Transition metal 2p excitaton of organometallic compounds studied by electron energy loss spectroscopy

ChemicalPhysics 147 (1990) 51-63 North-Holland

Transition metal 2p excitation of organometallic compounds studied by electron energy loss spectroscopy A.P. Hitchcock,

A.T. Wen and E. Rilbl

’

Department of Chemistry, McUaster University, Hamilton, Ontario, Canada L8S 41Ul Received 16 April 1990

Oscillator strengths for Mn, Fe, and Co 2p excitation of dicyclopentadienyl, carbonyl and mixed ligand complexes have been derived from inner shell electron energy loss spectra (ISEELS) recorded under electric dipole scattering conditions. The spectra are found to be surprisingly sensitive to the identity of the ligands and/or the character of the metal-Bgand bonding and apparently insensitive to the molecular symmetry or even the d count of the metal atoms. A simple model in terms of the relative energies of unoccupied orbitals of large 3d character involving in-line (“de) and off-axis (“da?) interactions with ligand orbitals is proposed. Extended Htickel (EHT) calculations have been used to aid assigmnent of the observed spectral features. The EHT results suggest that the metal 2p spectra should be more sensitive to the molecular symmetry than is actually observed. The 2p spectra are compared to the metal 3p spectra of the same species and to the metal 2p spectra of the pure metals and various halide and oxide compounds.

1 . Introduction

Inner shell excitation of transition metal carbonyls and metallocenes has been the subject of several recent studies both by inner shell electron energy loss [ l-3 ] and by X-ray absorption spectroscopy [ 4-61. Studies to date have focused on the ligand K edges (C Is, 0 Is). Although these are predominantly characteristic of the ligand, there are aspects sensitive to the metal-ligand bonding. Thus, in transition metal carbonyls [ 1,3] the oscillator strengths of C ls+x* transitions appear to decrease in proportion to the extent of px-dx backbonding. In metallocenes [ 21 the C 1s spectra show an additional lowlying feature relative to that expected for the free cyclopentadienyl ligand which is assigned to C 1s4e,, [M 3d,,] excitations. Electron density in this metal-ligand anti-bonding orbital is mainly local&d on the metal atom. This transition is observed in C 1s excitation because of the mixing of the metal 3d and carbon 2p, orbitals that occurs in the dx-px bonding ’ Permanent address: Institut fur Physikalische und Theoretische Chemie, Freie Universitit Berlin, Takustrasse.3, D-1000 Berlin 33, Germany.

interaction. The intensity of this feature decreases systematically in the order Fe.+Co+Ni, corresponding to the successive filling of the 4e1, orbital. In our more recent work [ 7,8] we have been examining the ligand core excitation spectra of complexes containing several different types of ligands (e.g., carbonyls and cyclopentadienyls) #‘. Comparisons with the spectra of species containing a single type of ligand have indicated that, although the positions and intensities of most features can be explained in terms of independent ligand contributions, the deviations from a simple additivity model provide information concerning the ligand-ligand interactions mediated by the metal atom. Thus, evidence is being accumulated that ligund core excitation spectroscopy is a useful and potentially sensitive probe of bonding in organometallic complexes. In this paper we focus our interest on transition metal 2p excitations in organometallic compounds. Through studies of several series of 18-electron car-. bony], cyclopentadienyl and mixed ligand complexes we have found that the metal 2p spectra are remarkably sensitive to the identity of the ligands. Extended *I Ref. [7]onCospecies,ref.

0301-0104/90/S 03.50 0 1990 - Elsevier Science Publishers B.V. (North-Holland)

[8]onFespecies.

52

A.P. Hitchcock et al. / Transition metal Zp excitation of organometallic cornpow&

Htickel calculations are used to assist the spectral assignments and to rationalize the spectral shapes. The results indicate that core excitation at the metal atom provides information complementary to that obtained from ligand core excitation. The sensitivity of transition metal inner shell excitation to the chemical environment of the metal is discussed in relation to the metal 2p spectra of solid metals [ 9,10 ] and inorganic metal compounds [ 111.

Mn,
2. Experimental The experimental apparatus and procedures have been described previously [ 12,13 1. Briefly, the energy distribution of a fast electron beam is measured after inelastic collision with the gas phase target. Electrons with a kinetic energy of 2.5 keV after collision which are scattered by 2 ’ are collected and analysed in a hemispherical electron energy analyser with an overall resolution of 0.7 eV fwhm. Under these scattering conditions electric-dipole-allowed eleo tronic transitions dominate. The samples were obtained commercially (Strem Chemicals) and were used without further purification. The low vapour pressure of the solids made it necessary to place many of the samples in an internal celI directly attached to the interaction region which was heated to 50-80°C with circulating water. The spectra were converted to absolute oscihator strengths using procedures described earlier [ 14 1. The metal 2p core excitation/ ionisation signal is isolated by subtracting a smooth background extrapolated from the pre-edge region. It is then normal&d to atomic oscillator strengths in the far continuum (25 eV above the La IP in each case), where the core ionisation oscillator strength is essentially that of the free atom. Due to the low intensity at these relatively high energy losses and associated difficulties with reliable background extrapolations we estimate the uncertainty in the absolute intensities to be about 30%. 3. BesuIts 3.1. Spectra Fig. 1 presents the Mn, Fe and Co b,, oscillator strength spectra of Mn2(CO)l(h Fe(CO), and

j! k..

