5556
J . Phys. Chem. 1990,94, 5556-5559
In other words, bond orders are not necessarily of small values of the corresponding Mulliken populations (-0.052 vs -0.002) in the case of small overlap populations. A good example for this encourages us to confirm the suggestion of Grev and Schaefer' is the diborane (BzHa) molecule, in which the Be-B bond order that a weak but characteristic S i 4 i bond may exist in cyclowas calculated to be of 0.52 by STO-3G basis.'O Concerning this disiloxane. two-electron three-center system, we refer here to a detailed study It is known that Mayer's indexes are close to the chemically of Mayer15 published very recently. According to his analytical expected values even in their absolute v a l u e ~ . & ' ~ JNevertheless, ~ treatment, it was concluded that the formation of a three-center their relatiw changes in a series of molecules of a similar structure two-electron bond necessarily involves the appearance of a sigcan give us additional information and may help in understanding nificant nonzero bond order between the outer atoms as well, even the structural features of the systems studied. It should, however, if the overlap of the orbitals centered on them, and so the overlap be noted that, because of their basis set dependen~e,*-'~.'~ only population between them, can be neg1e~ted.l~ the indexes obtained in the same basis set can be compared. (As Our Si-Si bond order calculated by the comparable STO-3G' a number of successful applications showed, a carefully chosen basis is about a third of the B-B bond order in diborane1°J5 basis set, in the present case STO-3G* or 6-31G*, can yield quite (0.15-0.17 vs 0.52),which shows that significant Si-Si (covalent) reliable valences and bond orders.) bond does not exist in cyclodisiloxane but the existence of a weak The results in Table I1 show that the Si-Si bond order in bond can be assumed. cyclodisilathiane (11) is considerably less than that in cyclodisiloxane (I). In agreement with the conclusion of Grev and Conclusions Schaefer,' this result suggests that the larger the electronegativity difference between the Si and X,the larger is the probability of According to Mayer's bond orders, a weak covalent Si-Si bond a dibridged *-complex with 'significant" Si-.Si bond order. can be assumed in cyclodisiloxane (I). This bond can be neglected In the case of 1,3-dioxetane (Table II), the bond order of the in cyclodisilathiane (11) and the analogue C - C bond does not exist C-.C bond is very small (even a negative value is obtained with in 1,3-dioxetane(111). These results are in accordance with the the extended 6-31G* basis), which suggests that there is no C-C assumption of Grev and Schaefer' but seemingly in contrast to bond in this molecule. the conclusion of refs 2-5. However, there are different aspects It is worth mentioning that the weak S i 4 i bond in I is not of bonding, reflected by different theoretical quantities. (The bond manifested in the deformation density map of OKeeffe and orders, for instance, reflect exchange effects but are insensitive Gibbs.* The latter, eventually, represents the electron accumufor charge accumulation.) Therefore, we think it is useful when lation in the bonding region and thus it conforms with the the same well-defined systems are investigated thoroughly by (negative) overlap populations.2 However, as was mentioned different approaches. earlier, the bond orders are related to the exchange effects in Acknowledgment. We thank I. Mayer for helpful discussions bonding between the atoms (exchange part of the second order density matrix) and measure the degree of covalent b o ~ ~ d i n g . ~ J ~and for reading the manuscript. Financial support of the Central Research Institute for Chemistry is also acknowledged. Regism NO. 1, 34392-10-4; 11, 287-67-2; 111, 287-50-3. ( I S ) Maytr, I. J . Mol. Struct.: THEOCHEM. 1989, 186, 43.
