Theoretical aspects of the photochemistry of organometallics. 3

Journal of the American Chemical Society 0 (proofing), ... of First-Row Transition Metal Compounds by Spin Component Scaled MP2 and MP3 Methods. Isabe...
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J . Phys. Chem. 1984, 88, 4805-481 1

IV. Concluding Remarks In this work, we have solved the surface-dressed optical Bloch for the of a strong driving field, An analytic form is obtained for the two-level adatomic fluorescence spectrum. It is shown that the surface-reflected field and surface plasmon resonance will strongly modify the spectrum. Due to the interference between the incident laser field and the surface-reflected field, an interesting surface-induced asymmetry OCCUrS in the side bands which is quite different from the ~ ~ s uW-Phase al collision-induced asymmetry. Surface-enhanced photochemistry iS an interesting phenomen0n,15316and we are looking at extensions of our model to other (15) A. Nitzan and L. E. Brus, J. Chem. Phys., 74,537 (1981); 75,2205 (1981).

4805

types of spectroscopic and dynamic problems. These include the cooperative s~ctroscopyof many interacting atoms near a surface and the excitation and dissociation processes of polyatomic molecules adsorbed on a metallic surface, taking into account surface roughness.

Acknowledgment. This work was supported in pari by the Air Force Office of Scientific Research (AFSC), Unitdd States ~i~ Force, under Grant AFOSR-82-0046, the Office of Naval Research, and the U S . Army Research Office. T.F.G. acknowledges the Camille and Henry Dreyfus Foundation for a Teacher-Scholar Award (1975-84) and the John SimonGuggenheim Memorial Foundation for a Fellowship (1983-84). X.Y.H. thanks Prof. M. G. Raymer for a useful discussion. (16) C. J. Chen and R. M. Osgood, Phys. Reu. Lett., 50, 1705 (1983).

Theoretical Aspects of the Photochemistry of Organometallics. 3. Potential Energy Curves for the Photodissociation of Fe(CO), Chantal Daniel, Marc BGnard, Alain Dedieu, Roland Wiest, and Alain Veillard* Laboratoire de Chimie Quantique, E.R. no. 139 du C.N.R.S., Universite L. Pasteur, 67000 Strasbourg, France (Received: November 22, 1983)

-.

The photodissociation of Fe(CO)s, Fe(CO), Fe(CO), + CO, has been studied theoretically through ab initio configuration interaction (CI) calculations of the potential energy surfaces which connect the ground and low excited states of Fe(CO)5 with the ground and excited states of the products. ‘The calculations were carried out for the dissociation of an equatorial ligand under C, constraint with a Gaussian basis set (13,8,6/9,5) contracted to [5,3,3/3,2]. Two different sets of CI calculations were performed. The first one (called the small CI) was used to construct the potential energy curves. It is restricted to the metal 3d electrons and based on a set of 15 active orbitals (mostly metal 3d and ligand T * ) , with respectively five and four reference states for the singlet and triplet calculations and ah single and double excitations within the set of active orbitals relative to these reference states. In order to describe more accurately the energetics of the reaction, a second set of calculations (called the large CI) involved the metal 3d electrons and the six electrons of the equatorial Fe-C bonds with a set of 47 active orbitals and all single and double excitations within this set relative to one reference state. The lowest excited state of Fe(CO)Sis calculated to be the ligand field (LF) state 3E’ at 33 850 cm-’ above the ground state lA1’, with four singlet statesbetween 35 000 and 41 000 crn-’. SCF calculations fo; Fe(CO), yield a 3B2ground state of C, symmetry. CI calculations for the dissociating system Fe(CO),-CO were carried out for a number of points along the reaction path (calculated at the SCF level) for the thermal dissociation of an equatorial ligand in the state ‘Al. It is found that a single potential energy surface connects the LF state 3E’ of Fe(CO), to the giound state 3B2 of the products Fe(CO), + CO. It is proposed that the photodissociation of Fe(CO)S occurs through excitation to a singlet state followed by population of the 3E’ state through intersystem crossing and subsequent dissociation to the products of the reaction along the 3B2potential energy surface. With the large CI the reaction is calculated to be endothermic by 36 kcal/mol (43 kcal/mol with the Davidson correction vs. an experimental value of 55 kcal/mol). A qualitative analysis indicates that the connection between the LF state 3E’ of Fe(CO)5 and the ground state of the products is independent of the initial assumptions, dissociation of an equatorial ligand under C2, or C, constraint, or dissociation of an axial ligand under C3, or C, constraint. The reverse reaction, the recombination of Fe(CO), with CO, is spin forbidden in the absence of spin-orbit coupling but becomes allowed with a very low barrier when spin-orbit coupling is introduced. Finally, the photochemical loss of a carbonyl ligand is briefly discussed for the other metal carbonyls M(CO), (M = Cr, Mo, W) and Ni(CO),.

The primary result of irradiating Fe(CO)S is believed to be that represented by1 Fe(CO),

2 Fe(CO), + C O

Scheme I X

(1)

The quantum yield for C O production in the condensed phase was found to be close to 1.z The reaction may proceed further according to eq 2 since Fe(CO), is known to be photosensitive with

respect to CO loss in low-temperature mat rice^.^ Photolysis of

Fe(CO)5 in the gas phase can even yield Fe atom^.^-^ Furthermore, photolysis of Fe(CO)S yields an exceedingly active catalyst.s Substituted iron carbonyls such as Fe(C0)4(C2H4) also undergo photochemical loss of do9so that reaction 1 may

(1) Geoffroy, G. L.; Wrighton, M. S. “Organometallic Photochemistry”; Academic Press: New. York, 1979. (2) Balzani, V.; Carassiti, V. “Photochemistry of Coordination Compounds”i Academic Press: London, 1970. (3) (a) Poliakoff, M. J. Chem. SOC.,Dalton Trans., 1974, 210. (b) Poliakoff, M.; Turner, J. J. J . Chem. SOC.,Faraday Trans. 2 1974, 70, 93.

