J. Phys. Chem. 1992, 96, 121-123
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The Vacuum-Ultraviolet Spectrum of Fe(CO)5: An Experimental Analysis Supported by a CASSCF CCI Study of the Rydberg States Antonio Marquez? Chantal Daniel,*,*and Javier Fernandez Sanzt Departamento de Quimica Fisica, Universidad de Sevilla, 41 01 2 Sevilla, Spain, and Laboratoire de Chimie Quantique, E.R. 139 du CNRS Institut Le Bel, F-67000 Strasbourg, France (Received: May 13, 1991)
A spectroscopic and theoretical study of the electronic structure of Fe(CO)s is reported. The vacuum-UV electronic spectrum of Fe(CO)5was recorded in the gas phase at room temperature in the region 210-1 10 nm. The spectrum is dominated by a strong absorption with a maximum at 194 nm, followed by a series of strong overlapped bands of increasing intensity. Ab initio CASSCF CCI calculationsof the lowest ligand field, the two first ionization energies, and the first s, p, and d terms of the two first Rydberg series were undertaken. The excellent agreement found between experiment and theory for the LF transition and IEs shows the quantitativeaccuracy of these calculations. The first Rydberg series arises from excitation of one 3d, electron toward 4s, 4p, and 4d atomic-like orbitals and ranges from 49600 to 61 800 an-’.The term value calculated for the 3d, 4s excitation (18 800 cm-I) is close to that deduced from the first Rydberg transition of iron. The second Rydberg series corresponds to excitation of the 3d, levels and falls in the region comprised between 64 100 and 71 800 cm-l.
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Introduction Metal carbonyl complexes presently compose the most important class of organometallic substances which have been the object of photochemical studies.I Indeed, metal carbonyls are among the most photoreactive transition-metal compounds, and they can be used to illustrate most of the known types of lowest energy electronic excited states as well as many of the known excited-state reaction pathways. Although the photochemistry ,of transition-metal carbonyls has constituted a subject of growing interest in the last few years, both from a mechanistic and synthetic viewpoint, most of the time the electronic spectra of these complexes are poorly resolved. Even though the near-UV electronic spectra of many transition-metal carbonyl complexes have been known for a long time,Z4 their interpretation is restricted to general characterizationsof the intense absorption bands as metal to ligand charge-transfer excitations (d T * ) . A detailed assignment of the weaker bands is usually tentative and often the subject to controversy. The UV spectrum of Fe(C0)5 was first reported by Dartiguenave, Dartiguenave, and Gray (DDG).3 This spectrum, recorded in solution, was poorly resolved and showed a strong band at about 50000 cm-I preceded by two weak shoulders at 41 600 and 35 5 0 0 cm-I. Based on an EHT analysis, DDG assigned the very weak absorption at 35 500 cm-’ to the 3d(e’) 3d(a’l) excitation, the two other bands being characterized as metal to ligand charge-transfer transitions. The second experimental analysis of the electronic spectrum of Fe(CO)5 was carried out recently in gas phase by Kotzian, Rosch, Schroder, and Zerner (KRSZ).5 The general features are similar to that recorded in solution and the assignment, based on INDO/S calculations, agrees with that one of DDG (spectrum dominated by MLCT transitions). Surprisingly, KRSZ did not assign the transition corresponding to the d d excitation, claiming that it is Laporte forbidden. In fact, Laporte’s rule applies only to octahedral complexes. Compared with the experiment, the theoretical transition energies appeared to be considerably shifted (about 150 nm for the lowest one) toward the visible. A previously published6 CNDO/CI calculation of the electronic spectrum of Fe(CO)5is in essential agreement with the results of KRSZ (spectrum dominated by MLCT transitions), but with a shift of the electronic transitions to the higher energies. The first ab initio CI calculations on the low-lying states of Fe(COIs were carried out by Daniel et al.’ Three were associated to allowed transitions: the LF, calculated at 40 840 cm-I, and two MLCT (40 990 and 46 210 cm-I). More recently, on the basis of CASSCF/CCI calculations, Veillard et predicted the LF transition to lie at 28 000-29 990
