E L S E V I E R Journal of Electron Spectroscopy and Related Phenomena 73 (1995) 157-162

J O U R N A L OF ELECTRON SPECTROSCOPY

and Related Phenomena

Variable photon energy photoelectron spectroscopy of M o z ( O 2 C C F 3 ) 4 ~

John Brennan, Glyn Cooper, Jennifer C. Green*, Martin P. Payne, Catherine M. Redfern Inorganic Chemistry Laboratory, South Parks Road, Oxford OX1 3QR, UK

First received 31 August 1994; in final form 28 September 1994

Abstract

Relative partial photoionization cross sections, branching ratios and/3 measurements are reported for Mo2(O2CCF3) 4 over the photon energy range 20 to 70 eV. The metal based 6 and a + 7r ionizations are strongly enhanced over the photon energy range 36-65 eV. The comparable size of the resonance in the two metal bands suggests similar d character for the associated MO. A smaller resonance is also found in the oxygen localized ionization bands. An additional resonance is identified for the 6 band at 24 eV which is accompanied by a change in/~.

Keywords: Molybdenum tetracarboxylate; Photoelectron spectroscopy; Resonance; Relative partial photoionization cross section

1. Introduction

Dimetal tetracarboxylate complexes have provided a rich area for study of meta l -meta l bonding since the presence of or-, 7r- and 0-orbital symmetry interactions was first proposed [1]. The M2(O2CR)4 (where M is Cr, Mo or W) species have proved to be particularly fruitful for investigating these interactions. The volatility of these com- pounds makes them suitable for gas phase photo- electron (PE) studies, thus allowing detailed experimental examination of the bonding and testing of theoretical models [2-19].

Whereas ionization energies are independent of the photon energy employed, relative band intensities vary with photon energy, as was demon-

~" Dedicated to the memory of Professor W.C. (Bill) Price. * Corresponding author.

strated early on by Price et al. [20]. Their pioneering use of both He I and He II photons for determina- tion of PE spectra exposed various empirical rules which assist ion state assignment. The advent of synchrotron radiation enabled more extensive studies of PE band intensities [21]. Variable photon energy PE spectroscopy of transition metal molecules has been used to give firm experi- mentally based band assignments, to demonstrate covalency and, in favourable circumstances, to give a quantitative estimate of the metal d character of molecular orbitals [22]. It has been applied, using photon energies between 40 and 90 eV, to a thin film of Mo2(O2CCH3) 4 on a GaAs substrate, to examine the metal based or-, 7r- and 6-ionizations of the quadrupole bond [23]. Use of a thin film avoids the deleterious broadening effect on PE bands which is normally associated with solid state studies [18].

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158 J. Brennan et al./Journal of Electron Spectroscopy and Related Phenomena 73 (1995) 157 162

We report here results we have obtained on relative partial photoionization cross sections (RPPICS), branching ratios (BR) and measure- ments of the angular parameter/3, over the range 21-70 eV for the closely related Moz(OzCCF3) 4 in the gas phase. The two compounds have very similar PE spectra in the low ionization energy (IE) region in the gas phase with the 6-ionization giving the first band, and the a- and ~r-ionizations overlapping to form the second band. In the PE spectrum of a thin film of Moz(OzCCH3) 4 the a- and 7r-ionizations separate slightly, the or- ionization lying to slightly lower IE. It is of interest to compare the results obtained by the two approaches on the metal-metal bonding elec- trons. Also this study extends the photon energy range over which the cross section behaviour is examined.

2. Experimental

Preparation of Mo2(O2CCF3) 4 was carried out by the literature method [24].

The PE spectrum of Mo2(O2CCF3) 4 was obtained using the synchrotron radiation source at the SERC's Daresbury Laboratory [25]. A full account of our experimental method has been given [26], and the apparatus and its per- formance is described elsewhere [27]. Hence only a brief account of experimental procedures i s given here.

The compound was introduced into the spectro- meter inside a 1/4 inch diameter copper tube sealed with a naphthalene plug. The copper tubes were surrounded by Semflex non-inductively wound heating wire which was used to vaporize the sample. A liquid nitrogen cooled cold finger was fitted to the spectrometer to prevent diffusion of compound into the pumps.