(4) Callear, A. B.; Oldman, R. J. Nature (London) 1966, 210, 730. ( 5 ) Karny, Z . ;Naaman, R.; Zare, R. N. Chem. Phys. Let?. 1978, 59, 33. (6) Hellner, L.; Masanet, J.; Vermeil, C. Nouu. J. Chim. 1979, 3, 721. (7) Nathanson, G.; Gitlin, B.; Rosan, A. M.; Yardley, J. T. J. Chem. Phys. 1981, 74, 361. (8) Mitchener, J. C.; Wrighton, M. S. J Am. Chem. SOC.1981,103,975. (9) Mitchener, J. C.; Wrighton, M. S. J . Am. Chem. Soc. 1983, 105, 1065.

Fe(CO),

Fe(C0)3 + CO

(2)

0022-3654/84/2088-4805$01 S O / O

0 1984 American Chemical Society

The Journal of Physical Chemistry, Vol. 88, No. 21, 1984

4806

be considered as the prototype for a rather general class of photochemical carbonyl eliminations and substitutions. Little is known about the mechanism of the photodissociation of Fe(CO)5, for instance about the nature of the excited states which may be involved in this process. It has been assumed that the photoreaction is associated with the lowest energy transition, one of the ligand field type corresponding to a one-electron excitation e’ a’’ involving the 3d levels of iron.1° No detailed theoretical study has been reported for the potential energy surfaces corresponding to this reaction; previous theoretical arguments were restricted to the consideration of the change induced in the electronic structure of Fe(CO), by the electronic excitation.1’-12 We present here a theoretical study of reaction 1, based on ab initio configuration interaction (CI) calculations of the potential energy surfaces which connect the ground and excited states of Fe(C0)5 with the ground and excited states of the products Fe(CO)4 CO.

-

+

The Calculations The SCF Calculations. The calculations were carried out for the dissociation of an equatorial ligand (Scheme I). Fe(CO)5 is a trigonal bipyramid13 and belongs to the point group D3h;its geometr was slightly idealized by assuming the same bond length of 1.82 for the equatorial and axial F e C bonds (experimentally, Fe-C, = 1.83 A and Fe-C,, = 1.81 A). Furthermore, all metal-carbon bond lengths, except for the dissociating bond, were kept equal to this value for all the points on the potential energy surfaces. Fe(C0)4 has the C , symmetry (cf. below), and we have assumed that this symmetry is retained along the reaction path. The choice of axes is indicated in Scheme I. The C - O bond length was set equal to 1.15 A. When the carbonyl ligand dissociates, the fragment Fe(C0)4 rearranges. We have assumed that the extent of this rearrangement depends only on the distance of the dissociating ligand and that it is the same for all the electronic states. We have calculated this rearrangement (defined by the two angles a and p; cf. 1) a t the S C F level for the potential energy surface ‘A, corresponding to the ground-state ‘A1’ of Fe(CO)5. Some justification for this assumption may be found in the fact that the states 3B2and ‘Al of Fe(C0)4 have comparable geometries (a = 140’ and p = 120’ for the state 3B2vs. a = 160’ and fi = 120’ for the state ‘Al; see below). The S C F calculations were carried out with the system of ~ the following Gaussian basis sets: for programs A S T E R I X , ~using iron a (13,8,6) set contracted to [5,3,3]15 and for the first-row atoms a (93) set contracted to [3,2]17 (namely, the contracted basis set is single-f for the core orbitals and the iron 4p orbital and double-{ for the valence orbitals except for the iron 3d orbital which is triple-c). Calculations were carried out for several points along the reaction path, corresponding to an iron-carbon distance of 1.82, 2.25, 3.00, 5.00, and 50.00 A. For each point, two SCF calculations were performed, one for the closed-shell configuration corresponding to the ground state of Fe(CO)5 (13al’)2(la2O2(10e’)2(8a2/1)2(3e”)2 in D3* symmetry or (23a1)2(3a2)2(11b1)2(1 1b2)2 in C, symmetry and the other one for the open-shell configuration corresponding to the ground state of Fe(C0)4 + CO.

1

~~

(10) Yardlsy, J. T.; Gitlin, B.; Nathanson, G.; Rosan, A. M. J . Chem. Phys. 1981, 74, 370. (11) Wrighton, M. S . Top. Curr. Chem. 1976, 65, 37. (12) Vogler, A. In “Concepts of Inorganic Photochemistry”; Adamson, A. W., Fleischauer, P. D., Eds.; Wiley: New York, 1975; p 269. (13) Beagley, B.; Schmidling, D. G. J . Mol. Strucf. 1974, 22, 466. (14) Bbnard, M.; Dedieu, A.; Demuynck, J.; Rohmer, M.-M.; Strich, A.; Veillard, A.; Wiest, R., unpublished work. (15) This basis set has been set up by adding a diffuse p-type function (exponent 0.15) and a diffuse d-function (exponent 0.09) to the (13,7,5) set of - - ref .- - I6 - -.

(16) Hyla-Kryspin, I.; Demuynck, J.; Strich, A.; Bbnard, M. J . Chem. Phys. 1981, 75, 3954. (17) Huzinaga, S . Technical Report, University of Alberta, Edmonton, 1971.

