Organometallics 1995, 14, 4513-4520
4513
Variable-EnergyPhotoelectron Spectroscopy of (q5-C5H5)M(q3-C3H5)(M = Ni and Pd): Molecular Orbital Assignments Xiaorong Li,? J. S. Tse,* G. M. Bancroft,*?tJ R. J. Puddephatt,"?? and K. H. Tans Department of Chemistry, The University of Western Ontario, London, Canada N 6 A 5B7, Canadian Synchrotron Radiation Facility, Synchrotron Radiation Centre, University of Wisconsin-Madison, Stoughton, Wisconsin 53589, and Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Ontario, Canada K I A OR6 Received April 11, 1995@ Variable-energy photoelectron spectra have been recorded between 21.2 and 70 eV photon energies for (q5-C5H5)Ni(q3-C3H5) and between 21.2 and 60 eV photon energies for (q5-C5H5)Pd(q3-C3H5). The ground-state electronic structures have been calculated with the Xa-SW method. Photoionization cross sections (0)have also been calculated for the valence ionizations using both the Gelius and Xa-SW methods, and the theoretical branching ratios (o&) have been compared with the observed photoelectron branching ratios ( A i m , A = band area). The assignments of the photoelectron spectra are based on comparison of the experimental and theoretical band energies and intensities. For both molecules, the lowest binding energy peak is assigned to the HOMO with mainly metal d-Cp n2 bonding character. The higher IPSfor the Pd 4d electrons lead to a different assignment for the second peaks in the two compounds; this orbital has mainly Ni 3d character for M = Ni but has high n-allyl character for M = Pd. The metal d orbitals and ligand n orbitals follow at higher binding energies.
Introduction
the relative intensities of photoelectron peaks usually vary markedly with photon energy. Comparison of the (q5-C5H5)M(q3-C3H5)(M = Ni and Pd) are mixedexperimental intensities (or relative intensities) with sandwich organometallic compounds whose photoelectheoretical values usually enables a much more confitron spectra and electronic structures have not been dent assignment of the photoelectron peaks than is reported. With the advent of synchrotron radiation possible just using experimental and theoretical enersources, it has become possible in the last 10 years to g i e ~ . l - In ~ this paper, we have carried out a variableobtain high-quality valence band photoelectron spectra energy gas phase photoelectron study of these complexes at continuously variable photon energy for small inorusing He I and monochromatized synchrotron radiation ganic molecules1 and transition metal c o m p l e ~ e s . ~ ~ ~ sources. We have also performed Xu-SW ground-state Because photoionization cross sections for different calculations and have compared the experimental relaatomic and molecular orbital vary greatly with photon tive intensity (branchingratio) variations with the ones energy (due, for example, to delayed onsets, shape from both the Gelius model and from Xu-SW photoresonances, Cooper minima, and many body effects): ionization cross section calculations. On the basis of these comparisons, spectral assignments have been + The University of Western Ontario. made for these compounds. 4 Steacie Institute for Molecular Science, National Research Council. 5 Canadian Synchrotron Radiation Facility. @Abstractpublished in Advance ACS Abstracts, August 15, 1995. (1)For example: (a) Yates, B. W.; Tan, K. H.; Bancroft, G. M.; Coatsworth, L. L.; Tse, T. S. J. Chem. Phys. 1986,83,4906. (b) Yates, B. W.; Tan, K. H.; Bancroft, G. M.; Coatsworth, L. L.; Tse, T. S.; Schrobilgen, G. J. J . Chem. Phys. 1986, 84, 3603.(c) Addison-Jones, B. M.; Tan, K. H.; Yates, B. W.; Cutler, J. N.; Bancroft, G. M.; Tse, J. S. J . Electron SDectrosc. Relat. Phenom. 1989. 48. 155. (d) Bozek. J. D.; Cutler, J. N:; Bancroft, G . M.; Tan, K. H.; Yates, B. W.; Tse, J: S. Chem. Phys. 1989, 132, 257. (2)(a)Cooper, G.; Green, J. C.; Payne, M. P.; Dobson B. R.; Hillier, I. H. J . Am. Chem. SOC.1987,109, 3836.(b) Cooper, G.; Green, J. C.; Pavne. M. P. Mol. Phvs. 1988.63. 1031.(c) Didziulis. S.V.: Cohen. S. L.{Butcher, K. D.; Solomon,'E. I. Inorg. Chem. 1988, 27, 2238.'(d) Didziulis, S. V.; Cohen, S. L.; Genvith, A. A.; Solomon, E. I. J. A m . Chem. Soc. 1988, 110, 250.(e) Butcher, K. D.; Didziulis, S. V.; Briat, B.; Solomon, E. I. J . Am. Chem. SOC.1990, 112, 2231.(flBrennan, J. G.; Green, J. C.; Redfarn, C. M. J. Am. Chem. SOC.1989,111, 2373. (g)Brennan, J. G.; Green, J. C.; Redfarn, C. M.; MacDonald, M. A. J. Chem. Soc., Dalton Trans. 1990, 1907.(h) Lichtenberger, D. L.; Ray, C. D.; Stepniak, F.; Chen, Y.; Weaver, J. H. J . Am. Chem. SOC.1992, 114, 10492.(i) Green, J. C.; Kaltsoyannis, N.; Sze, K. H.; MacDonald, M. J. Am. Chem. SOC.1994,116, 1994. (3)Li, X.;Bancroft, G. M.; Puddephatt, R. J.; Liu, Z. F.; Hu, Y. F.; Tan, K. H. J. Am. Chem. SOC.1994,116, 9543. (4)Berkowitz, J. Photoabsorption, Photoionization, and Photoelectron Spectroscopy; Academic Press: New York, 1979;pp 35-72.