HCo(CO), and HCo(CO), Revblted: Structure and Electronic States through ab InRlo Calculatlons Alain Veillard,* Chantal Daniel, and Marie-Madeleine Robmer E.R. No. 139 du CNRS, Institut le Bel, 4, rue BIaise Pascal, 67000 Strasbourg, France (Received: November I , 1989; In Final Form: February 2, 1990)
The structure and electronic states of HCO(CO)~ and HCo(CO)3 were investigated by means of contracted CI calculations using as reference wave function a CASSCF wave function. For HCo(CO), the trigonal-bipyramid structure of C, symmetry (with the H atom axial) is found to be more stable than the one of C, symmetry (with the H atom equatorial), by 15 kcal/mol. The )E state of HCO(CO)~ is calculated at 24 250 cm-I, and the 'A, 'Eexcitation energy is estimated to be in the range 34000-36 O00 cm-'. The ground state of HCo(CO), is assigned to the closed-shell state 'A, of the C, structure, with the 'A2 state (corresponding also to the C, structure) 7 kcal/mol higher and a trigonal-bipyramid structure (with the hydrogen atom and a carbonyl as axial ligands) in the closed-shell 'A' state 10 kcal/mol higher.
-
Introduction Cobalt tetracarbonyl hydride, HCO(CO)~,is a well-known compound, of importance as catalyst in the hydroformylation process.' Gas-phase electron diffraction experiments* have been interpreted on the basis of a molecular structure 1 of C, symmetry with the hydrogen atom occupying an axial site. The photochemistry of HCO(CO)~in argon and carbon monoxide matrices has been interpreted in terms of two concurrent reactions, one (1) Orchin, M. Acc. Chem. Res. 1981, 14, 259. (2) McNeill, E. A,: Schalet, F. R. J . Am. Chem. Soc. 1977, 99, 6243.
0022-3654/90/2094-5556%02.50/0
%
Oc,
'\
/---cO OC
I
co 1
4IO
0
H
co 2
of C O loss and the other corresponding to the homolysis of the metal-hydrogen 0 1990 American Chemical Society
HCo(CO), and HCO(CO)~Revisited hp
HCo(C0)d
CO
-+ hv 254 nm
H'
The Journal of Physical Chemistry, Vol. 94, No. 14, 1990 5557
+ HCo(C0)'
*CO(CO)~
(2)
Cobalt tricarbonyl hydride HCO(CO)~,on the other hand, is a largely hypothetical species. It is assumed to be the actual catalyst in the hydroformylation reaction. The indirect evidence concerning its existence has been summarized in ref 5 (see also ref 6). Both of these compounds have been the subject of theoretical investigations using semiempirical methods. (A complete list of references may be found in ref 5.) More recently, they have been studied through ab initio quantum chemical calculations. Those in our group were carried out mostly in order to understand the photochemistry of HCO(CO),.~** The work of Antolovic and Davidson (denoted below as AD)5*9aimed at understanding the catalytic activity of these compounds. The theoretical study of Versluis et a1.,I0 based on density functional theory, is also related to the hydroformylation process. Since both HCO(CO)~ and HCo(C0)' are among the simplest organometallics, one might hope that some agreement has been reached at the theoretical level regarding their structure and electronic states. This would be particularly important for HCo(C0)' given the paucity of experimental information. Unfortunately, this is not the case. We summarize below some of the points of disagreement. (i) In their first paper,5 AD found that, after complete geometry optimization, structure 2 for HCO(CO)~(of C, symmetry with the H atom equatorial) was more stable than structure 1 by about 1 kcal/mol at the SCF level. This conclusion was reversed in their second paper? with structure 1 becoming more stable than structure 2 at the CI level, but marginally (the energy difference being less than half a kcal/mol). On the other hand, Versluis et al. find that structure 1 is more stable than structure 2 by 15 kcal/mol at the HFS level (with full geometry optimization). Also, the large difference in the Co-H bond lengths optimized by AD for the two isomers (1.71 A for the isomer 1 versus 1.59 A for the isomer 2) seems rather surprising. The fact that the Co-H bond is much longer for isomer 1 is rather different from the results reported by Koga et