-1.

7go

_.

-1.

_.

.’

780 Energy

_.

.1

Bl CeV)

Fig. 1. Oscillator strengths for Mn Zp, Fe 2p and Co 2p (L& excitation of Mn2(CO)Io, Fe(CO), and Co2(CO)8, aligned at the main b feature. These oscillator strengths were derived from electron energy loss spectra recorded under dipole-dominated conditions (2.5 keV f4 electron energy, 2’ scattering angle and 0.7 eV fwhm resolution). The hatched lines indicate the location of the 2p IPs as determhed by XP!3. The indicated oscillator strengh for Mq(CO),,, and Co,(CO), are per metal atom, i.e. halfthat of the full molecule.

Co2 (CO) s, aligned at the most intense L3 excitation feature. The energies, term values and proposed assignments for the features in the metal 2p spectra of the Fe and Co carbonyls are presented in table 1. The corresponding information for Mn2 (CO ) r0as well as for Mn( CO)& has been given previously [ 11. The locations of the L-, and L2 ionisation potentials, as determined by XPS [ 15 ] are indicated in this fgure by the hatched lines. As expected, the Ls and L2 spinorbit components of the spectrum of each carbonyl species are similar in shape. Each exhibits a main peak about 5 eV below the IP along with a weak low energy shoulder. The b features are always broader because of the reduced core hole lifetime associated with the bLsM Coster-Kronig process.

53

A.P. Hitchcocket al. / Transitionmetal 2p excitationof organometalliccompounds Table 1 Energies (E, eV), term values, (T, eV) and proposed assignments for features in the metal 2p spectra of Fe(CO)S and -(CO), Assignment final orbital

Coz(co)s

Fe(COh

Co 2P,lZ

Co 2P,,,

Fe 2p,/z

Fe 2~3,~ E

T

E

T

E

T

E

T

109.7 711.8 ” 713.8 715.8

6.1 4.0 2.0

122.4 124.3 725.9 728.4

6.0 4.1 2.5

780.6 782.8 .’ 784.7 786.34

5.1 3.5 1.6

795.1 797.5 798.5 801.0

5.9 3.5 2.5

*) CalibrationzFe(C0)~: +178.0(2) eVrelativeto0 Is-# in CO1 (535.40 eV). b)TheFeandCo2pa,21PsarefromXPS [15].The2p,,, excitation pairs to the 2p,,, IPs.

“dx*” “do*” Rydberg IP b’

(533.82 eV) inFe(CO)S [S]. C&(CO),: +247.5(2) eVrelativeto0

IS--M?

IPs were estimated by adding the average spin-orbit splitting for related

A number of trends are observed in these spectra which are basic to metal 2p spectra of atoms, molecules and solids. The relative intensity of corresponding L3 and L features is approximately two, which is that expected from the statistical degeneracy of the La and L2 ion cores in the absence of atomic multiplet effects [ 10,111 (see also section 4.3 ) . The spinorbit splitting increases with atomic number. The absolute intensity decrease s from Mn+Fe-Co, i.e. with increasing transition energy. This may be related to the decreasing spatial extent of the core hole, as well as to the reduction in the partial density of unoccupied 3d states with increasing d count. Counteracting these trends is the shift of the molecular counterpart of the atomic “3p+ 3d” giant resonance [ 16 ] to lower energy (term value) with increasing 2 which sharpens and intensifies the near edge features. Fig..2 presents the spectra at the metal b3 edges of the Fe and Co metallocenes. The energies, term values and proposed assignments for the spectral features are presented in table 2. The metal 2p spectra of the two metallocenes exhibit two pairs of well-resolved resonances below each ionisation limit. Relative to the metal 2p spectra of the carbonyls, the metallocene spectra exhibit a larger splitting; the discrete structure spans a larger energy, and it is the lower energy rather than the higher energy band which has the higher intensity. The metal 2p spectra of the mixed ligand species, and (Cx= 1,3_cyclohexadiene) CxFe(CO)s CpCo ( CO) 2 ( Cp = cyclopentadienyl ) are presented in fig. 3 while the energies, term values and proposed

~‘~1 x I....,

700

.

800 Energy

.

..I

.

.