Experimental Section The compounds were synthesized by methods in the literat ~ r e .Samples ~ were purified by vacuum sublimation before recording the NMR and PE spectra and were stored at -78 "C under an inert atmosphere. The lH NMR spectra confirm the purity of the samples used in our work. All samples were introduced into the gas cell of the photoelectron spectrometers by sublimation. The sublimation temperatures were 0 "C for both compounds. The He I spectra of the compounds were obtained using a n ESCA 36 spectrometer with a resolution of -20 meV.6 The variable-energy spectra (between 30 and 70 eV for the Ni compound and between 35 and 60 eV for the Pd compound) were obtained at the Canadian Synchrotron Radiation Facility (CSRF) at the Aladdin storage ring using modified ESCA 36 spectrometer fitted (5) McClellan, W. R.; Hoehn, H. H.; Cripps, H. N.; Muetterties, E. L.; Howk, B. W. J . Am. Chem. Soc. 1961,83, 1601. (6)Bancroft, G.M.; Bristow, D. J.; Coatsworth, L. L. Chem. Phys. Lett. 1981, 82, 344.
0276-733319512314-4513$09.00/0 0 1995 American Chemical Society
Li et al.
4514 Organometallics, Vol. 14, No. 10,1995 with a Quantar no. 36 position sensitive d e t e ~ t o r . ~The ,~,~ Grasshopper grazing incidence monochromator has been previously de~cribed.~ Many broad scan and narrow scan variable-energy spectra were recorded a t a resolution of 100 meV. The He I spectrum was calibrated with the Ar 3ps/z line a t 15.759 eV. For the synchrotron radiation spectra, the Xe 5s band a t 23.397 eV was used as the calibrant. For the cross section analyses, many of the spectra were fit to Gaussian-Lorentzian line shapes using an iterative proced~re.~JO Peak positions, widths, and shapes were normally constrained to obtain consistent fits from one photon energy to another. Correction of the areas for the electron analyzer transmission was performed by dividing the computed area by the kinetic energy of the band. Experimental branching ratios (BRJ were obtained using the resulting band areas (Ai) and the branching ratio formula, BRi = A m .
Computational Details Valence orbital energies and compositions of (a5C5H5)M(q3-C3H5) (M = Ni, and Pd) were calculated using the Xa-SW method as described earlier.l' Structural parameters for (q5-C5H5)Pd(q3-C3H5) were taken from the reported crystallographic data.12 Since no structural information is available for (q5-C5H5)Ni(q3C3H5), ligand parameters were used as for (q5-C5H5)Pd(q3-C3H5),but the distance between the Ni atom and each ligand was decreased by 0.17 A, the difference in atomic radii between Pd and Ni.13 C, symmetry was assumed for both compounds, shown as follows:
3 Electronic structures for the fragments (q5-C5H5)M(M = Ni and Pd, in C5" symmetry) and q3-C3H5 (in CZ, symmetry) were also calculated respectively using the Xa-SW method. The structural parameters used for them were the same as those of these fragments in the (q5-C5H5)M(q3-C3H5)molecules. For (q5-C5H5)M(q3C3H5) structures, the y-axis was assumed to be perpenand z-axis perpendicular dicular to the mirror plane (a), to the cyclopentadienyl ring, with the metal atom (7)Bozek, J. D.; Cutler, J. N.; Bancroft, G. M.; Coatsworth, L. L.; Tan, K. H.; Yang, D. S. Chem. Phys. Lett. 1990,165,1. ( 8 )Liu,Z. F.; Coatsworth, L. L.; Tan, K. H. Chem. Phys. Lett. 1993, 203,337. (9)(a) Tan, K H.; Bancroft, G. M.; Coatsworth, L. L.; Yates, B. W. Can. J. Phys. 1982,60,131.(b) Bancroft, G. M.; Bozek, J. D.; Tan, K. H. Phys. Can. 1987,113. (10)Bancroft, G.M.; Adams, J.; Coatsworth, L. L.; Bennewitx, C. D.; Brown, J. D.; Westwood, W. D. Anal. Chem. 1975,47,586. (11)(a) Yang, D. S.; Bancroft, G. M.; Puddephatt, R. J.; Bozek, J. D.; Tse, J. S. Inorg. Chem. 1989,28,1. (b) Yang, D. S.; Bancroft, G. M.; Puddephatt, R. J.;Bursten, B. E.; McKee, S. D. Inorg. Chem. 1989, 28,872.(c)Yang, D. S.; Bancroft, G. M.; Puddephatt, R. J. Inorg. Chem. 1990,29,2118. (d) Yang, D. S.; Bancroft, G. M.; Dignard-Bailey, L.; Puddephatt, R. J.;Tse, J. S. Inorg. Chem. 1990,29,2487. (e) Yang, D. S.; Bancroft, G. M.; Puddephatt, R. J.; Tse, J. S. Inorg. Chem. 1990, 29,2496.Yang, D. S.;Bancroft, G. M.; Puddephatt, R. J.; Tan, IC H.; Cutler, J. N.; Bozek, J. B. Inorg. Chem. 1990,29,4956. (12)Minasyants, M. Kh.; Struchkov, Yu.T. Zh. Strukt. Khim. 1968, 9,481 (in Russian); J. Struct. Chem. (Engl. Transl.) 1968,9,406. (13)Sargent-Welch Scientific Com. Table of Periodic Properties of the Elements; Sargent-Welch: Skokie, IL, 1980.