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cm-l in agreement with the fact that the photochemistry of Fe(C0),9.lo starts at around 28 600 cm-I. In spite of this activity, both experimental and theoretical, many unknowns still remain regarding specially the vacuum-ultraviolet spectrum of Fe(CO)5 (between 110 and 210 nm). As far as transition-metal carbonyls are concerned, the more intense bands which characterize the electronic spectrum at higher frequencies have been generally interpreted in terms of metal to ligand or ligand to metal charge-transfer or interligand transitions. An exciting question in electronic spectroscopy deals with the Rydberg transitions. The possibility of finding Rydberg bands in the related part of the spectrum received much less attention.” The difficulties encountered in this field are both experimental and theoretical. Only a few transition-metal complexes can be put into the vapor phase at room temperature. Many of them decompose upon heating. In addition we have no secure information on the Rydberg term values. The vacuum ultraviolet spectra of Cr, Mo, and W hexacarbonyl derivatives have been published.’* The vapor-phase electronic absorption spectra of the tris(hexafluoroacety1)acetonate complexes of AI, Sc, V, Cr, Fe, and Mn have been recorded up to about 80000 cm-I in the same way as the far-ultraviolet spectra of three sandwich compounds of Fe, Co, and Ni.l3-Is The presence of d-s and d-p Rydberg transitions has been suggested in Cr(CO)6 on the basis of the study of solvent effects on the electronic spectrum of the related Cr(V6-C6H6)2 complex.I6 From the theoretical point of view, the empirical and (1) Geoffroy, G. L.; Wrighton, M. S . Organometallic Photochemistry; Academic Press: New York, 1979. (2) Beach, N. A.; Gray, H. B. J. Am. Chem. SOC.1968, 90, 5713. (3) Dartiguenave, M.; Dartiguenave, Y.; Gray, H. B. Bull. Soc. Chim. Fr. 1969,4223. (4) Gray. H. B.; Beach, N. A. J . Am. Chem. SOC.1963. 85. 2922. (5) Kothan, M.; Rosch, N.; Schroder, H.; Zerner, M. C. J.’Am. Chem. SOC.1989. 111.7687. (6) Dick, B.f Freund, H. J.; Hohlneicher, G. Mol. Phys. 1982, 45, 427. (7) Daniel, C.; Benard, M.; Dedieu, A.; Wiest, R.; Veillard, A. J. Phys. Chem. 1984,88,4805.
(8) Veillard, A.; Strich, A.; Daniel, C.; Siegbahn, P. E. M. Chem. Phys. Lett. 1981, 141, 329. (9) Seder, T. A.; Ouderkirk, A. J.; Weitz, E. J . Chem. Phys. 1986, 85, 1977. (10) Yardley, J. T.; Gitlin, B.; Nathanson, G.; Rosan, A. M. J . Chem. Phys. 1981, 74, 370. ( I 1) Robin, M. B. Higher Excited States of Polyatomic Molecules; Academic Press: New York, 1975; Vol. 11. (12) Iverson, A.; Russell, B. R. Chem. Phys. Lett. 1970, 6, 307. (13) Lussier, L. S.; Sandorfy, C.; Goursot, A.; PBnigault, E.; Weber, J. J. Phys. Chem. 1984,88, 5492. (14) Sandorfy, C.; Lussier, L. S . In Photophysics and Photochemistry in
the Vacuum Ultraviolet; McGlynn, S . P., Findley, G. L., Huebner, R. H., Eds.; D. Reidel: Dordrecht, Holland, 1985; p 819. (15) Sandorfy, C.; Lussier, L. S.; Richer, G.; Goursot, A,; PBnigault, E.; Weber, J. J. Mol. Struct. 1986, 141, 1.
0 1992 American Chemical Society
122 The Journal of Physical Chemistry, Vol. 96, No. 1 , 1992
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Marquez et al.
TABLE I: CASSCF Calculations state
Y
main confinuration
active mace
a'E' b'E' clE'
8e9a 8e9a 8e10a 8e10a 8e9a 8e10a 8e10a 8e10a 8e10a
d'E' e'E'
f'E'
CO Figure 1. Structure of Fe(CO)S.19
semiempirical methods are able to locate "in a rough manner", the lower Rydberg bands of a limited number of molecules.13J5 A more sophisticated computational strategy like the CASSCF/CCI method may offer one of the few theoretical possibilities for analyzing the upper part of the electronic spectrum of transition-metal carbonyl complexes. The aim of the present study is a detailed investigation of the far-UV electronic spectrum of Fe(CO)5. The first part is devoted to an experimental study of the vacuum-ultraviolet electronic spectrum of this molecule recorded in gas phase in the region going from 210 (47 000 cm-I) to 110 nm (90 900 cm-I). The second part of this paper deals with the excitation energies corresponding to the allowed transitions from the ground state to the lowest ligand field excited state and to the first and second Rydberg series (d 4s, d 4p, and d 4d) obtained from CASSCF/CCI calculations.