&!o W)

Fig. 2. Oscillator strengths for Fe 2p and Co 2p excitation in ferrocene and cobaltocene. See the caption to fig. 1 for experimental details.

assignments for the spectral features are presented in table 3. As with the carbonyl and metallocene species, these spectra exhibit transitions to two groups of states below the L3 and L2 edges. The splitting and the intensities of the lines differ from that seen in the

A.P. Hitckcock et al. / Transition metal 2p excitation of organometallic compounds

54

Table 2 Energies (E, eV), term values (T, eV) and proposed assignments for features in the metal 2p spectra of Fe(Cp), and Co(Cp)r ( Cp = cyclopemadienyl )

FdCph

Assignment final orbital

WCP)z

Co 2P,,2

Fe ~PW

Fe 2~312

co

ZP,,,

E

T

708.3

4.8

720.7

5.0

779.7

5.1

794.3

5.1

“dxW

710.8 713.6 713.1

2.2 -0.5

723.4 726.5 725.7

2.3 -0.8

782.4 785.4 784.8

2.4 -1.4

797.0

2.4

“de double exit. IP b)

T

E

T

E

E

T

799.4

‘) Calibration: Fe(Cp)2: + 172.9 eV relative to 0 ls+n* of C02. Co(Cp),: +244.3 eV relative to 0 IS-U? in C02. w The 2ps,, IPs are from XPS [ 151. The 2~,,~ IPs were estimated by adding the average spin-orbit splitting for related excitation pairs to the Zp,,, IPs.

1s -

(1. 3-CgHaIFe
;

L3 c

h ‘a>

a 7 Q z sF‘ c

L2 B

:, o-

I . . . . I . - .

I

780

.

800 Energy

.

1

.

820

(eV)

Fig. 3. Oscillator strengths for Fe 2p and Co 2p excitation in iron cyclohexadienyl tricarbonyl and cobalt cyclopentadienyl dicarbonyl. See the caption to tig. 1 for experimental details.

2p spectra of either of the single l&and species although the spectra of the mixed ligand species are closer to those of the carbonyls than to those of the

metallocenes. In both the Cp and mixed species the drop in absolute intensity between the Fe and Co species is greater in the Cp and mixed ligand complexes than in the carbonyl complexes. The difference in the metal atomic number dependence of the core excitation intensities shows that the 2p spectral intensities depend on the details of the metal-l&and bonding as well as on the contraction of the metal 2p orbital. These experimental observations suggest that the nature of the attached ligand and/or the type of ligand-metal bonding may be more important than symmetry or metal d count in determining the shape of metal 2p spectra of organometahic complexes. This seems to be an unexpected result. 2p+ 3d excitations at the metal atom should dominate metal 2p spectra. Thus the spectral appearance should be rather sensitive both to the d-electron count which, in a formal electron counting sense, varies from ds( Mn) to d’( Co) in these species, and to the ligand field splitting of the unoccupied 3d orbitals which should have a pattern that varies with the molecular symmetry. The experimental metal 2p spectra of these compounds do not reflect these factors in any obvious fashion. Instead the spectral shape appears to be primarily related to the nature of the attached ligands. The spectra of the three cat-bony1 species are similar to each other and the ferrocene and cobaltocene metal 2p spectra are alike. Those of CpCo( CO), and CxFe(CO)s are also rather similar to each other, in each case being intermediate between the M 2p spectra of the carbonyl and cyclopentadienyl complexes. Rationalisation of the strong ligand sensitivity of the

55

A.P. Hitchcock et al. / Transition metal 2p excitation of organometallic compounds

Table 3 Energies (E, eV), term values (T, eV) and proposed assignments for features in the metal 2p spectra of CxFe(CO)a (Cx= 1,3cyclohexadiene ) and CpCk ( CO)2 CxFe(CO)3

Assignment final orbital

wwCO)2

Fe 2~112

Fe 2p,/,

Co 2P,/2

Co 2p,/2

E

T

E

T

E

T

E

T

710.1

4.4

722.6

4.4

780.2

6.0

794.9

6.0

“dn+”

711.8” 713.1 714.5

2.6 1.4

724.3 725.4 727.0

2.7 1.6

782.4 786.0 786.25

3.8 0.3

797.0

3.9

“do*” Rydberg IP b)

800.9

*) Calibration: CxFe(CO)3: + 177.5(2) eV relative to 0 ls+nS(CO) of same species [8]. COCOS: +248.6(2) eV relative to the 0 ls-&(CO) ofsamespecies [7]. w The Co Zp,,, IPs are from XPS [ 15 1. The Fe 2p3,2 IP was estimated from the Fe 2~312IPofC,HsFe(CO)3 [15].The2p,,,IPswere obtained by adding the average spin-orbit splitting for related excitation pairs.