Daniel et al. (24a1)1(3a2)2(1 lbl)2(11b2)l The open-shell S C F treatment was based on the restricted Hartree-Fock formalism proposed by Guest and Saunders.18 All oneand two-electron integrals were computed with single-word accuracy on the Univac 11 10 (word of 36 bits). The S C F calculations were performed with double-word accuracy. The CI Calculations. CI calculations were carried out at two different levels. The first one (called the small CI) was used to construct the potential energy curves (ground and excited states) for reaction 1. For each point along the reaction path, two C I calculations were performed based on the orbitals from either the closed-shell or the open-shell S C F calculation, retaining the calculation which yielded the lowest total energy. These small CI calculations involved only the metal 3d electrons and were based on a set of 15 active orbitals. Of these 15 orbitals, four are doubly occupied in the closed-shell S C F configuration (10e’ and 3e” in D3h symmetry or 23al, 3a2, 1lbl, and 1 1b2in C, symmetry) while three are doubly occupied and two singly occupied in the open-shell S C F configuration (respectively 23a1, 3a2, 1lbl and 24al, llb2). These 15 active orbitals may be classified as 9 orbitals having mainly a metal 3d character, 1 orbital being essentially 4s and 5 orbitals mainly R* in character. Since the lowest excited states of Fe(C0)5 and Fe(C0)4 are ligand field d-d and metal-to-ligand L CT) d R* states, we felt that an charge-transfer (M appropriate description of the corresponding potential energy surfaces would require that the d and R* orbitals be well represented in the active set. We note that configurations involving excitation from 3d, to R*CO were found important for a description of the bonding in the ground state of NiC0.19 The singlet calculations used five reference states, one being the dominant S C F configuration and the four other ones corresponding to the singly excited configurations arising from the replacement 1Oe’, 15al’(dxz)for Fe(CO)5. The triplet calculations used four 3e” reference states corresponding to these singly excited configurations. All single and double excitations within the set of active orbitals relative to these reference states were considered. The number of configurations was 3415 and 5040 for the singlet and triplet calculations, respectively. The second series of CI calculations, called the large CI, were carried out only for the reactant Fe(CO)5 and for the products Fe(C0)4 + CO 50 A apart, since their aim was to describe rather accurately the energetics of reaction 1. During the course of this reaction, there are two major changes at the electronic level: one is a change in the electronic configuration of the metal 3d electrons, from a closed-shell singlet 3d8 configuration to an open-shell triplet configuration; the other one is the cleavage of the metal-carbonyl bond. In order to obtain an acceptable value for the relative energies of the reactant and products, one needs probably (i) a fair evaluation of the correlation energy associated with the broken pair of 3d electrons, which in turn requires that the active set includes a sufficiently large number of 3d, 4s, and 4p metal orbitals and (ii) an evaluation of the change in the correlation energy during the dissociation of the dative bond, which requires that the active set includes a number of orbitals localized in the region of the lone pair of the dissociating carbonyl. These large C I calculations involve the metal 3d electrons and the six electrons of the equatorial F e C bonds. They are based on a set of 47 active orbitals selected according to the above criteria. All single and double excitations within this set of active orbitals were considered relative to one reference state corresponding to the dominant Hartree-Fock configuration. The number of configurations was 10416 and 13 355 for the singlet and triplet calculations, respectively. The contribution of unlinked quadruple excitations was estimated according to the Davidson relationZoand was simply added to the SD-CI energy. The first series of CI calculations was made with a program written by Bdnard and Wiest2’ for single and double excitations

-

-

-+

(18) Guest, M. F.; Saunders, V. R.Mol. Phys. 1974, 28, 819. (19) Bagus, P. S.; Roos, B. 0. J . Chem. Phys. 1981, 75, 5961. (20) Langhoff, S. R.; Davidson, E. R.Int. J. Quantum Chem. 1973,7,999. (21) Bbnard, M.; Wiest, R.,unpublished work.

The Journal of Physical Chemistry, Vol. 88, No. 21, 1984 4807

Photodissociation of Fe(CO)S TABLE I: Calculated Excitation Energies of Fe(CO)< state 1Et! ’A1’ 3EtI

[A,’ LE’ LA;

’E!! 3A21 ’E‘ 3Al’ IA r t

E!

1

LE!! 3~

rr

LA

rr

3 ~ r r

’Al’’ )E’

excitation energy, cm-l 51 050

----

--

d-d

50 070 50 040 49810 46210 44 920 44 440 44 380 43 780

d

42 560 40 990 40 840

d

37 640 37 100 35 760 35 550 34 990 33 850

electronic excitation 3e” 15al’

type d d d

+

3e” 4e” 10e’ 9 a F 10e’- lle’ 10e’- lle’

R* R* ?r*

--

?r*

d -,R*

10e’

d-d d

3e” 10e’

R*

d-..rr* d

d-d d d d d d d-d

--

+

lle’ 15al’ lle’

R*

1 0 e ’ d lle’ 10e’-, lle’ 10e’ -,4e” 10e’ 15al’ 10e’ 4e”

R*

10e’

R*

10e’

R*

10e’

R*

10e’

R* R*

10e’

-----

4e” 4e” 4e”

4e” 15al’

2 .o

d [A]

-

I

\

1 ?.