located a t the origin. The exchange a-parameters used in each atomic region were taken from Schwarz's tabulation,14except for hydrogen, for which 0.77725 was used. Overlapping atomic sphere radii were used with the outer sphere radius tangent to the outermost atomic spheres. An I, of 4 was used around the outer-sphere region, whereas an,,I of 3,1, and 0 was used around M (= Ni and Pd), C, and H atoms, respectively. Photoionization cross sections were calculated for the outer valence levels of (q5-C5H5)M(q3-C3H5) using the Xa-SW cross section program of Da~enp0rt.l~ The calculations were performed with the converged Xa-SW HOMO transition state potential, modified with a Latter tail to correct for large r behavior. In addition to the parameters used in the Xa-SW calculations on molecular orbitals, the maximum azimuthal quantum number,,,,I for final states were extended to 7,4,2, and 1 around outer-sphere, metals, carbon, and hydrogen regions, respectively. In calculations of transition states, half of an electron is removed from the uppermost molecular orbitals. All symmetry-allowed photoionization processes based on the dipolar selection rule were included in the calculations.
Results (a) Photoelectron Spectra. In this paper, only the photoelectron spectra in the outer valence region for (q5C5H5)M(q3-C3H5)will be considered, since the relative energies of the nine valence orbitals of the 18-electron metal centers are of greatest interest. These orbitals are formed by combination of metal orbitals (8, p, and d) with ligand n orbitals. All ligand u MO's are omitted from the discussion since they have lower ground-state energies and there is much overlapping of the bands so that a reliable assignment cannot be made. For future discussion, the energy region encompassing the outer nine valence orbitals will be termed, somewhat arbitrarily, the "bonding valence region". The broad range He I spectra of the Ni and Pd compound are shown in Figures l a and 2, respectively. The first challenge is to determine which bands comprise the "bonding valence region". It can be assumed, on the basis of the Xa-SW calculations to be discussed (Tables 1-3) and on several precedents,16J7that the MO with mainly Cp-n1 character (lla') should have the highest binding energy among the nine outermost MO's. Hence, once the Cp-nl band is located, the "bonding valence region" can be defined. In the literature, this Cp n1 MO in the photoelectron spectra of (q5-C5H5)M(C0)z (M = Co and R h P and (q5-C5H5)NiN017is assigned to bands at ca. 12 eV and the band energy is expected t o be similar in t h e spectra of (q5-C5H5)M(q3C3H5). Therefore, band 7 of (q5-C5H5)Nitq3-C3H5)(12.20 eV) and band 9 of (q5-C5Hs)Pd(q3-C3H5)(11.99 eV) are assigned to the Cp-nl MO. Thus the "bonding valence region" for (q5-C5H5)M(q3-C3H5)comprises bands 1-7 for M = Ni and bands 1-9 for M = Pd. Hence, when M (14)(a) Schwarz, K. Phys. Rev. B 1972,5,2466. (b) Schwarz, K. Theoret. Chim. Acta (Bed.) 1974,34,225. (15)(a) Davenport, J. W. Ph.D. Dissertation, University of Pennsylvania, 1976. (b) Davenport, J. W. Phys. Rev. Lett. 1976,36, 945. (16)Lichtenberger, D. L.; Calabro, D. C; Kellogg, G. E. Organometallics 1984,3,1623. (17)Evans, S.;Guest, M. F.; Hiller, I. H.; Orchard, A. F. J. Chem. Soc., Faraday 2, 1974,70, 417.