metal 2p spectra is the goal of the remainder of this paper. 3.2. Extended Hikkzl calculations In order to gain insight as to the origin of the strong discrete resonances we have performed extended Hiickel calculations (EHT) of both the ground state and the metal core excited states of these species. For the latter, the equivalent ionic core virtual orbital model (EICVOM [ 171) ( (Z+ 1) analogy) is used to account for core hole relaxation and thus to give more accurate eigenvalues of the core excited states. For example Fe* (CO) 5 core-excited at the iron atom, is calculated as Co ( CO ) : . According to the EHT results the core hole relaxation at the metal atom can be quite large, although it depends on the metal atom involved. The difference between the ground state and Co excited state orbital energies is less than 1 eV (see table 4). However the EHT-predicted relaxation with the core hole at an iron atom is much larger. As an average over all unoccupied orbitals with appreciable metal 3d content there is 2 eV stabilisation in ferrocene and 1.5 eV in iron pentacarbonyl. By way of contrast, the el,(d,,) orbital energy decreases by only 0.5 eV in the equivalent core species corresponding to carbon 1s excited ferrocene while the e2,, and e2, ligand R* orbitals are stabilized by less than 0.2 eV [ 21. Thus it is important to use the equivalent core approximation in these calculations although it is possibly less important for metal core excitation

than for ligand core excitation since there is less change in the orbital character. The Hilckel parameters used for Mn, Fe and Co are those reported by Albright et al. [ 18 1. It is possible that the large difference between the EHT-predicted core hole relaxation of Fe and Co may be related to the uneven metal 3d orbital energies in this set of parameters. The fixed geometries used in the calculation are the equilibrium gas phase values where those have been reported [ 19,20,22-241. In some cases the gas phase geometry is poorly known. Co2( CO), is believed to exist as two non-bridged forms in equilibrium [ 2 11. In such cases we have explored the geometry dependence of the results by carrying out the Hiickel calculation for a number of plausible geometries. In the case of Co2( CO), the details of this investigation are reported below. The negative energy eigenvalues of the virtual molecular orbitals (MO) reported by the EHT calculations are interpreted as the term values for the M 3d+MO excitations. The sums of the squares of the coefficients for the M 3d atomic orbital contributions to the virtual MOs of the (Z+ 1) ionic species are taken as estimates of the relative intensity of metal 2p excitations to each unoccupied MO. A rationalisation for this simple procedure for estimating core excitation transition intensities has been presented earlier [ 251. Although it provides an adequate estimate of the shape of the spectra and the relative intensities of spectra of different compounds with the same metal atom, one needs to consider the

56

A.P. H&kcock et al. / Transition metal 2p excitation of o~~o~t~Iic

cornpow&

Table 4 Results of EHT calculations *) for cobalt compounds species

MO

Co(Cp,C’

CPQ(CC%~)

Cq(CO),

l)

e,

EHT c&u&ions

3d char.

Ground state

Co* excited state (Z+ 1) +

E

xc:,

-6.4 -6.6 -9.7

0.08 0 0.64

E

EC:,

other char. b,

-6.3 -6.5 -9.5

0.09(X2) 0 0.62(x2)

n*(CP_) @(CP’) -

0.04 0.04 0.41 0.36 0.18 0.35 0.22

-6.4 -6.4 -7.5 -8.4 -8.5 -8.7 -9.1

0.06 0.04 0.45 0.35 0.16 0.34 0.14

rc*(cp) 1.5 fl(cp) 1.5 +(Co),*(CP) x*(cO) 0.49 ti(CO) 0.57 @(CO) 0.39 *(CO) 0.28

ea

xy, x2-9

et, ela

=, P

a2 aI bz ai

xy x2-yZ yz x2- yz, .z2

a2

w

bl bz

xz YZ

-6.5 -6.5 -7.8 -8.6 -8.1 -8.9 -9.2

b 11 es b ; es b28

x2- y2 X2, yz x*-y2 x.G YZ =, YZ V

-4.73 - 8.28 -8.52 -8.69 -8.71 -9.09

0.37 0.09 (0) (0) 0.04 0.20

-8.06 - 8.43 -8.58 - 8.70 -8.79 -9.15

0.319 0.008(x2) 0.043 (0) 0.005(x2) (0)

Z(CO) rz*(CO) KJVco) 11c(CO) x*(CO) Ic*(co)

:,

x*-y2 a-z, Yx

-9.11 -9.33

0.08 0.01

-9.39 -9.43

0.066(x2) 0.047

K”(co) ?F(CO)

aIs bzs

z* xy

-9.24 -9.21

0.09 0.08

- 9.47 - 10.16

0.016 0.108

r*(Co) n*(co)

carried out with Albright parameters [ 18] for Co and Ni, using the EICVOM procedure for the core excited states

r171. ‘) The n* chkracter listed in this column is the sum of c$ for one C and one 0 (for x*(CO) ) and for all carbons in the case of r*(Cp). ‘) Co(Cp)2geometry (D&, seeref. [19]. dl CpCo(CO)lgcometry (apptwimately C&), see ref. [20]. Cl -(CO), geometry (D& This is the lower energy of two non-bridged isomers proposedto exist in the gas phase 1211. Its EHTpredicted spectmm is the one in best agreement with experiment. All of these orbitals have *(CO) ckracter.

