact such states are claimed to be responsible for an extra band in the 1s absorption spectra of copper complexes

1181. We have made preliminary 2h-2p CI calculations with additional excitations from valence levels in Ni(&H,),. The variations observed are of minor entity and in any case not likely to affect the picture obtained. We defer a detailed analysis of the effect of higher excitations to a further study. It is to be noted that the metal 2p shell spin-orbit coupling which in open shell systems, like the free atoms or ions embedded in a weak crystal field, can lead to a significant spin mixing and strong intensity redistributions [ 19 1, in the case of closed shell metal complexes cause probably only a duplication of the structures corresponding to the 2p,,, and 2p,,, couplings. The single excitation scheme adopted here has moreover the important advantage of a clearly defined ionization limit, which is the energy of the hole state single configuration, with respect to which excitation energies are conveniently referred (term values) thereby subtracting errors due to inaccurate calculation of the core ionization potential, for instance by incomplete relaxation, and facilitating comparison with experimental values. A technical difficulty arises in the evaluation of transition moments with respect to the ground state, as the algorithm presently available requires the use of a single set of orthogonal orbitals. We have therefore adopted ion SCF orbitals also for the description of the ground state, again employing a single excitation ( lh-lp) CI for describing relaxation to ground state, as already suggested for the calculation of the satellite intensities in the core photoelectron spectra [ 20 1. Energies so obtained are usually slightly lower than ground state SCF energy, indicating that the 1hlp wavefunction is comparable to the ground state SCF solution.

54

P. Decleva et al. /Chemical

Physics 16% (1992) 51-60

As a last technical point, for tetrahedral molecules we actually employed metal 2s’ SCF orbitals because of difficulty in preserving equivalence of the degenerate orbitals in the presence of a 2p core hole. While this may affect the energy of the 2p-’ conliguration, it is not expected to have a significant effect on the valence orbital relaxation, and therefore on the term values obtained.

the tetrahedral molecules, while more lines spanning a larger energy range are obtained for the iron compound. The first observation is the strong variation of the spectral pattern, in marked contrast with the similarity of the valence photoelectron spectra [ 71 for all molecules. In this case XAS data prove much more sensitive to the change in the electronic structure and the nature of the bonding. Let us consider first the two tetrahedral systems. In Ni(C0)4 there are two intense features about 3 eV below threshold, with the transition to the e orbital at higher energy and more intense than that into the tz orbital. In Cr(NO), the e final orbital is found at much lower energy, 5.5 eV (term value), with an intensity similar to that in Ni(CO),, while the t2 transition is pushed towards

3. Results and discussion Excitation energies (term values) and intensities for Ni(C0)4, Fe(C0)2(N0)2 and Cr(N0)4 are reported in table 1 and fig. 1. As can be seen the spectra are dominated by a small number of intense lines in Table 1 Term values (eV) and oscillator

strengths

Ni(CO)I

for metal 2p spectra of Ni(C0)4,

Fe(C0)2(N0)2

Fe(CC)a(N0)2

MO

T

fx lo*

t2

3.26

5.200

d*+rt*

e

2.65

8.482

d*+x*

a1

2.00

0.002

s*+x*

t2

0.26

0.100

x’

T

fx 102

b,

6.90 6.23

1.623 1.509

d’+x*NO

a2 bz

4.58 4.56 4.39

1.249 2.088 1.464

d’+x*NO

a2

3.70

1.474

d*+x*NO

aI

2.71 2.50 2.41 2.23 2.02 1.89 1.82 1.75 1.68 1.49 1.35 0.93 0.70 0.51 0.27 0.13 0.11 0.08 0.04

1.657 1.436 0.560 0.072 0.025 0.014 0.249 0.159 1.070 0.259 0.011 0.181 0.383 0.112 0.932 0.004 0.009 0.039 0.923

d’+a*CO

al b, a2 al al a1 In the MO composition

a)

Cr(NOL

MO

b, al aI a, a2 b, bz bz aI a2 at

and Cr(NO),

d’ and S* are the metal 3d and 4s orbitals

d’+n*CO

x’CO+x*NO d’ d*+x’CO

d*+x’CO,

NO

and I* is the l&and 2~.