Organometallics, Vol. 14, No. 10, 1995 4515
Photoelectron Spectroscopy of ($-CJ&)M($-C&ld
Table 1. Xa-SW MO Enel-gies and Compositions (n = 3 for Ni, n = 4 for Pd)
(%) for MCp (M = Ni and Pd) Fragments
~~
energy M M orbital (eV) (n l ) p (n 1)s
+
+
~
~
M
C
C
nd
2p
2s i n t e e o u t e e
NiCp 6e1 4al le2 5el
18.0
16.0
14.0
12.0
10.0
8.0
6.0
Binding Energy (eV)
4.9
3al
-3.35 -4.64 -4.82 -5.78 -8.82
6el 4al le2 5el 3al
-3.11 -4.81 -5.14 -6.22 -8.86
15.3 0.5
63.6 97.5 98.0 53.6 11.1 0.7 PdCp 53.1 3.9 94.0 97.3 56.6 15.3 3.1 1.4
1.3 2.3
2.3 7.2
31.3 20.5 0.5 7.0 2.7 6.4 44.4 0.5 18.6 85.6 0.2 30.7
4.7 0.7 0.5 0.7 1.1
31.4 0.8 2.5 40.0 74.1
5.7 1.1 0.5 0.7 0.9
0.1 17.7 0.1 5.2 3.9 0.8 10.2 0.3 16.3
inter = intersphere; outer = outer sphere.
9.0 8.6 8.2 7.8 Binding Energy (eV)
Figure 1. (a) Broad range He I photoelectron spectrum of (775-C5H5)Ni(r13-C3H5). Bands 1-7 are corresponding to the "bondingvalence region". The small fitted peak in band 1 is due to the vibrational splitting. (b) Narrow range He I photoelectron spectrum of (q5-C5H5)Ni(q3-C3H5),which shows bands 2-4. Band 2 is fitted into peaks 2A and 2B.
I 18.0
16.0
14.0
12.0
10.0
Binding E n e r g y
8.0
(eV)
Figure 2. Broad range He I photoelectron spectrum of (q5C5Hs)Pd(v3-C3H5). = Pd, there is a one-to-one relationship between the number of bands and the number of MO's in the outer valence region. But, when M = Ni, the number of bands (seven) is less than the number of MO's (nine). This means that overlapping of bands occurs for M = Ni, in agreement with the general observation that the PE bands are less widely separated for first-row transition metal molecules than second Representative variable-energy spectra for the Ni and Pd compounds are shown in Figures 3 and 4, respectively, and show dramatic intensity variations as a function of photoelectron energy. A qualitative estimate of the atomic orbital composition for each MO repre-
sented by its ionization band can be made very quickly on the basis of the comparison of the observed band intensity variation with Yeh and Lindau's cross-section curve for atomic subshells (Figure 5).18 Orbitals with mostly ligand C 2p character are expected to give bands whose intensities fall a t higher photon energy. It is apparent from Figure 3 that the relative intensity of bands 1, 3, and 6 of the Ni compound and bands 1, 2 and 8 of the Pd compound decrease with increasing photon energy, suggesting that they arise from the MO's with mostly ligand character. Also, the relative intensity of bands 2 and 4 of the Ni compound and bands 3-5 of the Pd compound increase with photon energy, indicating that they should be assigned to orbitals with mostly metal d character. The trend of intensity variations of band 5 of the Ni compound and bands 6 and 7 of the Pd compound are intermediate between the variations of the above two groups, showing that they are from MO's with mixed-metal d and ligand C 2p character. Although the intensity of both bands 1 and 2 of the Pd compound decrease with photon energy relative to bands 3-5, the first band shows an increase in intensity relative to the second band, demonstrating that the orbital associated with the second band has more ligand character than that of the first band. (b)Electronic Structures of (q6-C~&)M(qS-Cs&) (M = Ni and Pd) from Xa-SW Calculations. The Xa-SW orbital energies and compositions for M(q5C5H5) (M = Ni and Pd) fragments (with CsVsymmetry) in the outer valence are listed and plotted in Table 1 and Figures 6 and 7. The MOs in these regions are formed mainly between metal nd (n = 3 for Ni, n = 4 for Pd) and Cp n1 (a1 symmetry), Cp n2 (el symmetry, doubly degenerate) MOs, with a small amount of metal (n 1)s and ( n 1)p also involved. The calculated electron configurations for M(v5-C5H5) are as follows: (core)(3a1)2(5e1)4(le2)4(4a1)2(6e1)3. There are three types of orbital interactions (n,0,and 6) for M(T,I~-CE,H~).~~ Among them, 4al and 3al are u type MOs,with 4al having mainly metal nd,z character; and 3al having mainly Cp-nl character with some metal s, p constitution. The metal nd,z orbital is essentially nonbonding. The le2 MOs are of &symmetry with predominant
+
+
(18)Yeh, J. J.; Lindau, I. At. Nucl. Data Tables 1985, 32, 1. (19) Elschenbroich, C . ; Salzer, A. Organometallics: A Concise Introduction, 2nd ed.; VCH Publishers Inc.: New York, 1992; pp 318319.
