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Electronic structure and bonding of transition metal complexes MCO (M Ru, Os)
Journal of Molecular Structure, 198 (1989) 215-221 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
215
ELECTRONIC STRUCTURE AND BONDING OF TRANSITION METAL COMPLEXES MC0 (M=Ru, OS)*
A. DAOUDI:b, M. SUARD”, J.C. BARTHELAT” and G. BERTHIERd %t?partement de Chimie, Ecole Polytechnique, Route de Saclay, F-91 128 Palaiseau (France) bDkpartement de Chimie, Facultt des Sciences de F&, B.P. no. 1796, F&-Atlas (ktaroc) “Laboratoire de Physique Quantique, Universitg Paul Sabatier, 118, Route de Narbonne, F31062 Toulouse (France) dZnstitut de Biologiephysico-chimique, 13, rue Pierre et Marie Curie, F-75005 Paris (France) (Received 3 October 1988)
ABSTRACT Relativistic pseudo-potentials have been determined for ruthenium and osmium with the object of studying the electronic structures of RuCO and OsCO systems by SCF-CI calculations using the CIPSI iterative method. Linear RuCO and OsCO have 3C- ground states and ‘C- firstexcited states, bonded with respect to their corresponding asymptotes, as does their iron analogue FeCO; however, RuCO is found to be correlated with a Ru atom in the d* configuration, at variance with the d7s’ configuration of metal in FeCO and OsCO.
INTRODUCTION
In the recent past, a rather large number of papers have been devoted to the properties of coordination complexes MC0 including one transition metal and one carbonyl group. These compounds were originally suggested as model molecules for rationalizing the results of chemisorption processes on metallic surfaces, e.g. in the analysis of the IR spectrum of CO adsorbed on iron [l], but some of them can be observed in inert-gas matrices or even in the gas phase. In the same way, the problem of the bonding of one CO molecule to various transition metals M gave rise to many quantum-mechanical computations by semi-empirical or ab initio methods (for a list of experimental and theoretical references, see Bach et al. [ 21) . We have been concerned with the variation of the electronic structure of MC0 complexes when passing from the left to the right of row 3 in the Periodic Table [ 3-61 and noticed a change from high-spin to low-spin ground states along the series. This trend seems to be consistent with the few data available from ESR spectroscopy [ 2,7,8]. Some calculations were directed towards MC0 molecules including atoms *Dedicated to Professor W.J. Orville-Thomas.
0022-2860/89/$03.50
0 1989 Elsevier Science Publishers B.V.
of rows 4 and 5; they are essentially devoted to metals interesting for their catalytic activity, namely the homologues of nickel (Pd [9,10] and Pt [ 11,121) and lastly a homologue of cobalt (Rh [ 131). However, the variation of the electronic structures of MC0 molecules from the top to the bottom of a transition-metal series has not been investigated. The aim of this paper is to study the bonding of carbon monoxide to ruthenium and osmium atoms, as was carried out previously for the iron analogue. The molecular orbital method is used throughout, starting with MOs determined by appropriate SCF open-shell calculations and including CI effects by the variation perturbation CIPSI method [ 141. By analogy with FeCO both molecules were supposed linear. ATOMIC CONSIDERATIONS
As in our previous work, the computations were restricted to the valence electrons of RuCO and OsCO. The (K, L, M, 4s2, 4p6) core of Ru atom and the (K, L, M, N, 5s2, 5p6) core of OS atom, as well as the K shells of C and 0, are taken into account through non-empirical pseudo-potentials of Gaussian form [ 15,161. The inner shells of the heavy atoms are described by relativistic pseudo-potentials (Tables 1 and 2)) the parameters of which were determined in such a way that the theoretical results found for the orbital energies and wavefunctions of selected configurations of Ru and OS are correctly reproTABLE 1 Pseudo-potential for W,(r) = e -““Cci
1
r “l and Gaussian basis functions for Ru atom”
Pseudopotential parameters
1
ni
a
0
-1
1.548013 2 4
0
1
2
-2 -1 0
0.739772
0.426723
Basis functions
Ci
Exponent
Coefficient -0.476527 0.582097 0.773133
8.722279 18.910762 1.167810
s s St
0.362059 0.157259 0.043978
10.785682 4.541669 -9.702959
P P PI
0.216317 0.150616 0.031096
-0.358403 0.589470 0.815751
3.237356
d d d d’ d’ d”
13.409406 2.602323 1.327307 0.567365 0.209802 0.078007
- 0.007065 0.086313 0.348227 0.455928 0.280613 0.066006
“SCF valence energies of the lowest states of Ru (in a.u.): 5F(d’s’), - 16.304697; 5D (d%*), - 16.266822; 3F(d8), - 16.212189 (valence-electron calculations with ground-state orbitals).