al." for the isomers of the Rh complex Rh(H)(C2H4)(CO)2(PH3),with lengths of 1.567,1.562,and 1.542 A for axial Rh-H bonds versus 1.621,1.623,1.616,and 1.615 A for equatorial Rh-H bonds. (ii) In our work? the lowest excited states of H C O ( C O ) are ~ the 'JEstates corresponding to a dd u* excitation ( u and u* denote the molecular orbitals which are respectively bonding and antibonding with respect to the cobalt-hydrogen bond), and these states were computed respectively at 25 900 and 36000 cm-' for the triplet and the singlet. In the experimental spectrum, absorption begins around 36000 cm-l, the only resolved feature being a band at 44000 cm-l.4 The mechanism that we have proposed for the photochemistry of HCo(CO), was based on the potential energy curves computed for these two states together with the state 'A, corresponding to a u u* excitation. On the other hand, the excitation energies computed by AD in their second paper9 are much lower, being equal to 1 1 460 and 23 200 cm-'respectively for the triplet and singlet states. (These values have been obtained for the experimental geometry of the C, structure, so that they are directly comparable to ours.) (iii) In our earlier work on HCO(CO)~?we optimized the bond angles 6 of the C3, structure 3 and a and B of the C, structure 4 at the SCF level for the closed-shell singlet state and for various
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(3) Sweeny, R. L. Inorg. Chem. 1980,19, 3512. (4) Sweeny, R. L. Inorg. Chem. 1982. 21, 752. ( 5 ) Antolovic, D.; Davidson, E. R. J . Am. Chem. Sac. 1987, 109, 977. (6) Sweany. R. L.; Russell, F. N.Orgunometullics1988, 7, 719. (7) Daniel, C.; Hyla-Kryspin, I.; Demuynck, J.; Veillard, A. Nouu. J . Chim. 1985. 9, 58 1. (8) Veillard. A.; Strich, A. J . Am. Chem. Soc. 1988, 110, 3793. (9) Antolovic, D.; Davidson, E. R. J . Chem. Phys. 1988, 88, 4967. (IO) Versluis, L.; Ziegler, T.; Baerends, E. J.; Ravenek. W. J. Am. Chem. Soc. 1989.111.2018. (1 1) Koga, N.; Jin, S.Q.; Morokuma, K. J. Am. Chem. Soc. 1988, 110, 3417.
3
4
triplet states (with fixed bond lengths). Next, limited CI calculations were carried out for the lowest energy structures. It was found that the C3, structure has three states close in energy, a 3Az state (eop = l l O o ) , a 'E state (e = 90°), and the 'Al closed-shell state (e, = SSO). The singet states corresponding to the C, structure were found to be higher than the 'Al state of the C, structure at the SCF level, but a 3A' state (a= 0 = 130O) was found at a comparable energy at the CI level. The CI calculations slightly favor the C, structure with a 'A2 ground state, but since the number of configurations was rather limited, a IAl ground state could not be ruled out. In fact, in our later work we implicitly assumed for HCO(CO)~ a C, structure with a 'Al ground state, on the basis of CASSCF calculations unpublished at that time (cf. below). On the other hand, AD have considered two types of ~ t r u c t u r e s : ~those . ~ corresponding to a trigonal bipyramid (tbp) with a vacancy, like 5 (labeled H C according to the nature of the ligands in the axial positions), and those corresponding to a distorted tetrahedron. H
5
They first optimized these structures at the HartreeFock level with a relatively small basis set5 but later repeated some of these calculations, at the H F and CI levels, with a larger basis set.9 Their conclusions may be summarized as follows: (i) Of the three tbp structures HC, CC, and HV (HV corresponding to the C,, structure), HC and CC have comparable energies for the state IA' but a number of triplet states would be lower in energy (the lowest being the state 3A" of HC). (ii) For the C, conformation, there are two states below the IAl state and the potential energy curve as a function of the angle 0 shows a minimum at 114' for the lowest 'A2 curve and around 90° for the 'Al curve (in good agreement with our results7). (iii) They finally conclude that their calculations provide strong evidence for a triplet ground state of HCO(CO)~,probably of the broken-symmetry tetrahedral type. The study of Versluis