intrinsic atomic matrix elements ( ( 3d [ p J2p) ) in order to compare complexes with different metal atoms. In principle both 2p-+ 3d and 2~44s transitions at the metal atom are electric dipole allowed and thus might be important. However we have neglected the 2p-r4s component in this treatment. Generally one assumes that (rls]p]2p)dj:(3dlp]2p). Althougb this is certainly true in the 2p continuum [ 261 it may not necessarily be the case in the pre-edge region. Nevertheless, the 4s contribution to the low lying virtual orbitals predicted by the EHT calculations is very, very small such that (4s Ifi/ 2p > would have to be several orders of m~itude larger than ( 3d I p I 2p) for 4s to make a similar contribution to the 3d component. The results of the EHT EICVOM calculations for

the ground and Co-excited states of the Co species are presented in table 4. Corresponding results for the C 1s and 0 Is discrete excitations in the Co and Fe species are being reported elsewhere [ 7,8]. In order to more easily compare the EHT and experimental results we have constructed simulated metal 2p spectra by adding Gaussian lines of 2 eV fwhm located at the eigenvalues. The intensity (area) of each line is the sum of the squares of the 3d A0 coefficients. Fig. 4 compares the El-IT-stimulated spectra to the experimental L3 discrete structure in the three Co species. In this case the EHT calculation seems to be reproducing the experimental trends, aside from poor reproduction of the low energy shoulders in the CpCo(CO)zandCo2(C0)sspectra.Thereisalarge, relatively rigid shift of about 5 eV between the EHT

57

A.P. Hitchcock et al. / Transition metal 2p excitation of organometallic compound

Term

Value

(eV)

Fig. 4. Comparison of experimental L:,spectra (on a term value scale, TV=IP-E) and spectra simulated from the EHT results by adding Gaussians of 2 eV fwhm, intensity given by the sum of the squares of cM at positions given by the negative eigenvalues of the virtual orbitals in an EIVCOM calculation (see table 4). The vertical lines indicate the component lines.

and experimental energy scales similar to that observed in our earlier work on the C 1s spectra of the metallocenes [ 2 1. Aside from this there is reasonable agreement between the experimental and calculated spectral shapes. In particular the shift between M-Cp and M-CO to higher energy (lower term value) of the centroid of the discrete excitation is replicated in the calculations, as is the rough splitting and relative intensities of the two groups of states. This suggests that, even though multiplet effects are not being considered, the EHT molecular orbital description may be a useful tool for interpreting metal 2p spectra of organometallic compounds. From our experimental results we believe that the shape of the metal 2p spectra of covalent complexes with low and/or similar ligand fields should be characteristic of the ligand but not strongly dependent on either the metal atom or the geometric arrangement of ligands. The very close similarity between the Mn 2p spectra of Mn2(CO)io and BrMn(CO), [l] is further indication that even the addition of a second metal atom and a direct metal-metal bond is not sufficient to modify the basic dependence of the 2p spectra of the nature of the metal-ligand bonding.

However, the EHT results presented in Iig. 5 tell a very different story. This figure compares the results for several different geometries of Coz (CO) 8 [ 231 along with those for several other metal carbonyl species (HMn(CO)5, Fe(CO)5, HCo(CO),). Some of the numerical results are presented in table 5. In contrast to our experimental observations, these EHT results suggest that metal 2p spectra of transition metal carbonyl compounds should be strongly dependent on both the metal species and the detailed geometry of the complex. Interestingly although the EHT-predieted Co 2p spectra of HCO(CO)~ and Co2(CO)s differ and that of Coz (CO ) 8 varies considerably with geometry, the EHT-predicted spectrum of Co2( CO), that is in best agreement with experiment is that of a Ddh geometry which is believed to be the lower energy conformation in solution or matrix isolation and thus likely to be the most stable in the gas phase [ 2 11. The discrepancy between the experimental and EHT results could be resolved in several different ways. It could be related to the inadequacy of the EHT

------++----,, ,.,,,,,.,,,,,.,,,,,.,., 15

10

5

0

-5

”

Term Value

15

10

5

0

-5

&‘I)

Fig. 5. Plots of metal 2p spectra of various metal carbonyls predicted by EHT EICVOM calculations (see table 5). The trial geometries used for Co,( Co), are: C, is that found in the solid state.; C,( 1) is a symmetrised version of the same d&bridged structure; C,,(2) and C*(3) are related structures containing small distortions of the C-Co-Co an&s from the C,( 1) structure;and Dr.,, Dlb are two non-bridged structurespostulated to be the conformers in equilibrium in solution or in matrix isolation (thus similar to that expected in the gas phase).