fx 102

MO

T

t1

6.27

0.003

x’

e

5.50

8.817

d*+x*

aI

2.41

0.009

S’

tz

1.43

16.983

d*+n*

t2 e

0.85 0.59

12.201 0.025

d’

tz

0.18

2.208

d*+x*

P. Decleva et al. /Chemcal

Cr(NO>,

7

5

3 ’

E(eV)

Fig. 1. Calculated term values (eV) and oscillator strengths for Ni 2p, Fe 2p and Cr 2p excitation in Ni(C0)4, Fe(C0)2(N0)2 and Cr( NO)+ Calculated lines are convoluted with Gaussians of 0.35 eV fwhm.

threshold, split into two components, and shows an enormous gain in intensity. Before attempting to interpret this change on the basis of the change in bonding, let us remark that plain spectral pattern similarity could easily deceive in assignment suggesting a close correspondence between the doublet at 3 eV in Ni(C0)4 and that at 1 eV in Cr(N0)4. Qualitative energy level diagram for these molecules are reported in fig. 2, where only the most important interactions have been illustrated. Although the energy scale is arbitrary, levels spacing has been chosen to follow photoelectron spectroscopy data for the occupied part, and the present results for the virtual part. In Ni(C0)4 metal 3d level are quite low in energy, comparable to that of ligand o levels. This gives rise to the well known destabilization of the tz component in the tetrahedral environment. Further interaction with the higher lying ligand 27r* orbitals is slightly larger in the 2e symmetry, possibly because the HOMO tz orbital is partly mixed with ligand o components, and has lost some 3d character. This rationalizes both the e* lying at higher energy than t;, and its greater 3d character, consistent with the higher intensity of the relative transition. The situation changes drastically in Cr(N0)4, since

Physrcs 168 (1992) 51-60

55

now less bound metal 3d orbitals lie close to the ligayd 271 levels, which are partially occupied. Interaction with the lower lying o ligand levels is less relevant, but destabilizes as usual the 3d tz component. Now the lower lying 3d e orbital interacts little with the 2x e counterparts, giving rise to a predominantly 3d occupied e orbital, and a lower lying e* orbital of mainly 27t composition. Interaction within the t2 manifold is much stronger, possibly because of quasidegeneracy, giving the occupied tz (HOMO) of mainly ligand 2x composition, and a high lying tz component, of dominant 3d character. Because of closeness with the metal 4p levels, this orbital is then split into two components, the higher lying of larger 4p character (as shown by present MO composition). This accounts for the low lying e* final state, with intensity comparable to that found in Ni (CO),, and the high intensity high lying doublet oft; final states. By chance this affects little the position of the occupied orbitals, with NO 2n orbitals taking the place of metal 3d ones, so that the change goes almost undetected in photoelectron spectra. If the pattern so outlined is indeed correct, one may anticipate a much larger increase in the cross section of the e ionization with respect to that of the outermost tz in tuning photon energy to the 2p+3d (or more easily 3p-+3d) resonances, while their behavior is expected similar, or even slightly reversed in Ni ( CO ) 4. It is worth stressing again the sensitivity of the 2p+ 3d transition to the nature of the bonding, as it probes very selectively the 3d content of the unoccupied states. Although implicit in the structure of the wavefunction, this is not easily seen by other considerations. In a thorough study of the differences in the electronic structure of Ni(C0)4 and Cr(N0)4 [21] it was suggested that bonding in the latter is best viewed as donation of one electron from Cr to the (NO), moiety, and formation of five covalent bonds between the five corresponding singly occupied orbitals, with significant ionic character, rather than by promotion to the d”+2 state like in Ni (CO), due to the Pauli repulsion with ligand orbitals. In the end it was concluded that bonding in Cr( NO), is similar to that in Ni (CO ) 4, apart from a larger charge transfer into the NO 27t orbitals, suggesting therefore a uniform change of the 3d-2rr mixing in going from Ni to Cr, at variance with the strong difference in the nature of the t2 and e orbitals found in the present study.

P. Decleva et al. /Chemcal

56

(co)4

Ni(C0)4

-

4P

-

4s

-

3d

Ni

Phyws 168 (1992) 51-60

Cr

Cr(N0)4

(NO>4

Fig. 2. Molecular orbital diagrams for Ni(C0)4 and Cr(NO)+ Only the most important interactions between the fragment orbitals are indicated. Energy scale as well as relative position between Ni and Cr diagrams ISarbitrary.