217 TABLE 2 Pseudo-potential for W,(r) = e -a’* C ci rnLand Gaussian basis functions for OS atom”
I
Basis functions
Pseudopotential parameters
1
n,
0
-1
ff
Exponent
Coefficient
S’
0.398248 0.168437 0.051117
-0.531270 0.725576 0.680735
12.893662 9.962278
P P P’
0.698251 0.185694 0.061944
-0.116834 0.571504 0.564365
1.619018 18.735343 - 20.839960
d d d d’ d’ d”
3.540226 1.734071 0.892204 0.439686 0.224289 0.088364
- 0.040674 0.180544 0.393729 0.318210 0.249253 0.085821
Ci
1.911153
8.536155 21.750190 12.413212
2 4 1
2
0 4
-2 -1
1.490322
1.330173 0
-
s S
“SCF valence energies of the lowest states of OS (in a.u.): 5D(d6s2), - 14.922781; 5F(d7s’), - 14.896153; “F(d8), - 14.748771 (valence-electron calculations with ground-state orbitals).
duced. Reference all-electron SCF calculations were obtained using the analytical relativistic method developed in ref. 17. The orbital basis sets describing the valence electrons come from SCF atomic calculations performed with these pseudo-potentials. They include 3 primitive Gaussians contracted in (2,l) double-zeta form for the s and p orbitals and 6 primitive Gaussians contracted in (3,2,1) triple-zeta form for the d-shell of metal. Here, we have chosen a contraction scheme in which the Gaussian of lowest exponent for each sequence of primitive functions was kept free, in order that one rather diffuse atomic orbital can be really involved in the variational treatment. The presence of this type of basis function is known to be necessary for the representation of d orbitals of transition metals having near dns2, d n+lS1 andd”+2 configurations [ 181. Correct configurations are assigned to the ground states of Ru (4d7 5s’) and OS (5d6 6s’) by our SCF valenceelectron calculations. As a matter of fact, a balanced description of the various lowest configurations of metals is a relevant question in the present context because of their very unequal binding properties in MC0 molecules. If atom M is in a groundstate configuration dns2, rather inactive in chemistry, as is the case for most transition elements in row 3, its bonding with ligands will be considered easier as soon as M is supposed to be promoted in a more appropriate valence state, generally d n+lsl [ 191 (note that the question of whether M in a metal mono-
218
carbonyl studied as much as NiCO is near 3dg4s1 [20] or 3d’O [21] is still a controversial subject where the quality of the d-shell orbitals is the heart of the matter). For heavier transition elements in which the characteristic lowering of the d shell with respect to the next s shell in row 5 may be superseded by the relativistic stabilization of s shells and destabilization of d shells in row 6 [ 221, the effective valence states of M are specific problems to be solved one by one (for instance, metals in PdCO and PtCO, homologue of NiCO ‘C+, are near 4d lo [9] and 5d1’ [ 111, respectively). It can be added that the assignment of a well-defined configuration to the combined metal has, ideally, a meaning beyond the theoretical picture used to describe the chemical bond between M and its ligand. One should not only be guided by the results of the population analysis of the molecular wavefunction at the equilibrium geometry, but also determine which of the two states of the correlated with same symmetry, M (dn+lsl) or M (d”+2), is asymptotically MC0 along its potential energy surfaces. MOLECULARRESULTS
We proceeded to the study of RuCO and OsCO in the same way as for FeCO. The low-lying states of these compounds are determined by a second-order perturbation treatment starting with zeroth-order multi-reference wavefunctions constructed iteratively. The selection threshold of determinants (about 100) to be included in the CIPSI variational subspace S was less than 0.03 for the first-order perturbation corrections on wavefunctions. Figures 1 and 2 show typical potential energy curves obtained for the ground states and first-excited states of RuCO and OsCO, i.e. a 3C- triplet and a 5C- quintet as in the case of FeCO. Energy values including second-order perturbation corrections and some observables (bond distances, binding energies, charge distributions, etc.)