2 ?.

we have not completely explored the potential energy surface of Fe(C0)4 with C2, symmetry as a function of the angles a and B, the results of Table I1 point to a ground-state 3B2of C , symmetry (with a = 140’ and B = 120’) at the SCF level. Since it is known that the SCF calculation is usually biased in favor of the triplet relative to the singlet by the correlation error, the calculated energy difference of 0.031 au between the lowest triplet and singlet (IAl, C3, symmetry) states could well be recovered when the correlation energy is included in the calculation. For this reason we have carried out two CI calculations involving the metal 3d electrons, with respectively 33 and 43 active orbitals and one reference state. It turned out that the relative energy of these two states was not changed appreciably. Previous theoretical studies at the extended Hiickel level yielded conflicting results of D2d,D,,, and C2, geometries for the low-spin system but agreed on a C2, geometry for the high-spin system.2628 Experimentally, Fe(C0)4 has been shown to be p a r a m a g n e t i ~ ,and ~ ~ the analysis of the infrared spectrum indicates a C, structure with bond angles near 120 and 145°.30 Thus, there appears to be a good agreement between our theoretical results and the experimental information regarding the ground state of Fe(CO),. For the state ‘Al, the energy minimum was found for a = 160’ and B = 120’. The Potential Energy Surfaces f o r the Photochemical Dissociation of an Equatorial Ligand. Figure 1 shows the potential energy surface for the thermal dissociation of an equatorial ligand

-

-

(22) (a) Paldus, J. J . Chem. Phys. 1974, 61, 532!. (b) Paldus, J. Theor. Chem.: Adu. Perspect. 1976, 2, 131. (23) (a) Brooks, B. R.; Schaefer, H. F. J . Chem. Phys. 1979.70, 5092. (b) This program was adapted for the IBM computers by F. Brown and I. Shavitt. (c) It was interfaced for use in conjunction with the ASTERIX system of programs on the Univac 11 10 by J. Demuynck and R. Wiest. (24) Shavitt, I. Int. J . Quantum Chem. Symp. 1977, No. 11, 131. (25) Dartiguenave, M.; Dartiguenave, Y . ;Gray, H. B.Bull. SOC.Chim. Fr. 1969, 12, 4223.

5.0

energies of 35 760, 37 640,40 850, and 40 990 cm-’ for the lowest singlet states (the only symmetry-allowed transitions being from the ground-state ‘Al’ to the states ‘E’ and ‘Ai’; however, it is known that the symmetry-forbidden transitions are generally observed with a moderate intensity due to some coupling of the vibrational and electronic wave functions2). The Ground State of Fe(CO),. The geometry of Fe(CO), in its ground state has been determined at the SCF level. The S C F energies obtained for the geometries corresponding to the point groups Td,C, (I), and C3, (2) are reported in Table 11. Although

Results and Discussion The Ground and Excited States of Fe(CO)5. For the ground-state ‘Al’ of Fe(CO)5, the CI coefficient of the closed-shell d8 wave function was 0.96 in the small C I and 0.93 in the large CI. Other important configurations involve single or double replacement of the 3da orbital by a ligand a* orbital from the S C F configuration. We have reported in Table I the energies of the excited states relative to the ground state. We find that the lowest excited state at about 34 000 cm-’ is a triplet 3E’corresponding to the ligand field excitation e’ a’’. Next, between 35 000 and 50 000 cm-’ are found six triplet and six singlet states corresponding to ligand-to-metal charge-transfer (L M CT) states of the d a* type (the triplet state being usually at about 5OC-4000 cm-’ below the corresponding singlet state). Imbedded between is the singlet state ’E’ corresponding to the above-mentioned ligand field excitation. The fact that the lowest excited state of Fe(CO)5 is the 3E’state (which is probably the clue to the photochemical reactivity of Fe(CO),; see below) appears as a consequence of a large singlet-triplet splitting of 7000 cm-’. At somewhat higher energies (above 44000 cm-’) are found the states 3E’’ and ‘E’’ associated a,’. with the other ligand field excitation e” Unfortunately, direct comparison is precluded since the experimental spectrum of Fe(CO)S is poorly resolved. It exhibits a shoulder near 41 500 cm-’ and another one at 35 500 cm-’ but otherwise seems f e a t u r e l e ~ s . ~It~is expected that these details of the spectrum correspond to absorption from the singlet states. These values are in good agreement with the computed excitation

-

L.0

Figure 1. Potential energy surface for the thermal dissocietion of an equatorial ligand under C, constraint (state ]Al) as a function of the distance d of the ligand and of the angle a.

from a multireference wave function and based on the unitary group approach.22 The second set of C I calculations was carried out with the C I program developed originally by Brooks and SchaeferZ3for single and double excitations from a single-reference wave function, using the graphical unitary group approach.24 The nontheoretically oriented reader is warned that some of the calculations presented here (namely the small CI calculations) are survey in character and may not be quantitatively reliable (as it will be found for the energetics of the reaction). The small C I treatment may yield a qualitatively correct sequence of excited states for Fe(CO)5 but does not recover a significant portion of the valence correlation energy. For this reason we urge the reader to view the figures generated more as computational-state correlation diagrams than as surface cross sections.

-

3.0

I

(26) Burdett, J. K. J . Chem. SOC.,Faraday Trans. 2 1974, 70, 1599. (27) Elian, M . ; Hoffmann, R. Inorg. Chem. 1975, 14, 1058. (28) Pensak, D. A,; McKinney, R. J. Inorg. Chem. 1979, 18, 3407. (29) Barton, T. J.; Grinter, R.; Thornson, A. J.; Davies, B.; Poliakoff, M. J . Chem. SOC.,Chem. Commun. 1971, 841. (30) Poliakoff, M.; Turner, J. J. J . Chem. SOC., Dalton Trans. 1974, 2276.

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The Journal of Physical Chemistry, Vol. 88, No. 21, 1984

Daniel et al.

TABLE II: SCF Energies (in au and Relative to -1710) for the Various Geometries of Fe(COL angles' ff R symmetry state electronic config' 109O28' 1200 150' 130' 140' 150' 160' 140' 150' 160' 170°

e=

1000

-0.17273 -0.18270 -0.162 60 -0.184 74 -0.186 62 -0.185 95 -0.18373 -0.139 06 -0.141 61 -0.142 72 -0.141 58

"41

C3"

(x420i42(x2- Y2)2(xY)2 (xz)2cvz)2(x2- Y2)' (z )

1 for the

C2,symmetry; 0 as defined in

2 for the C, symmetry. bThe choice of axis is the one of Scheme I.