et a1.I0 by the HFS method was confined to the singlet states of HCO(CO)~.Optimization of the structures yielded the H C structure as the most stable one, the HV structure (of C3, symmetry) being 9 kcal/mol higher in energy. We have reinvestigated these three points, namely, the relative stability of the two structures of HCO(CO)~,the energy of the lowest excited states of HCo(CO),, and the structure and electronic states of HCo(CO),, through CI calculations based on CASSCF (complete active space SCF) reference wave functions. Method and Computational Details The following bash sets were used: for the Co atom a (1 5,11,6) set contracted to [9,6,3],12 for the first-row atoms a (10.6) set (12) This basis set is made from the (14.93) basis of Wa~hters'~ by adding an additional s function (exponent 0.3218), two diffuse p functions, and one diffuse d function. All these exponents were chosen according to the even tempered criterion of Raffenetti et al." (131 Wachters. A. J. H. J . Chem. Phvs. 1970.52. 1033. (14) Raffenetti, R. C.; Bardo, R. D.; Rkdenbe;g. K. In EnerBy, Srrucrun undReuctiuiry;Smith, D. W., b%Rae, W. B., Us.; Wiley: New York. 1973; p 164.
Veillard et al.
5558 The Journal of Physical Chemistry, Vol. 94, No. 14, 1990
contracted to [4,2],15 and for hydrogen a (6,l) set contracted to [3,1].16 This basis set is triple-f for the 1s shell of hydrogen and for the 3d and 4s shells of cobalt; otherwise it is double-{. In general, we have used either the experimental geometry2 (for isomer I of HCo(CO),) or assumed geometries based on the experimental bond len ths in HCO(CO)~ ( C d , 1.818 A, Co-C, 1.764 A, C-0 1,141 ) and on the bond angles optimized at the HF levels+" (with the COCOangle fixed at 180'). This approach (which has already given reasonable results1') is based on the following grounds: (i) optimization of the bond lengths a t the HF level for these metal carbonyls usually results in bond lengths that are too 10ng;59'0*~8-20 (ii) optimization of the bond lengths at the CASSCF and CI levels should yield accurate values,I9 but at a cost that exceeds our present computational resources; (iii) bond angles optimized at the HF level seem reasonably accurate, as judged from the results for H2Fe(C0)4.20 For the isomer 2 of HCo(CO), we have used the bond angles optimized at the HF level by Koga et al. for the compound Rh(H)(C2H4)(CO)2(PH3).11 (These values, shown in 6, are inter-
x
co I
co 6
mediate between those of AD and those of Versluis et a1.I0) Since no experimental bond length is available for an equatorial Co-H bond, this bond length has been optimized at the HF level for isomer 2 with an optimized value of 1.565 A. (Optimization of the Co-H bond length for isomer 1 yielded a value of 1.585 A, in reasonable agreement with the experimental value of 1.556 A. In order to have results comparable with those of our previous work? the calculation of the excitation energy to the 'E state was carried out for the experimental bond length of 1.556 A.) For HCo(CO), we have considered a number of structures: (i) the C,, structure (Co-H 1.556 A, Co-C 1.818 A) with the angle 6 optimized at the CASSCF level for the three states 'Al, 'E,and 3A2;(ii) the tbp structure HC (Co-H 1.556 A, C d 3 , 1.818 A, Co-C,, 1.764 A, with the bond angles optimized by AD5); (iii) four tetrahedral structures, of which two correspond to the values of a and 5, optimized in ref 7 (a= 140°, /3 = 120' and a = 130', p = 130') and the other two to the values optimized by AD (a = 1 ISo, @ = 124' and a = 97', @ = 114') (with the following bond lengths: Co-H 1.556 A, Co-C 1.818 A). For the tbp and tetrahedral structures of HCo(CO), calculations were first carried out at the SCF open-shell level. CASSCF calculations21were performed in order to generate reference wave functions and orbitals which were used in the multireference contracted CI (CCI) calculations.22 The reference configurations of the CCI calculation are the configurations that appear in the CASSCF wave function with a coefficient greater than 0.08. The number of reference configurations selected in this