R”?Fd’)
/:A -.-.
/
_ -0.83l
’ 3.2
+ COtI+)
Ruf’Fd’s’)
+
CO(‘Z+)
.7-
/’
’ ’ ’ ’ ’ ’ ’ ’ ’ ’ 3.4 3.6 3.8 4.0 4.2 4.4
’ ’ ’ ’ ’ 4.6 4.8 50
’ ’ ’ ’ ’ 5 2 5.4 5.6
’ 5.8
-
RW
Fig. 1. Potential a.u.).
energy curves for the two RuCO low-lying states
(C-O distance
frozen at 2.173
219 E
oS?Fd’s’)
+
CO(‘I+)
./ ./
3.2
3.4
3.6
3.8
4.0
4.2
4.4
4.6
0s?Fd7s’)
4.8
5.0
+
CO(‘Z+)
5.2
5.4
56
R(a.1
Fig.2. Potentialenergycurvesfor the two O&O low-lyingstates(C-O distancefrozenat 2.26 a.u.). are summarized in Table 3. The charge transfers between metal M and the carbonyl group CO are analyzed in terms of a, II, 6 charge distributions computed from the occupation numbers of the natural orbitals diagonalizing the one-electron density matrix of the S variational subspace (see the corresponding data for FeCO in ref. 6). At first glance, there is much resemblance between the description of RuCO and OsCO as low-spin ground states and high-spin first-excited states. These compounds are triplet ground states bound, with respect to their dissociation into metal and carbon monoxide, along their potential energy surfaces by a quantity D, which increases from ruthenium to osmium. The carbonyl groups have almost the same bond length (about 5% larger than in free CO) and the Ru-C and OS-C distances are not too different, as is observed in X-ray structures of Ru and OS related closed-shell complexes (for data on Ru and OS carbonyl compounds see refs. 23 and 24). The Aa and Arccharge transfers displayed in Table 3 arise from a population analysis of the natural orbitals in the S subspace; they are computed by taking the difference between the q0 and qn populations of carbon and oxygen in bonded MC0 and in free CO. Negative values of Aa, practically limited to the C atom, and positive values of Ax, distributed among both atoms of CO, indicate donation effects from ligand to metal and back-donation effects from metal to ligand respectively. For RuCO and OsCO, the Aa and An charge transfers balance each other rather well, as has already been found for FeCO, their magnitudes being a little greater for the osmium compound. On examining the electronic structures of several MC0 molecules, we discovered that the interplay of the Aa and Arc quantities can be correlated with the tendency of metallic
220 TABLE 3 Structure
parameters
of RuCO and OsCO molecules RuCo 3 x
RMC(a.=) ~CO
- 37.825 2.32
Q
-0.05 2.39 3.62 2.04
Q
5 1
3.57 2.25
::b
40 on 4s
M
(a.u.)
osco
4.05 2.26 - 37.788 0.52
Y-
5c3.41 2.27
- 36.489 3.12
3.52 2.28 - 36.481 2.64
0.02 2.22 3.73 2.07
0.14 2.39 3.47 2.00
0.17 2.35 3.46 2.02
0.35 2.44 1.21
0.24 2.64 1.12
0.20 2.41 1.39
0.15 2.49 1.36
-
C
1
(7” on
0
{
on 4n
-0.30 3.16 3.14
-0.22 3.14 3.08
-0.34 3.17 3.17
- 0.32 3.16 3.16
C
{
Aon Aqz
-0.37 0.12
-0.17 0.03
-0.40 0.30
- 0.32 0.27
0
1
Aq, Aiq,
-0.01 0.21
-0.03 0.15
0.00 0.24
-0.01 0.23
Q
“Values (in a.u.) of energies including Epstein-Nesbet second order corrections obtained for: RuCO “I- (R=3.55; r=2.26) and 5Z- (R=4.05; r=2.26); OsCO %- (R=3.40; r=2.26) and 5I(R=3.50; r=2.26). SCF energies are E (RuCO 3X-) = -37.395; E (OsCO3C-)= -36.033. bEnergy differences in eV between the geometry equilibrium and the point at RMc= 20.0 a.u.