TABLE III: CI Energy Values (in au and Relative to -1822) along the Potential Energy Curves of the Reaction Fe(CO)5 the Dissociation of an Eauatorial Ligand (Ground State and Ligand Field Excited States of Fe(C0)d 1.82 A

"42

-0.605 89

'Bl 'A2

-0.63601

'A, 'B2 'Al 3B2

-0.652 42

"41

-0.838 52' -0.822 18'

-0.684 27

From the closed-shell S C F orbitals.

-0.15566 -0.177 14

w

3E

'a and 0 as defined in

energy

109O28' 1200 150' 120° 120° 1200 1200 1200 1200 1200 120°

2.25 8, -0.715 18 -0.736 80 -0.754 28 -0.772 51 -0.74008 -0.774 23 -0.798 15 -0.838 64 -0.823 33' -0.83495'

3.0 8, -0.736 09 -0.758 57 -0.771 50 -0.792 75 -0.761 06 -0.801 87 -0.822 09 -0.859 43 -0.8 15 99' -0.841 21'

-

+ CO for

Fe(CO),

5.0 8,

50.0 8,

-0.747 75 -0.763 89 -0.776 58 -0.795 96 -0.78409 -0.8 15 72 -0.84003 -0.865 07 -0.806 80" -0.842 27'

-0.751 68 -0.769 23 -0.783 88 -0.802 82 -0.778 53 -0.810 33 -0.83475 -0.864 46 -0.809 00" -0.842 39'

'From the open-shell SCF orbitals.

in the state 'Al (corresponding to the ground-stpte 'Al' of Fe(CO)5) as a function of the distance of the dissociating ligand and of the angle a (the angle was kept to a fixed value of 120' corresponding to the value in both Fe(CO)S and Fe(C0)4). The potential energy curves reported below for the ground and excited states are sections of the corresponding potential energy surfaces along the reaction path shown in Figure 1. In Table I11 we have reported the calculated C I energies for the lowest potential energy curves (corresponding to the ground state and to the ligand field excited states of Ge(CO),) for the dissociation of an equatorial ligand. One will notice that, except for the reactant Fe(CO)5, the lowest CI energy corresponds to the SCF molecular orbitals from the open-shell calculation (this is understandable since in the open-shell calculation the five molecular orbitals with 3d chayacter are variationally optimized while in the closed-shell calculation only four of them are optimized, the fifth one being a virtual orbital). The potential energy curves are shown in Figure 2. Before we comment on the essential conclusion, let us examine first some inaccuracies of these potential energy curves. For instance, one detail of these curves, the slight minimum seen in the vicinity of 5.0 A, p a y be an artifact of the calculation since it is not clear whether it represents a true minimum (corresponding for ipstance 'to a dipole-dipole interaction) or whether it is a consequence of the truncation of the C I basis set. A more serious default is the greater stability of the products Fe(C0)4 + C O compared to that of the reactant Fe(C0I5. The reaction is calculated to be exothermic by 16 kcal/mol at the small CI level (and by about 2 kca'l/mol at the S C F level). Experimentally, the reaction is known to be endothermic by about 55 f 11 kcal/m01.~' Several reasons may contribute to this large error, like for instance the lack of bond length optimization. Increasing the basis set will act in the wrong way since the basis set superposition error biases the calculation in favor of Fe(CO)5. However, the main reasons for this large error must be (i) the

change in the correlation energy of the 3d electrons since Fe(CO)5 has a closed-shell structure while Fe(C0)4 has two unpaired 3d electrons and (ii) the change in the correlation energy of the dative bond between the carbonyl ligand and the metal (the correlation energy has been found to contribute to the total binding energy by as much as 32% in borazane and 62% in borane ~ a r b o n y l ~ ~ , ~ ~ ) . We come back to this problem in the next paragraph. Anyway, the most important feature of Figure 2, one which should remain unchanged in more refined treatments, is probably the existence of a single potential energy surface which connects, without any barrier, the ligand field excited state 3E' of Fe(CO)5 to the ground-state 3Bz of the products Fe(CO), + C O of the

(31) Engelking, P. C.; Lineberger, W. C. J . Am. Chem. SOC.1979, 101, 5570.

(32) Redmon, L. T.; Purvis, G . D.; Bartlett, R. J. J. Am. Chem. SOC.1979, 101, 2856. (33) Zirz, C.; Ahlrichs, R. J . Chem. Phys. 1981, 75, 4980.

a

-182260

-1BZZ.R

-1822.81

4

I

I

I

2.

3.

L.

5.

JkG3

Figure 2. Potential energy curves for the photochemical dissociation of an equatorial ligand under C2,constraint as a function of the distance of the ligand.