way was between 3 and 10. The CASSCF calculations for the closed-shell states have 10 electrons in 10 active orbitals (they are denoted lOalOe), namely, four 3d orbitals, four 4d orbitals which correlate them, and the u and u* orbitals. The CASSCF calculations for the states 'A2 (C3, structure), 3A" (tbp structure HC), and 'A' (tetrahedral structure) of HCo(C0)' are also of the lOalOe type (1 5 ) Huzinaga, S. Approximate Atomic Functions. Technical Report, University of Alberta, Alberta, 1971. (16) Huzinaga, S. J . Chcm. Phys. 1965.42, 1293. (17) Daniel, C.; Veillard, A. Inorg. Chem. 1989, 28, 1170. (18) Demuynck J.; Strich, A.; Veillard, A. Now. J . Chim. 1977, I , 217. (19) Lathi, J. P.; Siegbahn, P. E. M.; Almlbf, J. J . Phys. Chem .1985,89, 2 156. (20) Dcdieu. A.; Nakamura, S.;Sheldon, J. C. Chem. Phys. Lett. 1987, 141, 323. (21) Siegbahn, P. E. M.; Almlbf, J.; Heiberg. A.; R w , B. 0. J . Chem. Phys. 1981, 74,2384. (22) Siegbahn, P. E. M. Int. J . Quantum Chem. 1983, 23, 1869.
TABLE I: SCF and CCI Total Energies (in au) for HCo(CO), point principal group state configuration Escp ECl C,, 'A, d)d:u' -1 832.441 5 -1 832.780P -1832.7811' 'E d)d~02a*' -1 832.6706' Ca 'A, d8a2 -1832.4305 -1832.7557 "C0-H = 1.585 A. bCo-H = 1.556 A.
(with the doubly occupied orbitals correlated). For the 'E states of HCo(CO), and HCo(C0)' (C3"structure), the CASSCF calculations were of the type 9a8e, with the u orbital uncorrelated. (A 1lalOe calculation would be required in order to correlate the u orbital, but this calculation exceeded our present possibilities.) In the CCI calculations 10 electrons were correlated (the 3d electrons and those of the Co-H bond). The calculations included single and double excitations to all virtual orbitals except the counterparts of the carbonyl 1s and of the metal Is, 2s, and 2p orbitals. The number of configurations ranged from 221 352 to 2870210, but this number was reduced to at most 14000 by the contraction. The integral and SCF calculations were carried out with the System O f programs ASTERIX" and ARGOS.24 Results and Discussion The Structure o ~ H C O ( C OThe ) ~ total energies from the SCF and CCI calculations are reported in Table I for HCo(CO)+ Structure 1 appears more stable than structure 2 by 7 kcal/mol at the SCF level and 15 kcal/mol a t the CI level, in agreement with experiment. (This last value is decreased to 11 kcal/mol when 12 electrons are correlated in the CI calculation.) Our result at the CI level is close to the result of Versluis et a1.,I0 but appreciably higher than the value of 0.5 kcal/mol obtained by AD.9 The trend toward an increased stability of structure 1 when going from the HF level to the CI level is already found in the calculations of ADSv9and in those of Koga et al.I1 for the rhodium hydride. We find the Co-H distances to be quite similar in structures 1 and 2 (the axial bond in 1 being 0.02 A longer than the equatorial one in 2), in contrast to the large difference of 0.12 A found by ADS5Our Co-H bond length of 1.585 A in 1 is slighly longer than the experimental value of 1.556 A (the HFS method providing on the contrary a slightly too short distance of 1.52 AIo). The ' A , 3E Excitation Energy in HCo(CU)+ This excitation energy is computed at the CI level as 0.1 105 au or 24 250 cm-I. This result is close to our previous estimate8 of 25 900 cm-' but rather different from the value of 11 460 cm-' reported by AD.9 Since the separation between the two states 'E and 'E was calculated as 10 100 cm-l in our previous work* and l l 760 cm-' by AD,9 we may consider that this singlet-triplet energy gap must fall within the range 10000-12000 cm-I. If one uses the above value of 24 250 em-l for )Al 'E excitation energy, this would put the 'AI 'E excitation energy in the range 34000-36000 em-'. With the absorption in the experimental spectrum beginning around 36000 cm-I and only one resolved band at 44000 cm-', there are two possible ways to fit our theoretical results with the experimental data. The first one considers that the band at 44OOO cm-I corresponds to a metal-to-(carbonyl) ligand charge transfer (MLCT) and that the dd u* absorption is hidden below, somewhere between 36000 and 44000 cm-I. However, Sweany argue^^,*^ that weaker singlet-singlet absorption in this region should have been observable. The second way considers that the d, U* absorption either corresponds to the band at 44000 cm-I or is at higher energy. It seems difficult to choose between these two possibilities. One cannot assess the intensity of the dd u* band on qualitative grounds since this is neither a pure ligand-field