surfaces to dissociate chemisorbed molecules more or less easily according to the position of M in the Periodic Table (see refs. 25-28). Predominant Aa or predominant An transfers correspond to dissociative or molecular adsorption respectively, and a good balance between Ao and AZ occurs with metals that are at the borderline of the two processes. This is the case for the present series of elements: Fe is dissociative, Ru and still more OS are molecular. In spite of the foregoing, there is a difference between the RuCO and OsCO molecules in the nature of their dissociation products in the 3C- ground state where Ru is obtained in the d8 configuration and OS in the d7s1 configuration ( d7s ’ configurations are found for M in all “C - states). This change of configuration is not obvious from the population analysis of the equilibrium wavefunctions of MC0 molecules, because the contribution of s atomic orbitals to
221
the gross atomic population of M is always close to unity (0.8 in Ru and 1.2 in OS), but it becomes apparent if we consider the nature of the main configuration in the CI expansion of the multi-reference wavefunction, namely 93.7% of Ru(d8) and 0.4% of Ru (d7s1) for the 3C- state of RuCO, and 73.5% of Os(d7s’) and 18.6% of Os(d6s2) for the 3C- state of OsCO. We may attribute the fact that metal retrieves the same type of configuration, d7s ‘, in FeCO and OsCO, to the relativistic stabilization of s orbitals and destabilization of d orbitals previously mentioned. ACKNOWLEDGEMENTS
The authors wish to express their thanks to Professors C. Barbier (Lyon) and J.P. Flament (Palaiseau) for helpful discussions.
REFERENCES
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
G. Blyholder, J. Chem. Phys., 36 (1962) 2036; 74 (1966) 3134. S.B.H. Bach, C.A. Taylor, R.J. Van Zee, M.T. Vala and W. Weltner, Jr., J. Am. Chem. Sot., 108 (1986) 7105. A. Daoudi, M. Suard and C. Barbier, C.R. Acad. Sci., Ser. B, 301 (1985) 911; J. Chim. Phys., 84 (1987) 795. C. Barbier, G. Berthier, A. Daoudi and M. Suard, Theor. Chim. Acta, 73 (1988) 419. A. Daoudi, M. Suard, J.C. Barthelat and G. Berthier, C.R. Acad. Sci. Ser. B, 306 (1988) 1377. G. Berthier, A. Daoudi and M. Suard, J. Mol. Struct. (Theochem), 48 (1988) 407. P.H. Kasai and P.M. Jones, J. Am. Chem. Sot., 107 (1985) 813. R.J. Van Zee, S.B.H. Bach and W. Weltner, Jr., J. Phys. Chem., 90 (1986) 583. G. Pacchioni, J. Koutecky and P. Fantucci, Chem. Phys. Lett., 92 (1982) 486. J. Miralles, R. Caballol, M. Merchan and I. Nebot-Gil, unpublished communication, Workshop on Quantum Chemistry, Girona, Spain, 1988. H. Basch and D. Cohen, J. Am. Chem. Sot., 105 (1983) 3856. A. Gavezzotti, G.F. Tantardini and M. Simonetta, Chem. Phys. Lett., 129 (1986) 577. M.L. McKee and SD. Worley, J. Phys. Chem., 92 (1988) 3699. B. Huron, J.P. Malrieu and P. Rancurel, J. Chem. Phys., 58 (1973) 5745. Ph. Durand and J.C. Barthelat, Theor. Chim. Acta, 38 (1975) 283. J.C. Barthelat and Ph. Durand, Gazz. Chim. Ital., 108 (1978) 225. J.C. Barthelat, M. Pelissier and Ph. Durand, Phys. Rev. A, 21 (1980) 1773. P. Jeffrey Hay, J. Chem. Phys., 66 (1977) 4377. C.F. Melius, Chem. Phys. Lett., 39 (1976) 287. C.W. Bauschlicher, Jr., J. Chem. Phys., 84 (1986) 260. P.V. Madhavan and J.L. Whitten, Chem. Phys. Lett., 127 (1986) 354. P. Pyykko, Chem. Rev., 88 (1988) 563. M.R. Churchill and B.G. de Boer, Inorg. Chem., 16 (1977) 878,2397. M.R. Churchill, F.J. Hollander and J.P. Hutchinson, Inorg. Chem., 16 (1977) 2655. G. Broden, T.N. Rhodin, C. Brucker, R. Benbow and Z. Hurych, Surf. Sci., 59 (1976) 593. J.B. Benziger, Appl. Surf. Sci., 6 (1980) 105. W. Andreoni and C.M. Varma, Phys. Rev. B, 23 (1981) 437. A. Daoudi, These d’Etat, Universite de Paris-Sud, 1988.