The results obtained therefore indicate that XAS studies at the metal 2p edge are very effective in giving information on the key issue of d-n: backbonding in transition metal compounds. Let us briefly consider the intermediate member of the series, Fe(CO),(N0)2. While the presence of several structures is obviously related to the loss of tetrahedral symmetry, it is remarkable that both the total width of the spectral pattern and the total intensity are not just halfway with those of the extreme members of the series, again at variance with the photoelectron spectrum [ 7 1, which is most easily interpreted as splitting of the tz and e levels according to symmetry descent. The complexity and width of the spectral pattern may reflect the overlap of two structures, one associated with the CO ligand part, and the other to the NO part. Notably intensity does not vary linearly in the series either with formal d electron count, or with electron population, which are reported in table 2, both for ground and core hole states. The large increase in metal d population following core hole formation is remarkable. This amounts to a whole electron in Cr and Fe compounds, completely screening the metal charge due to the core hole. For Ni(CO), the d population increases by only 0.6 electrons, given the high occupation already present in the ground state. The charge appears to flow back from the p orbitals of the negatively charged oxygen atoms, and also from the N 2p orbitals in the presence of NO ligands. This gives

an indication of the strong relaxation which affects metal core hole formation in transition metal compounds, which is larger when the d shell is not completely filled, and which gives rise to intense satellites in the relative core photoelectron spectra. The possibility of corresponding satellites in XAS spectra has been suggested [ 22 1, but awaits further experimental and theoretical verification. We may comment here on previous results [ 51 obtained from EHT calculations employing the Z+ 1 (equivalent cores) approximation, which gave a much smaller relaxation effect. Apart from the crudeness of the approximation, employing just parameters for the next atom in the series, as seems implicit in the treatment of ref. [ 5 1, may not be entirely appropriate if charge independent parameters are employed for the diagonal elements H,,. Since the core excited electron lies in a virtual orbital of mainly ligand composition, outside a closed shell core, the physical situation is probably better described by the equivalent cores species corresponding to the valence ionized Z+ 1 atom, and energy parameter for the latter, closely related to valence ionization potentials, will be much larger. The drop in energy especially of metal 3d orbitals is then likely to give a significant relaxation of the valence orbitals. The variation of the metal atom in the series examined makes it difficult to ascertain which part of the observed intensity variation is due to molecular effects and which to the change in the atomic transi-

P. Decleva et al. /Chemrcal Physics 168 (1992) 51-60 Table 2 Mulhken population wavefunctions of Nt(C3H5L

Fc(CG)z(NG)z

Nt(C,H,)z

Pd(C&),

for ground and core hole SCF Fe(C0)2(N0)2, Cr(NO)+

and Pd(CA),

Ni(C0)4

Cr(NO),

analysis Ni(C0)4,

SCF (GS)

SCF (2~s’)

Ni(4s) No Ni(3d)

0.11 0.55 9.20

0.10 0.44 9.81

C (s) C(P) G (s) G (P)

3.60 2.24 3.80 4.39

3.58 2.21 3.82 4.24

SCF (GS)

SCF (2p-‘)

Fe(4s) Fc(4p) Fe(3d)

0.11 0.47 6.51

0.09 0.48 7.71

C C G G N N G G

3.65 2.26 3.81 4.31 3.74 3.51 3.89 4.30

3.63 2.25 3.82 4.23 3.80 3.15 3.93 4.06

SCF (GS)

SCF (2s~‘)

Cr(4s) Cr(4p) Cr( 3d)

0.03 0.95 4.51

0.01 0.89 5.53

N N G G

3.74 3.24 3.90 4.26

3.76 3.12 3.93 4.08

SCF (GS)

SCF (2p-‘)

N1(4s) Nr(4~) Ni(3d)

0.31 0.43 8.97

0.30 0.59 9.91

Cl(s) Cl(P) C2(s) C2(P)

3.23 2.71 3.22 3.06

3.24 2.72 3.26 2.80

SCF (GS)

SCF (3p-I)

Pd(5s) Pd(5p) Pd(4d)

0.10 0.15 8.90

0.15 0.34 9.46

Cl(s) Cl(P) C2(s) C2(P)

3.24 2.82 3.30 3.06

3.26 2.18 3.34 2.86

(s) (P) (s) (P) (s) (P) (s) (P)