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+
(23) Bcnard, M.; Daniel: C.; Dcdieu, A.; Demuynck, J.; Rohmer, M. M.; Strich, A.; Veillard, A.; Wiest, R. Unpublished work. (24) Pitzer, R. M. J . Chem. Phys. 1973, 58, 3111. (25) Daniel, C. Coord. Chem. Reu., in press. (26) Sweany, R. L. Private communication.
The Journal of Physical Chemistry, Vol. 94, No. 14, 1990 5559
H C O ( C O ) ~and HCo(CO), Revisited TABLE II: SCF and CCI Total Energies (in au) for HCO(CO)~ structure aeometr Y state EWE El-, 3 C,,, 9 85" 'AI -1719.7413 -1720.0667 -1720.0088 3 C3,, 9 = 90" 'E ''42 -1720.0548 3 C',, 9 = 1 IO0' 5
tbp (HC),C,
4
a
a a
a
'A' -1719.7444 3A" -1719.7661 = 140°, j3 120" 'A' -1719.7947 = 130°, j3 = 130°b 'A' -1719.7906 = 115', j3 124"' 'A' -1719.7859 = 97', j3 = 114" 'A' -1719.7747
-1720.0514 -1720.0008 -1720.0207 -1720.0072 -1720.0039
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OValue optimized at the CASSCF level. 'Assumed.
band nor a pure charge-transfer band. If the d, u* absorption corresponds to the band at 44000 cm-I or falls at higher energy, this means that our calculation is in error by 1 eV or more. Similar calculations on related compounds like HMn(CO)j27 and Fe(CO)527a have yielded excitation energies with an accuracy better than half an electron volt. The agreement between the present 'Al 3Eexcitation energy of 24 250 cm-' and our previous result of 25 900 cm-' is reassuring, since the previous calculation* was based on the use of a unique reference wave function (a CASSCF wave function for the state u u* ,A1) for the CI calculation of all the electronic states at a given geometry, an approximation which has been currently in use for the calculation of the potential energy surfaces of The Structure and Electronic States of HCO(CO)~.The total energies from the SCF and CCI calculations are reported in Table I1 for HCo(CO),. Optimization of the angle 8 for structure 3 at the CASSCF level gives the following values: 8 = 85' for the 'Al state, 8 = 90' for the 3Estates, and 8 = 1 10' for the ,A2 state. These are identical with the values obtained previously at the SCF level' and very close to the values of AD at the CI levelg (8 = 90' for the 'Al state and 8 = 114' for the 'A, state) and to the HFS valuelo (8 = 82' for the 'Al state). In agreement with the previous s t u d i e ~ ,the ~ , ~triplet states are lower in energy than the singlet states at the S C F level. For the tetrahedral structure 4, we considered four points with different values of the angles a and 0. The first two, defined by the angles a = 140°, 0 = 120' and a = 130°, 0 = 130°, correspond respectively to the lowest values of the energy at the SCF and CI level in our previous studye7The other two correspond to the angles a = 115', 0 = 124' and a = 97', 0 = 114' optimized by AD at the SCF level.' The lowest S C F energy is obtained for the angles a = 140' and j3 = 120°, in agreement with our previous result^.^ For the tetrahedral structure, CASSCF and CI calculations were carried out for the first three structures. (The fourth point, a = 97', 0 = 114', was discarded since the corresponding SCF energy was higher by 12 kcal/mol than the lowest one.) At the CI level, the lowest energy corresponds to the C,, structure 3 (8 = 85') in the closed-shell 'Al electronic state. There are two other points which are relatively close in energy, one being
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(27) Veillard, A.; Strich, A.; Daniel, C.; Siegbahn, P.E. M.Chem. Phys. Lett. 1987. 141, 329. (28) Msrquez, A.; Daniel, C. To be published.