Li et al.
4516 Organometallics, Vol. 14, No. 10, 1995
I
0
10.0
10.0
8.0
Binding Energy
8.0
Binding Energy
(eV)
(eV)
I 800
v)
2
400
2
0 200
0
0
0
8.0 10.0
Binding Energy
Binding Energy
(eV)
(eV)
Figure 3. Representative variable-energy photoelectron spectra of (v5-C5H5)Ni(v3-C3H5) of the first six bands at 21.2,35, 50, and 60 eV photon energies.
Y v)
J
0"
10.0
10.0
8.0
Binding Energy
8.0
Binding Energy
(eV)
(eV)
1200
1000 800
+ v) 3
eo0
0 0 400
200 0
70.0
8.0
Binding Energy
10.0
(eV)
8.0
Binding Energy
(eV)
Figure 4. Representative variable-energy photoelectron spectra of ( ~ , I ~ - C ~ H ~ ) P ~ ( of ~ ,the I ~ -first C~H eight S ) bands at 21.2, 40, 50, and 60 eV photon energies.
metal ndx2,2/ndxy character. They remain mainly nonbonding because the &type overlap with the Cp z-orbitals is weak. The MO's 6el and 5el have JC symmetry. They have significant metal nd,,lndy, and ligand C 2p character, suggesting that 6e1 is the out-of-phase and
5el is the in-phase interaction between metal nd,,lnd,, and Cp el orbitals. The electronic structure of the M(y5C5H5) fragment has been discussed before, and the orbital ordering from our X a calculation is in agreement with the literature.20
Photoelectron Spectroscopy of ($-C&JM($-C&J
VI
u lo-’
Organometallics, Vol. 14, No. 10,1995 4517
40 60 80 Photon energy (eV)
0
-2
-2 v
100
20
CpNi(ally1) I
Allyl
CpPd(ally1)
, I
11
16a’
Allyl X
I I
--I1
x
P
Figure 5. Photoionization cross sections of Ni 3d, Pd 4d, and C 2p atomic subshells from Yeh and Lindau’s calculation.18
NiCp
I
PdCp
1
c W
-6
-4
-- !I
lla’
\
-10
I
Figure 7. Molecular orbital diagrams of (v5-C5H5)Pd, v3C3H5, and,(v5-C5H5)Pd(y3-C3H5) from Xa-SW calculations. Dashed lines show the orbital correspondence due to bonding.
11
-10
; 12a’
I
1
Figure 6. Molecular orbital diagrams of (v5-C5H5)Ni,73C3H5, and (q5-C5H5)Ni(v3-C3H5) from Xa-SW calculations. Dashed lines show the orbital correspondence due to bonding.
The molecule (y5-C5H5)M(y3-C3H5) has C, symmetry. The correlation of M(y5-C5H5)MO’s on going from CsU C, symmetry is a1 a’ and e a’ a“. The configuration of n-allyl fragments in C, symmetry is (core)(a’)2(a’’)1or (core)(n1)~(~2)~. In forming (y5-C5H5)M(y3-C3H5)from the M(y5-C5H5)and q3-C3H5 fragments, no cross-interactions (a’”’’) are allowed. From the data in Tables 2 and 3 and Figures 6 and 7, it is clear that 12a’, loa”, and 8a” of (y5-C5H5)M(y3-C3H5)have more n-allyl ligand character. Since the ndxz,z/ndXy orbitals (9a” and 14a’, corresponding to e2 symmetry of M(y5C5H5)) are approximately perpendicular to the plane of the allyl and Cp ligands, they cannot interact effectively with the ligand orbitals. These two orbitals remain essentially nonbonding with metal d character. Besides the symmetry-match requirement, the energy of the interacting orbitals should be close. On the basis of these conditions, an orbital interaction occurs between the n2-allyl and an a” MO’s of M(v5-C5H5)which were
-
-
-
+
(20) Albright, T.A.;Burdett, J. K.; Whangbo, M.-H. Orbital Interactions in Chemistry; John Wiley & Sons: New York, 1985; pp 387388.