(s) (P) (s) (P)

51

tion moments. The variation of the latter is however expected to be smooth along the transition series. Certainly absorption data on the 2p edge of free atoms, which is just beginning to be explored by synchrotron radiation [ 231, as well as accurate theoretical calculations including relaxation, correlation and relativistic effects, would be very helpful in this respect. In order to analyze intensity behavior associated with the metal change in species containing the same type of ligand we have considered the Ni and Pd bisally1 compounds and we have computed transition moments for the Ni and Pd atoms, relative to the 2p+ 3d excitation starting from the ‘D 3d94s ( 4d95s) initial state to the 3P, 3D, 3F final states corresponding to the 2p-‘3d”4s (3p-‘4di05s) and 2p-‘3d94s2 ( 3p-‘4d95s’) configurations, for the Ni and Pd atoms. The results are reported in table 3. It is remarkable that the (n - 1 )p-+nd transition strength drops almost by a factor of three in going from Ni to Pd, while the opposite behavior is obtained for the much weaker (n- 1 )p+ (n-t 1 )s transition. The p+s transition is about ten times less intense than the p-+d in Pd, and almost two orders of magnitude less intense in Ni. We have then considered the two bis-ally1 complexes of Ni and Pd, whose photoelectron spectra have been thoroughly investigated [ 7,241, in order to consider also the variation associated with a change of ligands on the central atom, in particular in the case of Ni(C0)4 and Ni(C3H5)2 complexes. The results obtained are reported in table 4 and fig. 3. In comparison to Ni(CO), total intensity is reduced by a factor of two in the ally1 compound, implying that d occupation is higher in the latter. This appears at fist surprising, as the bonding is considered to be stronger Table 3 Term values (eV

) and oscillator strengths for Ni and Pd atoms r

fX102

Ni

13.12 12.64 12.06 11.03

0.4018 43.9026 0.2285 0.0800

2p+4s 2p+3d 2p+4s 2p+4s

Pd

13.72 9.37 8.63 7.87

16.2871 0.6020 0.9872 0.3811

3p+4d 3p-5s 3p+5s 3p+5s

P. Decleva et al. /Chenncal Physics 168 (1992) 51-60

58 Table 4 Term values (eV) and oscillator

strengths

for metal 2p spectra of Ni(C3HS)*

a)

Pd(&HS)z

N~(C~HS)Z

T

d*+x*

b,

4.18 4.15

1.205 1.108

d*+a*

0.092 0.020 0.064

(d+s)’

a8

2.23 2.16 2.12

0.067 0.005 0.008

(d+s)‘+x*

0.026 0.245 0.356

(d+s)*+n*

a,

1.22 1.15 1.11 0.91 0.84 0.81

0.063 0.199 0.042 0.010 0.006 0.011

(d+s)*+n*

0.63 0.56 0.52 0.46 0.39 0.35 0.25 0.19 0.16

0.011 0.006 0.089 0.002 0.017 0.013 0.069 0.388 0.162

(d+s)*

T

fx 102

b,

4.66 4.42

0.009 3.451

4.21

3.489

a,

1.85 1.76 1.61

a,

1.15 1.02 0.89

a,

a,

b,

a,

‘) In the MO composition

d* and s* are the metal 3d and 4s ortntals

5i

Pd(V’5)2

‘E I

,i\

Ni(C$&

h il

I 7

5

fx 102

MO

MO

N-

and Pd(&HS)z

3

’

E(eV)

Fig. 3. Calculated term values (eV) and oscillator strengths for Ni 2p and Pd 3p excitation m Ni(C3H5)? and Pd(C3H5)*. Calculated lines are convoluted with Gausslans of 0.35 eV fwhm.

in the carbonyls, pushing the Ni metal closer to the filled d” configuration. This is indeed shown by population analysis of the ground state, reported in table 2. However relaxation is larger in the ally1 complex, and gives relaxed hole d population closer to ten. It is

(d+s)*+rr*

d*+x*

d*+x*

and r[* is the ligand 2~.