-

The Journal of Physical Chemistry, Vol. 88, No. 21, 1984 4809

Photodissociation of Fe(CO)5 3ES

Reactant

0

Products

Figure 3. State correlation diagram with the singlet ground state of the reactant correlating an excited state of the products and a triplet excited state of the reactant correlating the ground state of the products. U

Spin-orbit couphng

Figure 5. The ligand field orbital of symmetry a,’ in Fe(CO),. Scheme I1

P

c

UrirnoLccular dissociation

+

F e ( C O ) L +CO

state being responsible for the photochemical reaction, “the spin-forbidden 3Tlgstate being the major state that is responsible for photochemical reactivity .... The implication is that direct irradiation of the L F bands is followed by efficient intersystem crossing to the lowest triplet state”.37 Also, for Cr(bpy),’+ (bpy Fe = 2,2’-bipyridine) in aqueous solution, a model has evolved which Figure 4. The proposed mechanism for the photochemical dissociation assumes that the lowest excited states (,T,/’E) are populated by of Fe(CO),. nearly quantitative intersystem crossing from the very short-lived lowest quartet state (4T2)with back intersystem crossing not being reaction (the ground state of CO being ,Al). Thus, the state ’E’ an important pro~ess.’~ is most probably involved in the photochemistry of Fe(CO)5, in The Energetics of the Dissociation Reaction. As mentioned agreement with our initial prediction from a state correlation above, the more serious default of the small CI calculations is the diagram based on the symmetry and spin-conservation rules34 calculated exothermicity of the reaction. The large CI calculations (Figure 3). In this diagram a reactant with a singlet ground state were intended to remedy this default. The reaction is now found has a triplet excited state which correlates with the ground state to be endothermic by 35.7 kcal/mol (Fe(CO)5 EcI = -1822.9958 of the products, with the thermal reaction being spin forbidden au, Fe(C0)4 + C O a t 50 8, Ec, = -1822.9390 au) and 42.8 (we discuss below the effect of spin-orbit coupling on this forkcal/mol with the Davidson correction.20 These values compare biddenness). This type of diagram appears relatively common rather well with the experimental value of 55 f 11 kcal/m01.~~ in organometallics photochemistry, and we have already met it Sherwood and Hall have calculated a dissociation energy, at the in the following photochemical reactions: (i) carbonyl el‘imination S C F level, of 49.8 kcal/mol for Cr(C0)6 and of 30.8 kcal/mol from Fe(CO), and from M o C ~ , C O , ’ ~(ii) reductive elimination (to be compared with the experimental value of 33.0 of molecular hydrogen from Fe(C0)4H2and M o C ~ ~ Hand ~ , ~ ~for* Cr(C0)6f ~ ~ kcal/mol for the latter).39 (iii) declusterification of the trinuclear clusters R u ~ ( C O ) , ~ , The Stereochemistry of the Carbonyl Photoelimination. In H C C O ~ ( C O )and ~ , H3Re3(C0),,.36 our earlier analysis of the carbonyl p h o t ~ e l i m i n a t i o n ~and ~ J ~in We may consider the state 3E’of Fe(CO), as the photoactive the calculations reported above, we have assumed that the leaving excited state in the sense that there is an adiabatic surface which carbonyl is an equatorial one. Then a natural hypothesis is that allows this excited state to evolve directly to the ground state of C,, syrnmetry.is retained along the reaction path, and this greatly the primary products. But this does not imply that the state 3E’ simplifies not only the analysis but also the calculations. However, is reached directly through the irradiation, since the transition this assumption is at variance with a remark12 that, if the pho‘Al’ 3Er is spin forbidden. Although this forbiddenness may toreaction is associated with the ligand field transition e’ a,!, be removed through spin-orbit coupling, it is usually considered the promotion of an electron into the strongly antibonding dXz that, in the 3d metal complexes, population of spin-forbidden states orbital (Ox being the C3axis) should cause the release of an axial by direct light absorption can be seldom achieved.37 A more likely ligand. We want to show in this section (i) that this argument possibility is that the state 3E’ becomes populated through inis erroneous, namely that the promotion of an electron into the tersystem crossing from a spin-allowed singlet excited state of antibonding d,z orbital weakens equally the axial and equatorial Fe(CO)5. Since the 3E’ state is the lowest excited state, the bonds, and (ii) that our analysis can be extended to the case where molecule gets trapped in the potential well of this state. From the leaving carbonyl is an axial one without changing the conthere it can dissociate to the products of the reaction along the clusions. 3B2potential energy surface (Figure 4). First, examination of the virtual 3d orbital of symmetry a,’ A related mechanism has been proposed for the Co(1II) pho(either from the above S C F calculation or from an extended to substitution^^^^^ with the lowest spin-forbidden ligand field excited Hiickel calculation) shows that this orbital is antibonding for both the axial and equatorial ligands (Figure 5) (the coefficients being (34) Veillard, A. N o w . J . Chim. 1981, 5 , 599. (35) Vejllard, A.;Dedieu, A. Theor. Chim. Acta 1983, 63, 339.

-

(36) Veillard, A,; Dedieu, A. Nouu. J. Chim., in press.

(37)Zinato, E. In “Concepts of Inorganic Photochemistry*;Adamson, A. W., Fleischauer, P. D., Eds.; Wiley: New York, 1975;p 143.

-

(38) Jamieson, M.A.;Serpone, N.; Hoffman, M . Z . Coord. Chem. Reo. 1981, 39, 121.

(39)Sherwood, D. E.; Hall, M. B. Inorg. Chem. 1983, 22, 93.

4810 The Journal of Physical Chemistry, Vol. 88, No. 21, 1984 TABLE IV: Correlation Table for the Electronic States of Fe(CO)5 under Various Point GrouDs“

Daniel et al. TABLE V Correlation Table for the Ground State of Fe(C0)4 under C,,, and C. Svmmetrv“

“p ~

C2” ’B2

~~

CJa, =

C,(u,l = y 0 z ) ’A’

3A/?

XOZ)

” Reference 4 1.

“ Reference 4 I .

A”, ,A’;,E:E”A

:

p

t

:

,

A

2

,

&

a

’A‘,

FCKO)~

Fc(COi+CO

Figure 7. State correlation diagram for the dissociation of an equatorial carbonyl ligand from Fe(C0)5without (a) and with (b) spin-orbit couFigure 6. State correlation diagram for the dissociation of an axial carbonyl ligand from Fe(CO)5under C3, constraint.

pling.