originally the 6e1 orbital in C5u symmetry. There are two MO’s formed for (y5-C5H5)M(y3-C3H5)by this interaction. They are loa” and a virtual MO in a” symmetry (Figures 6 and 7). The remaining valence molecular orbitals, 16a’, 15a’, Ba”, and 13a‘ for (y5-C5H5)M(y3C3H5), are mainly M(y5-C5H5) orbitals and have little allyl character. The 12a’ and l l a ’ MO’s are mainly nl orbitals of allyl and Cp, respectively, with small interactions with frontier metal d, s, and p orbitals in a‘ symmetry. The Xa-SW calculations show that, for the ligand orbitals, the n2 orbitals of Cp and allyl are largely involved in the bonding, and the n1 orbitals are less involved. The crystal structure of (y5-C5H5)Pd(y3-C3H5) has shown that the planes of the allyl and cyclopentadienyl ligands are not parallel; the angle between them is 19.5”. The end carbon atoms of the allyl group are 0.24 A closer to the cyclopentadienyl plane than the middle one,12 suggesting that the n2 orbital of allyl, whose electron cloud is mainly located at the end carbons, is largely involved in the bonding. (c) Theoretical Calculations for Photoelectron Branching Ratios. We obtained theoretical valence molecular orbital photoionization cross sections using both the GeliusZ1and Xa methods,15and then obtained branching ratios (BR, = a m ) , where ai is the calculated cross section for the ith orbital, t o compare with the experimental BRi values (ADA). In the Gelius treatment, the cross section of an individual MO is assumed to be proportional to the sum of the atomic cross sections (a&) of its components weighted by the “probability” (P&)i of finding in the ith molecular orbital an electron belonging to the atomic orbital Aj: (21)(a)Gelius, U. Electron Spectroscopy; Shirley, D. A,, Ed.; North Holland Amsterdam, 1972; pp 311. (b) Bancroft, G. M.; Malmquist, P.-& Svensson, S.; Bailier, E.; Gelius, U.; Siegbahn, K. Znorg. Chem. 1978,17, 1595.
Li et al.
4518 Organometallics, Vol. 14, No. 10,1995
Table 2. Xa-SW MO Energies and Compositions (%I for CpNi(ally1)(“BondingValence”) CP
Ni
allyl
orbital
energy(eV)
4p
4s
3d
C2p
C2s
16a‘ loa“ Ea’ 9a” 14a’ 8a” 13a’ 12a’ lla’
-2.86 -3.76 -4.32 -4.61 -4.82 -5.45 -5.63 -7.12 -8.62
2.6 9.0 0.3 0.3 0.2 0.3 2.5 6.9 1.1
0.6
63.2 16.5 92.2 95.0 95.0 63.8 46.9 12.4 2.6
27.8 39.4 0.9 3.2 2,2 25.0 46.0 16.5 70.0
0.1
3.4
0.4 1.8 13.3
HIS
C 2p
4.8 33.8 2.1 0.8 2.3 9.2 3.5 61.6 12.0
0.3 0.1 0.1
0.5 0.4 0.1 0.3
C2s
0.2
0.6
0.3 0.3 0.5
HIS
inter.
outer.
0.8 0.8 0.6 0.6 0.1 0.8 0.2 0.3
17.7 34.2 9.0 6.9 7.7 15.9 23.0 26.4 32.2
0.4 4.4 0.2 0.2 0.2 0.7 1.6 2.8 2.4
Table 3. Xa-SW MO Energies and Compositions (%) for CpPd(ally1) (“BondingValence”) Pd orbital
energy (eV)
5p
16a’ loa” 5a‘ 9a” 14a‘ 8 a” 13a‘ 12a‘ 1la‘
-2.22 -3.25 -4.03 -4.51 -4.78 -5.49 -5.51 -7.02 -8.26
6.5 20.0 1.1 0.9 0.7 0.6 6.2 13.2 4.0
5s 0.8 10.2 0.1
0.5 2.2 16.2
CP
allyl
4d
C 2p
C 2s
57.6 10.9 82.4 91.4 93.0 59.6 43.9 14.9 4.9
28.0 35.3 1.6 4.7 2.9 24.2 41.4 11.7 65.8
0.1
H Is
C2p
C2s
H 1s
inter.
outer.