215
ELECTRONIC STRUCTURE AND BONDING OF TRANSITION METAL COMPLEXES MC0 (M=Ru, OS)*
A. DAOUDI:b, M. SUARD”, J.C. BARTHELAT” and G. BERTHIERd %t?partement de Chimie, Ecole Polytechnique, Route de Saclay, F-91 128 Palaiseau (France) bDkpartement de Chimie, Facultt des Sciences de F&, B.P. no. 1796, F&-Atlas (ktaroc) “Laboratoire de Physique Quantique, Universitg Paul Sabatier, 118, Route de Narbonne, F31062 Toulouse (France) dZnstitut de Biologiephysico-chimique, 13, rue Pierre et Marie Curie, F-75005 Paris (France) (Received 3 October 1988)
ABSTRACT Relativistic pseudo-potentials have been determined for ruthenium and osmium with the object of studying the electronic structures of RuCO and OsCO systems by SCF-CI calculations using the CIPSI iterative method. Linear RuCO and OsCO have 3C- ground states and ‘C- firstexcited states, bonded with respect to their corresponding asymptotes, as does their iron analogue FeCO; however, RuCO is found to be correlated with a Ru atom in the d* configuration, at variance with the d7s’ configuration of metal in FeCO and OsCO.
INTRODUCTION
In the recent past, a rather large number of papers have been devoted to the properties of coordination complexes MC0 including one transition metal and one carbonyl group. These compounds were originally suggested as model molecules for rationalizing the results of chemisorption processes on metallic surfaces, e.g. in the analysis of the IR spectrum of CO adsorbed on iron [l], but some of them can be observed in inert-gas matrices or even in the gas phase. In the same way, the problem of the bonding of one CO molecule to various transition metals M gave rise to many quantum-mechanical computations by semi-empirical or ab initio methods (for a list of experimental and theoretical references, see Bach et al. [ 21) . We have been concerned with the variation of the electronic structure of MC0 complexes when passing from the left to the right of row 3 in the Periodic Table [ 3-61 and noticed a change from high-spin to low-spin ground states along the series. This trend seems to be consistent with the few data available from ESR spectroscopy [ 2,7,8]. Some calculations were directed towards MC0 molecules including atoms *Dedicated to Professor W.J. Orville-Thomas.
0022-2860/89/$03.50
0 1989 Elsevier Science Publishers B.V.