not completely clear whether intensity should be related to initial or final state d population, and although it seems more correct referring to the ground state ones, correlation appears here closer with relaxed core hole values. The larger amount of virtual d component present in Ni (CO), is probably a sign of the stronger d-x backdonation typical of carbonyls, which relieves the excess of d charge, favoring the 2p+ 3d excitation. The most striking difference in the two nickel compounds is however the change in the spectral pattern, with a single intense absorption band in Ni( C,H,), shifted to lower energy with respect to Ni(C0)4, confirming the strong influence of the type of ligands and the nature of the bonding on the spectra, already stressed in previous work [ 5 1. The pattern obtained may appear surprising, in view of the reduced symmetry, which splits d orbitals into five nondegenerate components, and could be expected to lead to several features widely spread like in the Fe( CO),( NO)* compound. In fact it may be easily understood in

P. Decleva et al. /Chemrcal Physics 168 (1992) 51-60

terms of an MO diagram for the bis-ally1 compound, reported in fig. 4. The Ni 3d levels lie slightly below the highest occupied x orbital (7a,) of the bis-ally1 skeleton, and are split by interaction with the lower lying a8 and b, ligand levels. A single strongly interacting xc*level (b,) is available on the ally1 moiety, through which backdonation is possible, giving a single b; virtual level of appreciable d character, available for the 2p+ 3d excitation. Two weak features are also apparent close to threshold, which are associated with the mixing of Ni 3d and 4s and ligand ~(a~) orbitals. The structure obtained for bis-ally1 palladium is extremely similar, apart from the reduction in intensity of the strongest feature by a factor of three, which is quite similar to the reduction in atomic transition moment already observed for the free atom. This implies that the degree of x backbonding is very similar in both compounds, despite the different ground state electronic configuration in Ni and Pd atoms. Also the term value of the transition stays almost constant, as is expected from the nature of the b: final state, which is predominantly composed of the rr* ally1 orbital. So the bonding situation appears very similar in both systems. Here the diversity of the metal orbitals appears instead in the occupied levels (since the d shell is almost completely filled) as is apparent from the

59

valence photoelectron spectrum which shows a distinct stabilization of the d manifold with respect to the HOMO 7a, ally1 orbital [ 241. In order to test the influence of additional correlation we performed a more extensive calculation on Ni ( C3H, )2, allowing further 1h- 1p excitations from the highest eleven valence orbitals to the lowest three virtuals. The results are essentially unchanged, apart from a modest reduction in the intensities (oscillator strength for the main transition is now 6.24X 10m2), and a shift to lower energies, which however affects only slightly the energy separations, and is attributed to an inconsistent determination of the corresponding threshold. In fact we have employed the same 2p-’ SCF value for the series limit, while a 2h-lp result relative to the same excitations employed for the transition energies calculations would have been more balanced, giving a lowered threshold. This points out the need of a carefully balanced treatment of both excitations and ionization if accurate term values are sought. At the lowest level this is certainly satisfied by 1h-lp CI with respect to the SCF 2p-’ energy. Although further investigation is certainly needed, the agreement between the two calculations makes us confident on the essential soundness of the data obtained at the present level.

4. Concluding remarks

3d

Ni

NiGJ&.)~

G+Jz

G3M

Fig. 4. Molecular orbital diagram for Ni(C3H5)2. Only the most important interactions between the fragment orbitals are indicated. Energy scale is arbitrary.

The results obtained from the present calculations support and extend the general conclusions reached in a previous experimental study [ 51 about the usefulness of absorption spectroscopy in the region of the 2p edge in transition metal compounds. The spectra show strong sensitivity to the nature of the ligands and the bonding situation, rather than being primarily dependent on the local symmetry or formal number of d electrons. The spectral pattern gives important information on the d orbital content of the virtual states, notably on the extent of backbonding in ICligand complexes, and can be rationalized by appropriate MO interaction diagrams. A relatively simple ab initio treatment, provided due care is paid to basis set selection and appropriate treatment of the strong relaxation present in such systems, appears adequate for a detailed interpretation of these spectra giving

60

P. Decleva et al. /Chemrcal Phyncs 168 (1992) 51-60

access to a thorough analysis of a wide range of complex systems.

[ lo] A.K. Rappe,

T.A. Smedley

and W.A. Goddard,

[ 121 S.P. Walch, C.W. Bauschlicher Acknowledgement

Thanks are due to CNR and MURST of Italy for financial support. Free computer time on the CRAY Y-MP/864 of CINECA (Bologna) is gratefully acknowledged. The authors thank Professor M. Ohno for useful discussion.

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