the C,, structure (e = 110') in the state which is 7 kcal/mol higher and the other being the tbp structure H C in the closed-shell 'A' state which is 10 kcal/mol higher. The ,A' state of the tetrahedral structures and the ,A" state of the tbp structure H C are consistently higher, by 29 kcal/mol or more. Some experimental evidence for the existence of HCo(CO), in different configurations was given by Sweany and Russell? They find that photolysis of H C O ( C O ) in ~ an argon matrix yields two isomers of HCo(CO),, the one of C,, symmetry being more prominent in the spectrum. Our conclusions regarding the ground state of HCo(CO), are different from those reached either by AD9 or by Versluis et al.IO When the calculations of AD favor a triplet ground state, probably of the tetrahedral type: we find a singlet ground state of the C3"symmetry. The investigation of Versluis et al. was confined to the singlet states of HCo(CO):, but favored the tbp H C structure by 9 kcal/mol over the C,, structure, when we find the latter more stable than the former by 10 kcal/mol.
Conclusion We have reinvestigated the structure and electronic states of HCO(CO)~ and HCo(CO), through CASSCF and CCI calculations, using moderately large basis sets. Our results may be summarized as follows. (i) For HCO(CO)~,the tbp structure of C3, symmetry (with the H atom axial) is more stable than the tbp structure of C , symmetry (with the H atom equatorial) by 15 kcal/mol, in agreement with the experimental structure. (ii) The 3E state of HCo(CO), is calculated at 24 250 cm-I, and the 'Al 'E excitation energy is estimated to be in the range 34000-36000 cm-I. (iii) The ground state of HCo(CO), is assigned to the closed-shell 'Al state of the C3, structure, with the Occurrence of another state (the 3A2state associated with the C,, structure) and another structure (the tbp structure H C with a closed-shell 'Af state) relatively close in energy (10 kcal/mol or less). Our results should be considered as tentative. One limitation of this study is the use of experimental or assumed geometries. Geometry optimization at the HF level yields metal-carbon bond lengths that are too long, and geometry optimization a t the CASSCF or CI level exceeds presently our computational resources. However, we doubt that our conclusions would be drastically altered by using geometries optimized at the CASSCF or CI level, since the changes would probably be minor. Improving the accuracy of the calculations could be achieved by (i) expanding the basis set to include higher spherical harmonics (d functions on first-row atoms and f functions on the cobalt atom) and (ii) increasing the number of electrons that are correlated. It would certainly be worthwhile to investigate the interconversion of the different structures, that is, mapping the potential energy surfaces rather than calculating a few points.
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Acknowledgment. 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. We thank Dr. R. Sweany for some useful comments. Registry No. HCo(CO),, 16842-03-8; H C O ( C O ) ~60105-25, I.