of rows 4 and 5; they are essentially devoted to metals interesting for their catalytic activity, namely the homologues of nickel (Pd [9,10] and Pt [ 11,121) and lastly a homologue of cobalt (Rh [ 131). However, the variation of the electronic structures of MC0 molecules from the top to the bottom of a transition-metal series has not been investigated. The aim of this paper is to study the bonding of carbon monoxide to ruthenium and osmium atoms, as was carried out previously for the iron analogue. The molecular orbital method is used throughout, starting with MOs determined by appropriate SCF open-shell calculations and including CI effects by the variation perturbation CIPSI method [ 141. By analogy with FeCO both molecules were supposed linear. ATOMIC CONSIDERATIONS
As in our previous work, the computations were restricted to the valence electrons of RuCO and OsCO. The (K, L, M, 4s2, 4p6) core of Ru atom and the (K, L, M, N, 5s2, 5p6) core of OS atom, as well as the K shells of C and 0, are taken into account through non-empirical pseudo-potentials of Gaussian form [ 15,161. The inner shells of the heavy atoms are described by relativistic pseudo-potentials (Tables 1 and 2)) the parameters of which were determined in such a way that the theoretical results found for the orbital energies and wavefunctions of selected configurations of Ru and OS are correctly reproTABLE 1 Pseudo-potential for W,(r) = e -““Cci
1
r “l and Gaussian basis functions for Ru atom”
Pseudopotential parameters
1
ni
a
0
-1
1.548013 2 4
0
1
2
-2 -1 0
0.739772
0.426723
Basis functions
Ci
Exponent
Coefficient -0.476527 0.582097 0.773133
8.722279 18.910762 1.167810
s s St
0.362059 0.157259 0.043978
10.785682 4.541669 -9.702959
P P PI
0.216317 0.150616 0.031096
-0.358403 0.589470 0.815751
3.237356
d d d d’ d’ d”
13.409406 2.602323 1.327307 0.567365 0.209802 0.078007
- 0.007065 0.086313 0.348227 0.455928 0.280613 0.066006
“SCF valence energies of the lowest states of Ru (in a.u.): 5F(d’s’), - 16.304697; 5D (d%*), - 16.266822; 3F(d8), - 16.212189 (valence-electron calculations with ground-state orbitals).
217 TABLE 2 Pseudo-potential for W,(r) = e -a’* C ci rnLand Gaussian basis functions for OS atom”
I
Basis functions
Pseudopotential parameters
1
n,
0
-1
ff
Exponent
Coefficient
S’
0.398248 0.168437 0.051117
-0.531270 0.725576 0.680735
12.893662 9.962278
P P P’
0.698251 0.185694 0.061944
-0.116834 0.571504 0.564365
1.619018 18.735343 - 20.839960
d d d d’ d’ d”
3.540226 1.734071 0.892204 0.439686 0.224289 0.088364
- 0.040674 0.180544 0.393729 0.318210 0.249253 0.085821
Ci
1.911153
8.536155 21.750190 12.413212
2 4 1
2
0 4
-2 -1
1.490322
1.330173 0
-
s S
“SCF valence energies of the lowest states of OS (in a.u.): 5D(d6s2), - 14.922781; 5F(d7s’), - 14.896153; “F(d8), - 14.748771 (valence-electron calculations with ground-state orbitals).
duced. Reference all-electron SCF calculations were obtained using the analytical relativistic method developed in ref. 17. The orbital basis sets describing the valence electrons come from SCF atomic calculations performed with these pseudo-potentials. They include 3 primitive Gaussians contracted in (2,l) double-zeta form for the s and p orbitals and 6 primitive Gaussians contracted in (3,2,1) triple-zeta form for the d-shell of metal. Here, we have chosen a contraction scheme in which the Gaussian of lowest exponent for each sequence of primitive functions was kept free, in order that one rather diffuse atomic orbital can be really involved in the variational treatment. The presence of this type of basis function is known to be necessary for the representation of d orbitals of transition metals having near dns2, d n+lS1 andd”+2 configurations [ 181. Correct configurations are assigned to the ground states of Ru (4d7 5s’) and OS (5d6 6s’) by our SCF valenceelectron calculations. As a matter of fact, a balanced description of the various lowest configurations of metals is a relevant question in the present context because of their very unequal binding properties in MC0 molecules. If atom M is in a groundstate configuration dns2, rather inactive in chemistry, as is the case for most transition elements in row 3, its bonding with ligands will be considered easier as soon as M is supposed to be promoted in a more appropriate valence state, generally d n+lsl [ 191 (note that the question of whether M in a metal mono-
218
carbonyl studied as much as NiCO is near 3dg4s1 [20] or 3d’O [21] is still a controversial subject where the quality of the d-shell orbitals is the heart of the matter). For heavier transition elements in which the characteristic lowering of the d shell with respect to the next s shell in row 5 may be superseded by the relativistic stabilization of s shells and destabilization of d shells in row 6 [ 221, the effective valence states of M are specific problems to be solved one by one (for instance, metals in PdCO and PtCO, homologue of NiCO ‘C+, are near 4d lo [9] and 5d1’ [ 111, respectively). It can be added that the assignment of a well-defined configuration to the combined metal has, ideally, a meaning beyond the theoretical picture used to describe the chemical bond between M and its ligand. One should not only be guided by the results of the population analysis of the molecular wavefunction at the equilibrium geometry, but also determine which of the two states of the correlated with same symmetry, M (dn+lsl) or M (d”+2), is asymptotically MC0 along its potential energy surfaces. MOLECULARRESULTS
We proceeded to the study of RuCO and OsCO in the same way as for FeCO. The low-lying states of these compounds are determined by a second-order perturbation treatment starting with zeroth-order multi-reference wavefunctions constructed iteratively. The selection threshold of determinants (about 100) to be included in the CIPSI variational subspace S was less than 0.03 for the first-order perturbation corrections on wavefunctions. Figures 1 and 2 show typical potential energy curves obtained for the ground states and first-excited states of RuCO and OsCO, i.e. a 3C- triplet and a 5C- quintet as in the case of FeCO. Energy values including second-order perturbation corrections and some observables (bond distances, binding energies, charge distributions, etc.)