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a' A? blA7 CIA?
r'i f / ///
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Computational Method
The calculations were carried out at the experimental geometry of Fe(CO)5 (D3*symmetry with 0,the C3&)Ig (Figure 1). A 0 basis sets appropriate for the calculation of the excitation energies to the valence states were augmented in order to take into account the diffuse character of the Rydberg orbitals which are vacant in the ground and valence states but singly occupied in the Rydberg states. For this reason we have used the following Gaussian basis sets: for the transition metal atom a (15,11,6) set contracted to [9,6,3I2Osupplemented with diffuse componentsz2(s, f = 0.014, 0.030,O.OlO; d, I= 0.037, 0.013, 0.0042), 0.0046, 0.0015; p, for the first-row atoms a (10,6) set contracted to [4,2].24 Complete active space SCF (CASSCF) calculation^^^ were carried out to obtain wave functions which are used as references
r=
(16) Wittmann, G. T. W.; Krynauw, G. N.; Lotz, S . ; Ludwig, W. J . Organomet. Chem. 1985, 293, C33. (17) Dognon, J. P. ThPse ri I'Uniuersitd de Pau, France, 1983. (18) Ballofet, G.; Romand, J.; Vodar, C. R. Acad. Sci. 1961, 252, 4139. (19) Beagley, B.; Schmidling, D. J . Mol. Struct. 1974, 22, 466. (20) This basis set is constructed from the (14,9,5) basis of Wachters2' by adding an additional s function (exponent 0.2985), two diffuse p functions, and one diffuse d function.22 (21) Wachters, A. J. H. J . Chem. Phys. 1970, 52, 1033. (22) The corresponding exponents were chosen according to the eventempered criterion of Raffenetti et aL2' (23) Raffenetti, R. C.; Barda, R. D.; Rudenberg, K. In Energy, Structure and Reactivity; Smith, D. W., Mc Rae, W. B., Eds.; Wiley: New York, 1973; p 164. (24) Huzinaga, S. Approximate atomic functions. Technical Report; Universitv of Alberta: Alberta. 1971. (25) Sbgbahn, P. E. M.; Almlof, J.; Heiberg, A,; Roos, B. 0. J . Chem. Phys. 1981, 7 4 , 2384.
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Experimental Section
The spectrum of Fe(CO)5 was recorded at room temperature with a laboratory-madevacuum-ultraviolet spectrometer described elsewhere." In brief, the radiation produced in a windowless BRV spark source'* is dispersed by a monochromator with a holographic grating on a toric support. A beam splitter separates the reference and the sample. The cell containing the Fe(C0)5 vapor was equipped with lithium fluoride windows, limiting transmission to about 105 nm. The vacuum-ultraviolet (VUV) light source is converted to UV-visible photons using a sodium salicylate coated window on the exit of the cell and monitored by a photomultiplier. The whole machine is run entirely by a personal computer. Fe(C0)5 was commercially available (Aldrich-Chemie) and purified before introduction into the cell by sequential trap to trap distillations.
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yl
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44
.
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u
76
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00
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Figure 2. Vacuum-UV spectrum of Fe(CO)s.