58

A.P. Hitchcork et al. / Transition metal 2p excitation of organometallic compouruh

Table 5 Results for EHT calculations (EIVCOM) of the 2p excitation of Mn, Fe and Co carbonyl complexes in various geometries Molecule

Molecular orbital d char.

Fe(CO)S b,

ai e’ e’ e” b, aI br e e aI

HCo(C0)4=’ (Cd”)

22 ZY. x2-Y2

zy, x*-y2 xz, YZ x2-y* Z2

w xz, YZ xz, YZ Z2

b, e aI

x2- y2 xz, YZ 22 xz, YZ

$

w C”

C A B, C A, B B C A C C A

I&

*)

E -5.08 -8.23 -8.73 -9.04

0.710 0.290(x2) 0.013(x2) 0.333(x2)

-0.78 -2.89 - 7.90 - 7.92 -8.31 -8.82

0.94 0.84 0.46 0.28(x2) 0.092(x2) 0.006

-3.5 -8.2 -8.4 -8.7 -8.8

0.74 0.13(x2) 0.30 0.06(x2) 0.35

-4.13 -4.84 -7.60 - 8.65 -8.78 -8.97 -9.07 -9.43 -9.59 -9.90 -10.13

0.044 0.025 0.019 0.012 0.016 0.033 0.020 0.061 0.049 0.122 0.049

a) The “intensity” listed is the sum of the squares of the 3d A0 coe.fficients for the indicated orbitals. Only those virtual orbitals with a 3d contribution greater than 0.005 are listed. w Geometry from ref. [ 22 1. c, Geometry from ref. [ 23 1. d)Geometry from ref. [ 271. This di-bridged structure is a close approximation to that of the solid state as determined by X-ray crystallography. =) Orbitals labelled A contain mostly contributions from the z2, x2 - yz AGs, those labelled B contain mostly xy AOs, while those labelled C contain mostly xz and yz AOs.

calculations and/or the neglect of multiplet effects. In this regard, we note that, relative to a recent discrete variational Xcx (DV-Xa) calculation of iron pentacarbonyl [ 28 1, the EHT orbital energies for the Fe-excited species are about 5 eV lower (in part because of core hole relaxation), but the relative energies and Fe 3d content of corresponding orbitals are very similar. An alternative explanation of the apparent discrepancy between our experimental and the EHT results is that the range of compounds studied

to date is not sufficiently broad to detect the intrinsic variability of metal 2p spectra. A wider base of experimental results and more sophisticated quantum calculations are both required in order to resolve this discrepancy.

A.P. Hitchcock et al. / Transition metal 2p excitation of organometallic compounds

4. Discussion 4. I. A qualitative model The dominant observation in this work is that on average, the virtual orbit& with large 3d contribution are lower in energy in the metallocenes than in the carbonyl complexes. This may be understood in a simple “chemically intuitive” fashion in terms of the relative magnitudes of x/x* and o/g splittings (see fig. 6 ) . If all other factors are similar (group orbital energies and sizes) then there is a qualitative expectation that a x/x* splitting is generally smaller than a o/cP splitting because the latter arises from stronger, along-bond interactions. Thus organometallic complexes with only x l&and-metal bonding, such as the metallocenes, would be expected to have the virtual MOs of appreciable M 3d content at lower energy than organometallic complexes with a large o ligand-metal bonding, such as the metal carbonyls. Although this simple concept ignores many of the common themes in descriptions of the electronic structure of transition metal compounds, such as crystal field splitting and atomic/molecular multiplets for the open shell configurations, it has some support in recent discussions of the L,, X-ray absorption spectra of 3d0 compounds [ 29 1. In that work the doublet character of the metal L3 and LZ spectra of a number of octahedrally coordinated oxide and fluoride compounds was interpreted in terms of tzg-e, splitting of the classical crystal field model, modified by atomic-like multiplet effects. The connection to the present interpretation is that, in all of these o-like bonding cases (FeTiO,, ScZ03, ScF3 and CaF2) the

Fig. 6. Sketch of the relative energies of molecular orbitals resulting from on-axis (do-pa) and off-axis (dn-pr ) interactions. The strongeron-axis interaction results in the “da*” orbital occurring at higher energy than the “dP orbital.