Synchrotron radiation from the 2 GeV electron storage ring at the SERC's Daresbury Laboratory was monochromated using a toroidal grating monochromator and was used to photoionize gaseous samples in a cylindrical ionization chamber. The photoelectrons were energy analysed using a three-element zoom lens in conjunction with a hemispherical electron energy

analyser. For RPPICS measurements the analyser was positioned at the "magic angle" to eliminate the effects of the PE asymmetry parameter /3 on signal intensity. For /3 measurements PE spectra were collected at the magic angle and at 180 ° . Multiple-scan PE spectra were collected at each photon energy required. The decay of the storage ring beam current was corrected for by linking the scan rate with the output from a photodiode positioned of intersect the photon beam after it had passed through the ionization region.

Because the spectral bands are well separated the intensities of bands A - E were determined by integration after subtraction of a linear back- ground. There was difficulty estimating the background at the high IE end of the spectrum and for photoelectrons at low kinetic energy. Thus, the integrated areas and RPPICS derived for band E (the C2s orbitals) were erratic and will not be discussed further. Uncertainties in the background for bands C and D at 21 eV mean that data is only reported for these bands between 24 and 70 eV.

The sensitivity of the photodiode to different radiation energies was determined by measuring the np -1 PE spectra of neon, argon and xenon. These were also used to characterize and correct for a fall off in analyser collection efficiency at PE kinetic energies below 15 eV. Photoionization cross sections for the rare gases were taken from the literature [28,29].

Sample pressure fluctuations were corrected for by collecting a "standard" calibration spectrum before and after each data spectrum. The inte- grated intensities of the bands in these spectra were then used a a relative measure of the sample density in the ionization region.

The band intensities per scan, corrected for photodiode sensitivity, analyser collection effi- ciency and pressure fluctuations give RPPICS as a function of photon energy.

Variation of cross section with angle is given by the expression

d~ij(e) oij@) [1 ' /3ij(e) ( 3 P c o s 2 0 + 1) ] (1) df~ - 47r [ + T J

The measured band intensities at the magic angle and at 180 ° were used to derive/3 values.

J. Brennan et al./Journal of Electron Spectroscopy and Related Phenomena 73 (1995) 157-162 159

3. Results and Discussion

The M O 2 0 8 C 4 c o r e of M o 2 ( O 2 C C F 3 ) 4 has D4h

symmetry. Each molybdenum atom has four d electrons to contribute to metal-metal bonding and a metal-metal o'271"462 quadrupole bond is achieved. Though this is the ground state con- figuration, ab initio calculations have shown that extensive configuration interaction improves the wavefunction significantly [7].

A P E spectrum of Moz(O2CCF3) 4 obtained with synchrotron radiation of energy 50 eV is shown in Fig. 1. Band A is assigned to ionization from the M o - M o bonding orbital• The assignment of the second band B remained in dispute for several years [10,11,30]; SCF-X~-SW results assigned band B to the 7r orbital [5,31], whereas the ab initio calculations suggested that it should be assigned to both the 7r and 0" orbitals [6,8,9,32]. PE work on W 2 0"2714(~2 [12,33] and 0"271-4 [34] dimers give firm support to the (0" + 70 assignment. The ionization energies and full assignments for Mo2(CCF3) 4 [11] are given in Table 1.

Cross sections and branching ratios (BR) for bands A and B are given in Table 2 and are shown in Fig. 2. The angular parameter/3 is also given in Table 2 and plotted in Fig. 3. After an overall initial decrease in photon energy showing a minimum at 36 eV, the RPPICS for both the 6 and (0" + 70 bands show a maximum at a photon energy of 50eV. Decay appears to be complete around 65eV. This is a result of Mo4p--+ 4d

Counts 41.0 515.

KE /eV 37-0 33.0 28.0

C

B • "

8.0 12.0

.'... O !

E

16h.O 2(~.0 I E / e V

Fig. 1. PE spectrum of Mo2(O2CCF3)4 acquired with synchrotron radiation of 50 eV.

Table 1 Band assignments for Mo2(O2CCF3) 4

Band label IE/eV Band assignment

A 8.67 M - M 6 B 10.44 M - M ~ + 7r C 13.0 (mean) O2p D 17.2 (mean) F2p (C2p) E 20.8 C2s

resonant absorption followed by super Coster Kronig (SCK) decay resulting in another channel for 4d ionization [35]. The maximum occurs close to the 4p binding energy of free molybdenum atoms (4p3/2 42eV, 4pl/2 45eV) [36]. In addition to the maximum at 50 eV, the resonance in both the A and the B cross section appears to have shouders at about 42 and 56 eV. The resonance region may be taken as extending from 36 to 65 eV. Onset of the resonance is also marked by an increase in the/3 value for both bands A and B (Fig. 3).