5.8 31.9 3.2 1.5 2.9 13.3 6.5 56.3 7.9
0.1 0.7 0.2 0.1
1.2 1.1 1.0 1.3 0.3 1.5 0.7 0.7 0.1
12.0 21.4 7.6 3.9 4.2 7.4 11.5 6.6 13.8
8.2 5.1 0.7 0.3 0.3 1.3 2.1 2.7 2.7
~
0.5 0.1 0.9 0.7 0.1 0.6
0.2
0.1
8 0
where (P~j)iis given approximately by the orbital composition from our Xa calculations, and uh are the theoretical atomic cross sections as a function of photon energy. In this work, Yeh and Lindau’s data,18obtained by the Hartree-Slater central field method, were used. A qualitative guide to the variations in molecular cross sections and branching ratios can be obtained by looking at the important atomic cross sections in Figure 5. Thus all metal d orbitals show a large increase in cross section above threshold, before decreasing in markedly different ways at higher energies. In contrast, the C 2p orbital shows a Iponotonic decrease in cross section over the whole range. This behavior gives rise to the changes in the ratio of the M ndlC 2p cross sections, which are reflected in the branching ratio changes. The theoretical branching ratios for MO’s in the valence spectra of (q5-CsH5)M(q3-C3H5)are plotted in Figures 8 and 9 (solid line, Xa calculation; dashed line, Gelius model treatment). They demonstrate that the general trends of experimental and theoretical branching ratios of the metal d based M O s (Ea’, 9a”, and 14a’) increase from 21.2 t o 70 eV, and the curves for the other MO’s either decrease or show little change on going from low to high photon energy.
0.7 0.1 0.8 0.4
4 t
4 0 1
1
Band 2
._
0
v F
20
40
60
80
10
-
0 0
20
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40
60
80
1
Band 4 140’
Band 3
e
20
i t
4 0 1
m
0
80
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80
0
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40
60
80
20
40
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80
8a”+ 130‘
e
30
m E
e
m
IO
Discussion 0
(a) Spectral Assignment. “he spectral assignments are based on the generally good agreement between the trends of theoretical branching ratios (Gelius and XaSW) with the experimental ones (Table 4 and Figures 8 and 9). The agreement is just as good as we have observed in previous ~tudies.l-~Jl The assignments for M = Pd are more straightforward and so are discussed first. It is clear from Figures 4 and 9 that bands 3-5 increase in intensity with increasing photon energy and they must be assigned to the MO’s Ea’, 9a”, and 14a’ which are calculated to have >8O% Pd 4d character. Bands 1, 6,and 7 have
0
20
40
60
Photon energy (eV)
Photon energy (eV)
Figure 8. Experimental branching ratios of PES bands and theoretical branching ratios of molecular orbitals for (v5-C5H5)Ni(v3-C3H5)(circles, triangles, and squares, experimental data; solid line, Xu-SW data; dashed line, Gelius model treatment). intensities which are less dependent on photon energy and so are assigned to MO’s 16a’, 8a“, and 13a’ which are calculated to have 44-60% Pd 4d character and have metal ligand bonding character. Bands 2,8, and 9 show the greatest decrease in intensity with photon
Organometallics, Vol. 14, No. 10,1995 4519
Photoelectron Spectroscopy of ( ~ / ~ - C S H S ) M ( T ~ - C ~ H , , 30 n
30
25
Band 1
1
v
0
20
e .-rP
15
P
e
m
20
tc
Band 2
-
20
-
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lo
- 10 -
5
5
- 5 -
0
0
15
- -----
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1
Band 5
60
80
20
0
40
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i
?i"l/
-
9a"
1
0
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30 n
25
Y
v
.0
a. .