59

higher energy feature of e, symmetry in Oh (which would be labelled “do*” in the present qualitative description) is more intense (and broader) than the lower energy feature of tlg symmetry in 4. As the authors conclude [ 29 1, the higher energy and greater breadth of the e, feature relates to the fact that the dZ2and dxz_,,2 orbitals of e, symmetry point at, and thus have larger overlap (hybridisation) with, the ligands. One phenomenological difference between the present results on d’-d’ complexes is that, in these spectra, the higher energy component of the Ls speo trum is not noticeably broader than the lower energy component. However, the observed spectra are the result of a complicated overlap of a number of states with different bonding character so it is not surprising that the details of these spectra differ from those of the more simple do complexes. According to the simple model we have proposed, the aromatic (Cp) contribution (“drr*“) occurs at lower energy (or higher term values), whereas the CO related orbitals (“do*“) are at higher energy. As outlined in fig. 6, the relative energies of these components reflect the fact that there is more efficient metal 3d electron overlap with carbonyl ligands than with Cp ligands. This is consistent with the orbital energies involved. The average of the EHT-calculate ICand n* energies of ground state Cp- is - 9.3 eV while that for CO is - 12.4 eV. Thus t(2pa) of CO is much closer than e( 2~7~)of Cp- to the 3d orbital energies (Mn (-11.6eV),Fe (-12.7eV),Co (-12.1 eV), based on the Albright EHT parameters [ 181) and thus there will be a stronger interaction. The resulting competition of different ligands for bonding/antibonding interaction with the metal 3d orbitals could be related to ligand basicity, as well as size effects of ligands with respect to orbital overlap. These effects in turn may be related to the different kind of ligand binding to the central metal, i.e. ICversus o interaction and corresponding bond length arguments, which complement the spatial and energetic overlap arguments. Recently the Fe Lr3 spectrum of iron pthalocyanine has been studied experimentally [30] and through crystal field calculations [ 3 11. That spectrum is consistent with the qualitative model proposed here in that a doublet structure is observed, i.e. there are both “dx” and “do*” features with the higher energy “do*” feature being more intense, con-

60

A.P. Hitchcock et al. / Transition metal 2p excitation of organometallic compounds

sistent with the geometric structure. The theoretical study [ 3 I] attempted to derive a description of the ground state as an equilibrium between quintet and triplet spin states by matching multiplet calculations of line shapes and the La/L2 branching ratio with the observed spectrum. Although a conclusive answer was not obtained, the complexity of b, spectra of openshell systems is well illustrated. The concept that we have proposed, i.e. a simple relationship between the energies of core excited states and the separation of bonding/anti-bonding orbital pairs, is closely related to the “weak bond” effect which we have discussed earlier [ 321. It would suggest that it may be possible to derive the average degree of x or o character of the metal-l&and bonding from metal 2p spectra. Of course, detailed correlation of this simple concept with more sophisticated MO treatments of the electronic structure of these complexes [ 27,29,32-341, acquisition of the spectra of a wider range of compounds, and rigorous tests of the predictive powers of this concept are needed before concluding that ligand sensitivity is the dominant aspect of the metal 2p spectra of non-ionic organometallic compounds. In particular the disagreement with the EHT results is disturbing and suggests that an intrinsic sensitivity of the metal 2p spectra to the overall molecular symmetry is being masked, perhaps by our experimental as well as the fundamental (natural linewidth) resolution limitations of the spectroscopy. High resolution X-ray absorption studies [29,35] of these species (in either the solid or gas phase) would be useful in this regard. 4.2. Mixed &and complexes: additivity? The metal 2p spectra of CpCo( CO)z and CxFe(CO)% Cx= 1,3-cyclohexadiene (fig. 3, table 3) are somewhat similar in shape to those of the carbonyls but the lower energy peak has greater intensity. Since the lower energy peak is more intense in the cyclopentadienyl complexes while the higher energy feature is more intense in the carbonyls, we expect that the lower energy feature will be related to 2p excitation to orbit& of mainly metal-Cp character and the higher energy feature to orbitals of mainly metalCO character. This interpretation is supported by the EHT calculation of CpCo (CO), (table 4) which indicates that the b2 orbital at -7.5 eV, which is the

7

~....l.‘.-l’.--l-.‘.l-.’

$ 8: Q

1

cp COCCO),

:

Ir-5780.._. 790

000

Energy

Cd)

I . . . . I .._.

770

I..,

010

Fii 7. Comparison of the experimental Co I43 spectrum of COCOS with a simulation which is the sum of one-quarter of the Co L, spectrum of dicobaltoctacarbonyl and one-half of that of cobaltocene.

lowest energy orbital with appreciable Co 3d character, is mixed mainly with n*( Cp) while ah of the higher energy unoccupied orbitals of appreciable d character are mixed with x* (CO). Therefore we assign the lower energy shoulder in the metal 2p spectra of the mixed ligand species to excitations to orbitals having x* (Cp ) character. This interpretation suggests that the metal 2p spectra of mixed ligand complexes could be considered in some sense to be a weighted sum of spectra individually characteristic of M-Cp and M-CO bonding. We have explored the applicability of this additivity concept by comparing the COCOS spectrum to the weighted sum of one-half of the cobaltocene and onequarter of the cobalt carbonyl Co Lz3 spectra (fig. 7 ) . As expected, the simulated spectrum is in semiquantitative agreement with experiment, although the simulated spectrum shows a distinct splitting into two bands whereas there is only a low energy shoulder in the experimental spectrum of CpCo (CO), which is more intense but less well resolved than that found in the Co Lz3 spectrum of Co2(CO)s. 4.3. Comparison to related core excitation spectra of metals and metal compounds The metal Lj spectra of solid pure metals and metallic alloys, recorded at resolutions comparable to