The 4p ~ 4d resonant absorption may be localized on one of the metal centres or it may be delocalized throughout the molecule. Group theoretical treatment of the problem [25] shows that both approaches give essentially the same result. In D4h symmetry (the delocalized case) the

Table 2 Relative partial photoionization cross sections (RPPICS), branching ratios (BR) and /3 values as a function of photon energy hu for Mo2(OzCCF3) 4

hu RPPICS BR /3

A B A B A B

21,0 213 (5) 1340 (11) 0.14 0.86 0.51 0.20 24.0 457 (10) 1089 (16) 0.30 0.70 -0.02 -0 .04 30,0 179 (7) 542 (10) 0.25 0.75 0.44 -0.01 33,0 121 (3) 495 (8) 0.20 0.80 0.24 -0.13 36.0 74 (2) 269 (3) 0.22 0.78 0.32 -0 .14 39.0 153 (4) 353 (5) 0.30 0.70 0.10 -0 .06 42.0 279 (8) 727 (13) 0.28 0.72 0.20 0.17 45.0 314 (9) 900 (16) 0.26 0.74 0.68 0.36 47.5 423 (10) 1069 (16) 0.28 0.72 0.51 0.27 50.0 489 (12) 1186 (18) 0.29 0.71 0.64 0.50 53.0 374 (10) 1009 (16) 0.27 0.73 0.60 0.32 56.0 321 (5) 964 (9) 0.25 0.75 0.77 0.53 60.0 196 (3) 629 (6) 0.24 0.76 0.54 0.55 65.0 110 (3) 419 (5) 0.21 0.79 0.67 0.66 70.0 146 (6) 631 (13) 0.19 0.81 0.59 0.20

160

e -

.9 tJ (/I

(/1 i/1 o

L)

0 . D

4.a

e- t- tJ t "

t ~

1600

1400

1200

1000

800

600

400

200

0 20

I • A RPPICS - S

30 40 50 60 70

Photon energy

0 .9

0 .8

0 .7

0 .6

0 .5

0 .4

0 .3

0 .2

0.1 2O

J. Brennan et al./Journal of Electron Spectroscopy and Related Phenomena 73 (1995) 157-162

• A R P P I C S D A b e t a

6 0 0 . . . . ~ . . . . , . . . . ~ . . . . , . . . . ~ . . . .

• A/(A+B) I [] B / ( A + 8 )

30 40 50 60 70 Photon energy / eV

Fig. 2. R R P I C S and BR of the 6 (A) and a + 7r (B) ionizat ion

bands o f Mo2(O2CCF3)4.

t/) L) if_ 13t. r~

<

t/) L)

t~ ev,

rw

5 ~

300

200

100

0 , , : , I L , , , I , , , , I , , , , I , ~ , , I , ~ , 20 30 40 50 60 70

Photon energy

- • B R P P I C S [ ] B b e t a

1400 . . . . , . . . . ~ . . . . , . . . . , . . . . ~ . . . .

1200

1000

800

600

4OO

200 , , , , I , , , , I , , , , l , , , , L , , , , I , , , , 20 30 ~ 50 ~ 70

Photon energy

1

0 . 8

0.6

0.4

0.2

0

80 -o . 2

1

0.8

0.6 Q3

0.4 D)

0.2

0

8 0 - 0 - 2

Fig. 3. C o m p a r i s o n of the var ia t ions in the angu la r p a r a m e t e r f3

with the var ia t ions in R R P I C S for the 6 (A) and tr + ~r (B)

ionizat ion bands o f Mo2(O2CCF3) 4.

allowed excited states for absorption are two 1A2u states and three 1Eu states. Thus a complex profile, consisting of overlapping Fano profiles [37,38], is expected for the cross section in the resonance region.

Inspection of the 6/(or + ~r) BR (Fig. 2) over the whole resonance region between 36 and 65eV shows that there is only a small variation, suggesting the resonance is comparably strong in both bands. If anything the 6 ionization shows a marginal gain over the (~ + ~r) ionization band.