in the contracted configuration interaction (CCI) calculations.26 Our interest will center mostly on the Rydberg states accessible through allowed transitions from the ]A'] ground state ((3dJ4(3dd4)to the lowest ]E' and l A F states. These states correspond to 3d 4s, 3d 4d, and 3d 4p excitations. For each excited state one CASSCF calculation was carried out with the main configuration corresponding to the required state. The active space will include the four 3d orbitals occupied in the ground state and the four orbitals which correlate them. The number of active orbitals will vary from nine to ten, depending on the nature of the singly occupied orbital (degenerated or not) in the electronic configuration of the excited state. Details of the CASSCF calculations are given in Table I, in which the active space is described by the number of electrons correlated (ne) and the number of active orbitals (na). These CASSCF wave functions were used as reference wave functions of the subsequent CCI calculations. For each excited state a multireference CCI calculation was performed including all the configurations which appear with a coefficient larger than 0.08 in the CASSCF wave function. Eight electrons are correlated (the 3d electrons) in these calculations. Single and double excitations to all virtual orbitals, except the counterparts of the carbonyls 1s and of the metal Is, 2s, and 2p orbitals, are included in the CCI calculations. The number of configurations ranged from 85 729 to 995 995, but this number is reduced to at most a few thousands by the contraction. The integral calculations were carried out either with the system of programs ARGOSZ8or with the system of programs ASTERIX.~'
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Results and Discussion Fe(CO), Vacuum-UV Spectrum. The vacuum-UV spectrum of Fe(C0)5 recorded at room temperature is reported in Figure (26) Siegbahn, P. E. M . Int. J . Quantum Chem. 1983, 23, 1869. The original program was interfaced for use in conjunction with the ASTERIX system of programs on the C r a ~ 2 ~by' C. Daniel, M. Speri, and M . M. Rohmer. (27) Ernenwein, R.; Rohmer, M. M.; Btnard, M. Comput. Phys. Commum 1990,58,305. Rohmer, M. M.; Demuynck, J.; Btnard, M.; Wiest, R.; Bachmann, C.; Henriet, C.; Ernenwein, R. Comput. Phys. Commun. 1990, 60, 127. Wiest, R.; Demuynck, J.; Btnard, M.; Rohmer, M. M.; Ernenwein, R. Comput. Phys. Commun. 1991, 62, 107. (28) Pitzer, R. M. J . Chem. Phys. 1973, 58, 3111.
The Vacuum-Ultraviolet Spectrum of Fe(CO)5 TABLE II: Calculated CCI Excitation Energies (cm-’) to the Rydberg and Ligand Field States of Fe(CO)I (Allowed Transitions Only)
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one-electron excitation in 3d, alE’ x’A’, blE’ xlA,’ 3d, clE’ x‘A,’ 3d, 34 d’E’ x’Al’ x’Al’ 3d, elE’ xlAl’ b’A”2 6 xlA,’ flE‘ xlA,’ c1A’I26 x‘Al’
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3 4 3 4 3d, 3d,
lE’ 1st IP 2E” 2nd IP
3d, 3d,
the principal configuration 29 100 3dX2 49 600 4s 56 200 4d, 4P* 59 900 4d, 61 800
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4Px 4Pz 4d, 4d,
64 100 66 300 70 100 71 800
m
64 400 (68 400)’ 79600 (79 100).
m
‘In parentheses are reported the values obtained at the CASSCF level.
2. The region of lower energies shows an asymmetric band lying between 170 and 210 nm (the absorption continues in the classical UV up to 300 nm). This band appears to be very intense with a e, z 20000 M-’ cm-’ and shows the maximum a t 194 nm. Detailed analysis reveals a prominent peak at 197 nm and two shoulders at 202 and 187 nm (very weak). This band overlaps in the region of high energies to a second set of strongly overlapped bands lying between 180 and 150 nm. These absorptions are less intense (emax z 7000 M-I cm-l) and the main peaks appear at 169 and 163 nm. Beyond 150 nm a series of strong structureless absorptions reach the limit of our spectrometer. Fe(CO), Rydberg Excited States. The calculated excitation energies to the ligand field and Rydberg states accessible through allowed transitions (‘A’! ‘E’ and ‘A’] ‘A;) are reported in Table 11. The excitation energies to the ionic states ZE’and ZE” corresponding to the loss of one electron from the 3d, and 3d, orbitals, respectively, leading to the first and second vertical ionization potentials (IP) were also computed. At the CASSCF level the fist IP is calculated at 68 400 cm-’ (8.60 eV) in excellent agreement with the experimentz9 (8.60 eV), while at the CCI level it is somewhat lower (8.00 eV). The CCI value is close both to the MCPF value of 7.75 eV obtained by Barnes et al.30and to a more recent experimental data of 7.877 eV quoted in ref 30. For the second IP, both the CASSCF and CCI values (79 100 cm-l or 9.81 eV and 79 600 cm-’ or 9.87 eV, respectively) agree with the photoelectron experiment (9.86 eV). The lowest state denoted a ‘E’ and corresponding to a 3d, 3d,2 excitation with a CI coefficient on the main configuration greater than 80% is calculated at 29 100 cm-I. This value is in excellent agreement with the fact that both the electronic spectrum and the photochemistry of Fe(CO)5 start around 28 600 These results make it clear that the calculations reported in the present study are quantitatively accurate. To the state of our knowledge, these are the best calculated values obtained both for the ionization potentials and the lowest metal d-d excitation in Fe(C0)5.5~7J*30 The next states characterized as Rydberg states correspond to 3d 4s, 3d 4p, and 3d 4d excitations (with a CI coefficient on the main configuration greater than 80%). The promotion of
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(29) Lloyd, D. R.; Schlag, R. W. Znorg. Chem. 1969,8, 2544. (30) Barnes, L. A,; Rosi, M.; Bauschlicher, C. W. J. Chem. Phys. 1991, 94(3), 2031.