Scheme IV

Scheme I11

C /O

0

I

I

C

+

C/O

+

CS

CS

D3h

slightly larger for the axial ligands than for the equatorial ligands). Let us repeat our previous analysis,, but now with the assumption that the reaction corresponds to the dissociation of an axial ligand with the C, symmetry retained along the reaction path (Scheme 11). By doing so, we assume that the product Fe(C0)4 has the C3, symmetry, an hypothesis which we know to be incorrect but which is not really restrictive since the C,, structure is only 6 kcal/mol higher than the C2, structure (Table 11) and most probably Fe(CO), could easily rearrange from C, to C2,. The two ligand field excited states of Fe(CO)5 are now in C,, symmetry (Table IV). The ground state of Fe(CO), in C,, symmetry is a ,E state (Table 11). The corresponding state correlation diagram, shown in Figure 6 , is closely related to the one obtained previously for the dissociation of an equatorial ligand and the conclusions are similar: (i) the reaction should be thermally forbidden; (ii) the photodissociation occurs through the ligand field state ,E’ (this correlation between the 3E’ state of Fe(C0)5 and the ground state of the products being the consequence of a natural correlation at the level of the molecular orbitals40). Let us suppose now that the reaction corresponds still to the dissociation of an axial ligand but with only one plane of symmetry, the plane of the figure in Scheme 111, retained along the reaction path (C,symmetry with Fe(CO), having the C2, symmetry) (in this way we do not assume any more that the ligand will leave along the C, axis). There are two possible ways to distort the fragment Fe(CO), from its original geometry in Fe(CO)5 to the geometry of the product. If we assume first a deformation of the fragment Fe(CO), as shown in Scheme 111, the ground state of Fe(CO), is 3A’ and correlates with the component 3A’ of the excited-state ,E’ (Tables IV and V). If we assume now a deformation of the fragment Fe(CO), as shown in Scheme IV, the ground state of Fe(C0)4 is 3A’’and will also correlate with the lowest ligand field state ,E’. (40) Bigot, B.; Devaquet, A,; Turro, N. J. J. Am. Chem. SOC.1981,103, 6.

(41) Wilson, E. B.; Decius, J. C.; Cross, P. C. “Molecular Vibrations”; McGraw-Hill, New York, 1955; p 333.

A similar conclusion would be reached for the dissociation of an equatorial ligand with the C, symmetry retained along the reaction path (instead of the C,,symmetry assumed previously). Our main conclusion, that the lowest ligand field excited state ,E’ correlates with the ground state of the products, appears independent of the assumption on the nature of the dissociating ligand (equatorial or axial) and on the symmetry retained along the reaction path (C2, or C, for an equatorial departure, C3”or C,for an axial departure). The Effect of Spin-Orbit Coupling and the Reverse Reaction. Since the ground state is ]Al’ for Fe(CO)5 (]Al in C, symmetry) and ,B2 for the products Fe(C0)4 CO, we concluded from the state correlation diagram for the dissociation of an equatorial ligand under C, symmetry that the thermal dissociation is both spin and symmetry forbidden (Fe(CO), has a boiling point of 103 OC4*). This conclusion remains unchanged for the dissociation of an axial ligand under C3, symmetry. The thermal dissociation remains spin forbidden when the symmetry constraint is lowered to C,. Then the reverse reaction, the recombination of Fe(CO), with CO, should be equally forbidden. However, this reaction takes place at very low temperature.,, The key to this dilemma is probably to be found in the effect of spin-orbit coupling. Figure 7a represents the state correlation diagram for the dissociation of an equatorial ligand under C,, symmetry, in the absence of spin-orbit coupling. Figure 7b corresponds to the same diagram now with the spin-orbit coupling operative (for the determination of the symmetry properties when spin-orbit coupling is operative, see ref 44). The reverse reaction appears now to be symmetry allowed as the result of an avoided crossing, probably with a very low barrier, while the direct reaction remains probably hindered by a large barrier. Lowering the symmetry also contributes to make the reverse reaction symmetry allowed, at least with respect to orbital symmetry. For instance, this can be achieved, for the dissociation of an equatorial ligand, by retaining only one plane of symmetry

+

(42) Cotton, F. A.; Wilkinson, G. “Advanced Inorganic Chemistry”;Wiley: New York, 1980; p 1051. (43) Poliakoff, M.; Turner, J. J. J . Chem. Soc., Dalton Trans. 1973,1351. (44)Herzberg, G. ”Molecular Spectra and Molecular Structure”; Van Nostrand: Princeton, NJ, 1966; Vol. 111, p 14 f

The Journal of Physical Chemistry, Vol. 88, No. 21, 1984 4811

Photodissociation of Fe(CO)5

\\

lT1'

blE

lT1g

y

blE

-

'A1

lA19

L

SP

tbp Cr(CO)5 sp

Cr(CO)8

M (C:O), t C 0

M (Cole

-

a

C r (CO

Cr(CO&sp

b

-

Figure 8. The state correlation diagram for the photochemical reaction M(CO)6 M(CO)5 CO as proposed in ref 46.

Figure 9. Two possible correlation diagrams for the photochemical reaction M(CO)6 M(CO)5 CO.