Band A (ionization from the 6 orbital) also shows a maximum at a photon energy of 24 eV (Fig. 2). The value of fl for band A is also found to vary strongly at 24 eV (Fig. 3). It seem likely that both these features are due to a shape resonance [39,40] at this photon energy. The electrons ionizing from the 6 orbital can access q~ (f) type

free electron waves inaccessible to the 7r and electrons.

Fig. 4 shows RPPICS and BR for bands C and D, the oxygen and fluorine based ionizations. Their RPPICS are dominated by the large fall-offin cross section at low photon energies characteristic of ionization from 2p orbitals [41]. Both the O2p and the F2p RPPICS seem to show a slight increase in the photon energy region centred on 50eV. This feature is more pronounced in the O2p (band C) cross sections where a gradual rise is seen from 39 to 50 eV. The BR plot (Fig. 4) shows that the increase is not as great as in bands A and B, but is larger than that found for band D. The carboxylate oxygen orbitals can interact with the metal orbitals; consequently these orbitals have some metal character and so may mimic the behaviour of the metal band RPPICS.

J. Brennan et al./Journal of Electron Spectroscopy and Related Phenomena 73 (1995) 157-162 161

t- O

. 1 4 - a

O ' )

( / )

tt) O

35000

30000

25000

20000

15000

10000

5000

0 20

, , ' I ' ' ' ' I ' ' ' ' I ' ' ' ' I ' ' ' ' I ' ' ' '

x~ : A RPPICS \ o B RPPICS \ o C RPPICS

CS

[ ~" ' , - , = ' : : _ _ - - . , , ,

30 40 50 60 70 80

P h o t o n e n e r g y

1 ' ' ' ' 1 . . . . I ' ' ' ' l ' ' ' ' l ' l ' ' l ' ' ' '

O . i

t-- . 1 t- U ¢.-

0.8

0.6

0.4

0.2

0 20

~ o o

= (A+B)/(A+B+C+D) D C/(A+B+C+D) o D/(A+B+C+D)

30 40 50 60 70 80

P h o t o n e n e r g y

Fig. 4. RPPICS and BR for ionization bands A - D of Mo2(O2CCF3)4.

Comparison of our .experimental results with those obtained on the thin film of Mo2(O2CCH3) 4 show the results on the metal localized ionizations to be very similar. The com- parison is somewhat hindered by the differences in the photon energy ranges used (21-70 eV by us and 40-90 eV by Lichtenberger et al. [23]) and by the fact that in the case of M o 2 ( O 2 C C H 3 ) 4 the second band area was deconvoluted into a 7r and acr band. Lack of numerical data makes the combined area difficult to reconstruct. The ligands also differ in the two studies but our band C assigned primarily to O localized ionizations shows a similar varia- tion, including a weak resonance, to the combined areas of bands 4 -6 of Moz(OzCCH3) 4 [23], as is expected from the similar bonding roles of the associated electrons.

Table 3 Calculated 4d character for the 6(bzg), 7r(e.) and a(alg) metal metal bonding orbitals of Mo2(O2CH)4

Method 6/% 7r/% a/% Reference

Hartree-Fock-Slater >75 >60 >75 [15] Ab initio SCF 87 90 91 [7] Ab initio S C F - M O >75 >75 >75 [8] Hart ree-Fock ab initio 84 86 76 [9] SCF-Xc~-SW 89 65 75 [5]

There is a difference in interpretation however, probably as a result of the fact that the Mo2(O2CCH3) 4 study commences at 40eV, an energy we find to be part way into the resonance region. The Fano minimum which we find to be at 36 eV is not detected in the former study. As a result Lichtenberger et al. [23] conclude that the cross section enhancement for the 7r-ionization (or the 7r and a bands combined) is over twice as great as for the 6-ionization. We find on balance that inspection of the branching ratios leads to the opposite con- clusion, i.e. that the resonance is marginally stronger in the 6 band than in the 7r + a band.

The symmetries of the meta l -meta l bonding orbitals allow mixing the components of the ligand sets; thus the metal character can vary. Most of the calculations predict the metal d content of all three MOs to be very high. Typical values in the literature for the 4d content from calculations on molybdenum carboxylate dimers are summarized in Table 3. The general pattern is very similar values for the three metal-metal bonding MOs consistent with the results of our variable photon energy PE study showing p ~ d resonances of comparable strength in both metal localized PE bands.

Acknowledgements

We thank the SERC for financial support and the staff at the SERC Daresbury Labora tory for their assistance.

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