A.P. Hitchcocket al. / Transitionmetal 2p excitationof organometaliccompouna%

that employed in the present work, exhibit a single broad band (the 2p33,2+3d “white line”) for all elements [ 9,10 1. Thus there is a dramatic difference between the 2p spectra of metals and those of organometallic compounds. At higher resolution [ lo] it is found that this band has considerable multiplet fine structure which varies greatly and systematically with increasing d count and the detailed environment of the metal atom (e.g., compare metal oxide and metal halide spectra [ 111) . A single band at low resolution (which exhibits multiplet fine structure at higher resolution) is also observed in the 3d spectra of the lanthanide series [ 36 1. In this case the 3d+4f transition reflects the systematics of the filling of the f orbital. The trends in the spectra of transition metals are considerably different from those of the metal 2p spectra of organometallics. Instead of a strong sensitivity to the metal atom we find a chemically perturbed 3d electron shell, which is.influenced predominantly by the nature of the attached ligands, i.e. the spectra of species with the same ligands look almost identical and there is no obvious dependence on the metal species or the formal electron count of the metal. In the spectral reported in this paper the ratio of the intensity of corresponding L3 and L features is approximately two, which is consistent with the statistical degeneracy factors of LS and Lz singly ionised cores. However, previous studies of the b, spectra of pure metal and metal halides/oxides have found LJ Lz intensity ratios which are very different from 2 [ 9111. In particular they are much smaller in the early (e.g., < 1 in Ti) and much larger in the late transition elements ( % 3 in Ni) . The explanation of non-statistical LS/Lz intensity ratios has been a theme of b3 spectroscopy for about ten years. A strong multiplet dependence of the LS and Lz intensities accounts for this effect in the metals and metal halides/oxides [ 10,111. In open shell systems the ratio has been proposed as a means of determining the composition of mixed-spin configuration ground states [ 30,37 1. Perhaps the b3 spectra of organometallic species have L3/Lz ratios close to statistical because the multiplet effects are much smaller. Alternatively it may simply be the case that the Mn, Fe and Co complexes we have examined are sufftciently close to the centre of the transition metal series that the intensity ratios are naturally around 2 (although the LJL, intensity ratio for metallic iron is 3 [ lo] ).

61

The K-shell core excitation spectra of many 3d transition metal complexes, including metallocenes [ 381 and other organometallic compounds [ 391, have been studied in recent years. The most intense features are related to 1~4~ excitations and thus are somewhat less interesting with regard to probing the bonding However, a low energy “ls+3d” feature is often observed. The intensity of this transition, which is electric-dipole forbidden in the atomic and certain high symmetry molecular environments, generally reflects the degree of symmetry lowering at the metal site. In some cases [40], splittings have been detected in the “Is+ 3d” region which may be cdmpared to the bonding-related splitting of the unoccupied d orbitals which is probed so effectively by L3 spectroscopy. However, because of the higher transition energy (4- 10 keV for the K edge as opposed to 0.4-0.9 keV for the L, edge) and associated large natural linewidths, as well as the strong dependence of the intensity on the local symmetry, the K-edge spectra are generally less useful as a probe of unoccupied 3d electronic structure. One might expect that the metal 3p spectra would show a similar local structure sensitivity to that found in the metal 2p spectra. However, the natural linewidth is much larger ( > 1 eV) and the spin-orbit splitting is much smaller ( l-2 eV). Both of these factors result in a blurring of spectral detail such that the observed spectra are much less sensitive to the local

Fig. 8. Comparisonof the background subtmcted metal 3p elee tron energy loss spectra of the indicated Fe and Co compounds. The hatched lines are the estimated 3p ionisation potentials [4 11.

62

A.P. Hitchcock et al. / Transition metal 2p excitation of organometallic compounrls

structure than are the metal 2p spectra. Thus in the present series of compounds relatively little difference is found between the carbonyl or cyclopentadienyl species, or between compounds involving different metals (see fig. 8 ). We do note that there is a trend to a sharper edge structure in the carbonyl species. This is not to say that studies of the metal 3p spectra will always be unrewarding. In fact Grass0 et al. [ 421 have shown considerable differences in the 3p photoabsorption spectra of Mn, Fe and Ni thiophosphates ( NPS3).

5. sumlnary The metal 2p spectra of a number of organometallit complexes have been reported. This brief and certainly incomplete survey reveals a spectroscopy that potentially may be a very useful tool for characterising metal environments (possibly identifying ligands) and for investigating metal-l&and bonding from the perspective of the metal 3d contribution to the virtual orbitals. Further systematic explorations of metal 2p spectra are certainly of great interest and are being pursued vigorously in our laboratory.

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