(Cssymmetry) during the course of thle reaction (this plane becoming the equatorial plane of Fe(CO),). Then the energy surfaces connected with the ground st,ates of the reactants and product belong to the same representation A' of the point group

state becomes 'B2 'E. They correlate with two 'E excited states of Cr(CO), corresponding the first one to the electronic configuration b:e3ai (this one will be denoted here a'E and was calculated by Hay at 1.49 eV above the ground state) and the other one to the electronic configuration b:e3b', (denoted b'E, it is expected to be at higher energy). It is believed, since the early semiempirical calculations of Gray and Beach,47that the lowest ligand field singlet of M(CO)6 corresponds to the state lTlg.l The potential energy surface connecting the 'T,, and 'T1, excited states of Cr(CO)6 to the singlet excited states 'E of Cr(CO), will satisfy one of the following: either (i) there is a natural correlation40 between the states 'T,, of Cr(CO), and a'E of Cr(CO), and another correlation between the states lT2, of Cr(C0)6 and b'E of Cr(CO), (Figure 9a) or (ii) the state 'T,, correlates with blE and the state IT2, with a'E; this will result in an avoided crossing (Figure 9b). In any case the state 'T,, will correlate with the lowest excited state a'E of Cr(CO),, and we tentatively assign the photoactive excited state of Cr(C0)6 as the 'Tlg state. Experimental data support this conclusion. Burdett et al.46mention that UV photolysis of Mo(CO)6 occurs with irradiation at a wavelength of 314 nm (3.95 eV), which is close to the reported excitation energy of 30 150 cm-' (3.74 eV) for the transition IA,, lT1, but well below the excitation energy of 37 200 cm-' (4.61 eV) for the transition 'A,, 'T2,.' One will notice that the photochemical elimination of CO from Ni(CO), (reaction 5) represents another case where the products should be initially in an excited state. The ground state of Ni(CO), is a 'A, state, and one may reasonably suppose that the ground state of Ni(C0)3, assuming a D3h symmetry, is a lAl' state corresponding to a closed-shell 3dI0 configuration (in analogy with the 'Z' ground state with a 3d'O configuration predicted for NiC048). The 'A, ground state of Ni(CO), must correlate with the 'Al' ground state of Ni(C0)3, and the potential energy surface of the electronically excited Ni(CO), must correlate with electronically excited Ni(CO),. However, in contrast to Cr(C0)6, no excited state of the d-d type is available for Ni(CO),, and a guess of the photoactive excited state will have to await an accurate calculation of the excited states and corresponding potential energy a*(CO) C T excited states have been considered surfaces. d as responsible for this photodissociation on the basis that all absorption bands in the experimentally accessible region of the a*(CO) C T transitions.12 However, spectrum belong to d examination of the S C F wave function for the ground state of N i ( C 0 ) t 9 shows a low-lying virtual orbital with 4s character, and one may anticipate low-lying excited states of the type 3d 4s.

+

CS.

The Photochemical Loss of a Carbonyl Ligand from Other Metal Carbonyls M(CO),. The photochemical loss of a carbonyl ligand is a very general reaction which is found for a large number of metal carbonyls.' The reaction is known for the following unsubstituted metal carbonyls.' M(CO)6 [M(CO),]-

hv 7 M(C0)5 + CO

hv

[M(CO),]-

Ni(CO),

-

+Co

Ni(CO),,

M = Cr, Mo, W

(3)

h4 = V, Nb, Ta (4)

+ CO

(5)

The details of the mechanism of reaction 3 have been worked out theoretically by Hay4, and mostly by B'urdett et al.46in order to explain a complicated set of experimental findings obtained from the matrix experiments. Hay pointed aut that photodissociation of Cr(C0)6 should yield Cr(CO)5initially in an excited electronic state since the 'Al, ground state of Cr(C0)6 correlates with the 'A, ground state of Cr(CO),, then the potential energy surface of the electronically excited hexacarbonyl must correlate with electronically excited pentacarbonyl. Hay assumed that photodissociation occurs within the singlet manifold and that the excited pentacarbonyl is in the lowest excited state ('E for square pyramid and IE' for trigonal bipyramid). Burdett et al. assumed that the photoactive excited state of M(CO)6 is the IT2, state (although they also mention a variant of their mechanism which involves the triplet manifold) and analyzed in much detail the pathways interconnecting the ground-state 'A, and excited-states 'E for the square-pyramid conformation of M(CO)5 with the excited state 'E' of the trigonal bipyramid, as shown in Figure 8 (this figure represents the state correlation diagram corresponding to the molecular orbital diagram given in Figure 9 of ref 46). In what follows we shall concentrate on that part of the state correlation diagram connecting M(CO)6 to M(CO:15 assumed to be a square pyramid (this corresponds to the least-motion path for the departure of the carbonyl ligand). The likely candidates for the photoactive excited states are the two ligand field states T1, and T2, corresponding to the electronic configuration t+&e:, either singlet or triplet.'*4 Let us assume that the photochemical elimination occurs through a singlet state, either 'TI, or IT2, (Burdett et a1.& have given some argument in support of the singlet hypothesis, based on the fact that no increase in photosensitivity is observed on moving from Cr(CO), to W(CO), as would be expected if the triplet route were significant). If one assumes that the C , symmetry is retained along the reaction path, the ITl, state becomes 'A2 'E in C,,, symmetry and the IT2,

+

-

-

-

-

-

Acknowledgment. We are grateful to Dr. B. Brooks and Prof. I. Shavitt for making available the CI program based on the GUGA approach. We thank Dr. B. Levy for some useful information. Registry No. Fe(CO)5,13463-40-6;Fe(C0)4, 15281-98-8; CO, 63008-0.

-(45) Hay, P . J. J . Am. Chem. S O ~1978, . 100, 2411. (46) Burdett, J. K.; Grzybowski, J. M.; Perutz, R. N.; Poliakoff, M.; Turner, J. J.; Turner, R. F. Inorg. Chem. 1978, 17, 147.

(47) Gray, H. B.; Beach, N. A. J . Am. Chem. SOC.1970, 92, 7312 (48) Rives, A. B.; Fenske, R. F. J . Chem. Phys. 1981, 75, 1293. (49) Daniel, C., unpublished results.