The Journal of Physical Chemistry, Vol. 96, No. 1, 1992 123
the optical electron from the 3d orbitals to the Rydberg orbitals gives rise to nine allowed Rydberg transitions3’separated into two series depending on the original orbital (3d, and 3d,). The excitation energies to these states range from 49 600 to 61 800 cm-l for the first series (below the first ionization potential) and from 64 100 to 71 800 cm-’ for the second series (below the second ionization potential). The value of 56200 cm-’ obtained for the xIA’~ clE’ transition is comparable to the atomic energy level of 5 1000 cm-’ 32 associated with the Rydberg state corresponding to a 3d 4d excitation in the iron atom. However, a direct comparison with the atomic splittings is hazardous since the metal atom in the complex is described by a 3d8 electronic configuration corresponding to an excited state 4.1 eV higher than the atomic ground state 3d64sZ. A term value of 18800 cm-’ is obtained for the transition X’A’~ blE’ corresponding to a 3d 4s excitation. This is in rather good agreement with the range for the term value of the first Rydberg state in the iron atom32which is 20 895-18 105 cm-I. The transitions corresponding to 3d 4p and 3d 4d excitations are characterized by a term value comprised between 12 80015 000 and 6100-9100 cm-I, respectively. Except for the transitions xIA’~ clE’ and x’A’~ dlE’, these values follow the well-established term values trends, namely a decrease of these one when going from 4s to 4d bands. Because of the low intensity of the Rydberg bands,” it is probable that the first series calculated between 49 600 and 61 800 cm-l do not account for the strong absorption centered at 194 nm (51 500 cm-I) in the electronic spectrum. This band could be assigned as a metal to ligand charge-transfer or internal ligand transition which would cover the first Rydberg series. Further investigations in order to perform an assignment of the valence transitions in Fe(CO)5 are now in progress. The second Rydberg series is predicted to range from 64 100 to 71 800 cm-’ and could correspond to the f i s t weak absorptions observed beyond 150 nm (66600 cm-I). The excellent results obtained for the excitation energies to the ligand field state corresponding to the allowed transition ‘Alt I’E and to the ionic states ZE’and ZE”corresponding to the first and second IP, respectively, show that the theoretical level used in the present work is quantitatively accurate. However, this sophisticated computational method is far from being systematic since the difficulty of the CASSCF convergence, even for states as those considered here with a leading configuration in the CI wave function.
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Acknowledgment. We thank M. GelizB and A. Dargelos for their help in recording the VUV spectrum. The calculations have been carried out on the CRAY-2 computer of the CCVR (Palaiseau, France) through a grant of computer time from the Conseil Scientifique du Centre de Calcul Vectoriel pour la Recherche. This work was partially supported by the Direction General de Investigacion Cientifica y T h i c a (grant PB86-0140). Registry No. Fe(CO)5, 13463-40-6. (3 1) Only eight Rydberg transitionsare reported in Table 11. Our attempts to converge the CASSCF procedure on the Rydberg state corresponding to the 3d 4d,2 excitation were unsuccessful. Namely the presence of the state correspondingto the 3d 3d,2 in the same symmetry causes the higher root to collapse into the lowest one. (32) Moore, C. E. Afomic Energy levels; Nat. Bur. Stand. Circ. No. 467; US GPO: Washington DC, 1949.
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