e
20
01
15
C
10
.r
e
m
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25
12a'
5 0 20
0
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Photon energy (eV)
-0-00
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40
60
Photon energy (eV)
80
0
20
40
60
Photon energy (e")
80
0
20
40
60
80
Photon energy (eV)
Figure 9. Experimental branching ratios of PES bands and theoretical branching ratios of molecular orbitals for (q5C5H5)Pd(v3-C3H5)(circles, triangles, and squares, experimental data; solid line, Xu-SW data; dashed line, Gelius model treatment). Table 4. Assignments for Photoelectron Spectra of CpM(ally1) (M = Ni and Pd) molecular orbital
X a ground-state
16a'
-2.86 -3.76 -4.32 -4.61 -4.82 -5.45 -5.63 -7.12 -8.62
enerm (eV)
band assmt
vertical IP ( f 0 . 0 1 eV)
1 3 2A 2B 4 5A 5B 6 7
7.06 8.43 8.02 8.10 8.76 9.42 9.77 10.84 12.20
6
C pNi( allyl)
loa" 15a' 9a" 14a' 8a" 13a' 12a' lla'
C pPd( allyl) 16a'
loa" 15a' 9a" 14a' 8a" 13a' 12a' lla'
-2.22 -3.25 -4.03 -4.51 -4.78 -5.49 -5.51 -7.02 -8.26
1 2 3 4 5 6 7 8 9
7.10 7.99 8.99 9.36 9.74 10.10 10.45 11.05 11.99
8 h
2 -
v
0 ._ c
C
P)
c
g
10
t 0
.0 .-N C
120'
4-
0 -
12
1 la'
CpNi(ally1)
energy and are assigned to the ligand bands loa", 12a', and lla', respectively. Figure 9 shows that this assignment gives good agreement between the observed and calculated branching ratios, while Table 4 indicated a perfect correlation between the sequences of experimental and calculated MO energies. Both the energy and intensity calculations put this assignment on a very firm footing. The assignments for M = Ni are more difficult. Because of band overlaps, deconvolution of the bands is necessary and creates some uncertainty in the experimental branching ratios. However, from Figures 3 and 8 it is clear that band 2 gives the greatest increase in intensity with photon energy and that band 4 is the only other band to show an overall increase in intensity. Band 2 (peaks 2A and 2B in Figure lb) is assigned to both MOs 15a' and 9a", while band 4 is assigned to MO
.-..- _- __ _ _ - -~ ____ __--
130' 120'
1 la'
CpPd(ally1)
14
Figure 10. Correlation of the ionization potentials for the complexes with M = Ni and Pd. 14a'. These are all calculated to have 92-95% Ni 3d character. Bands 1 and 5 do not change in intensity greatly as a b c t i o n of photon energy and are assigned to M u s 16a' and 88'' 13a', respectively, having 4764% Ni 3d character. Bands 3,6, and 7, which decrease in intensity with increasing photon energy, are then assigned to MO's loa", 12a', and lla', respectively, having 516% Ni 3d character. (b) A Comparison of the Assignments for M = Ni and Pd. A correlation of the MO energies for the complexes with M = Ni and Pd is shown in Figure 10.
+
4520 Organometallics, Vol. 14, No. 10, 1995 The relative IPSfor M = Ni are therefore 16a' > 15a', 9a" > loa" > 14a' > 8a", 13a' > 12a' > 1la' In contrast, the calculated Xa sequence for M = Ni and Pd, and experimental IP sequence for M = Pd, are 16a' > loa" > 15a' > 9a" > 14a' > 8a" > 13a' > 12a' > lla' The major energy differences between the two compounds are for the MO's with greatest metal nd character, especially for the MO's Ea', 9a", and 14a'. These differences are not predicted by the ground-state Xa calculations (Tables 2-4). There are two possible explanations. First, it is possible that relaxation effects are greater for the Ni 3d MO's than for the Pd 4d MO's. There is support for this interpretation from studies of photoelectron spectra of organometallic derivatives of other first- and second-row transition metal complexes.16,22The second interpretation is that the groundstate energies are actually Ni 3d > Pd 4d and that the Xa calculations fail to reflect this difference. Support for this interpretation is found from a study of the photoelectron spectra of M(q3-C3H& (M = Ni, Pd, and (22) (a) Calabro, D. C.; Lichtenberger, D. L. Inorg. Chem. 1980,19, 1732. (b) Lichtenberger, D. L.; Kellogg, G. E. Acc. Chem. Res. 1984,3, 1623.
Li et al.
FW3 Of course, it is also possible that both factors play a part.
Conclusion The photoelectron spectra of (q5-C5H5)M(q3-C3H5)(M = Ni and Pd) have been recorded and assigned by using the variable-energy technique and the theoretical calculations. Both experimental and theoretical relative intensities for the mainly ligand and metal valence photoionizations show distinct trends, which puts the spectral assignments on a firm footing. The comparison of the assignments for M = Ni and Pd (in this work and our previous work3)suggests that the orbital relaxation and ground-state energy orderings (both are Ni 3d > Pd 4d) may play parts for the observed ionization difference between Ni 3d and Pd 4d.
Acknowledgment. We are very grateful for the financial support of NSERC (Canada), for the continued assistance from the staff at the Aladdin Synchrotron, and for the other assistance from Mr. Y. F. Hu. We also acknowledge the support of NSR Grant No. DMR9212658 to the Synchrotron Radiation Centre. OM9502603