R”?Fd’)
/:A -.-.
/
_ -0.83l
’ 3.2
+ COtI+)
Ruf’Fd’s’)
+
CO(‘Z+)
.7-
/’
’ ’ ’ ’ ’ ’ ’ ’ ’ ’ 3.4 3.6 3.8 4.0 4.2 4.4
’ ’ ’ ’ ’ 4.6 4.8 50
’ ’ ’ ’ ’ 5 2 5.4 5.6
’ 5.8
-
RW
Fig. 1. Potential a.u.).
energy curves for the two RuCO low-lying states
(C-O distance
frozen at 2.173
219 E
oS?Fd’s’)
+
CO(‘I+)
./ ./
3.2
3.4
3.6
3.8
4.0
4.2
4.4
4.6
0s?Fd7s’)
4.8
5.0
+
CO(‘Z+)
5.2
5.4
56
R(a.1
Fig.2. Potentialenergycurvesfor the two O&O low-lyingstates(C-O distancefrozenat 2.26 a.u.). are summarized in Table 3. The charge transfers between metal M and the carbonyl group CO are analyzed in terms of a, II, 6 charge distributions computed from the occupation numbers of the natural orbitals diagonalizing the one-electron density matrix of the S variational subspace (see the corresponding data for FeCO in ref. 6). At first glance, there is much resemblance between the description of RuCO and OsCO as low-spin ground states and high-spin first-excited states. These compounds are triplet ground states bound, with respect to their dissociation into metal and carbon monoxide, along their potential energy surfaces by a quantity D, which increases from ruthenium to osmium. The carbonyl groups have almost the same bond length (about 5% larger than in free CO) and the Ru-C and OS-C distances are not too different, as is observed in X-ray structures of Ru and OS related closed-shell complexes (for data on Ru and OS carbonyl compounds see refs. 23 and 24). The Aa and Arccharge transfers displayed in Table 3 arise from a population analysis of the natural orbitals in the S subspace; they are computed by taking the difference between the q0 and qn populations of carbon and oxygen in bonded MC0 and in free CO. Negative values of Aa, practically limited to the C atom, and positive values of Ax, distributed among both atoms of CO, indicate donation effects from ligand to metal and back-donation effects from metal to ligand respectively. For RuCO and OsCO, the Aa and An charge transfers balance each other rather well, as has already been found for FeCO, their magnitudes being a little greater for the osmium compound. On examining the electronic structures of several MC0 molecules, we discovered that the interplay of the Aa and Arc quantities can be correlated with the tendency of metallic
220 TABLE 3 Structure
parameters
of RuCO and OsCO molecules RuCo 3 x
RMC(a.=) ~CO
- 37.825 2.32
Q
-0.05 2.39 3.62 2.04
Q
5 1
3.57 2.25
::b
40 on 4s
M
(a.u.)
osco
4.05 2.26 - 37.788 0.52
Y-
5c3.41 2.27
- 36.489 3.12
3.52 2.28 - 36.481 2.64
0.02 2.22 3.73 2.07
0.14 2.39 3.47 2.00
0.17 2.35 3.46 2.02
0.35 2.44 1.21
0.24 2.64 1.12
0.20 2.41 1.39
0.15 2.49 1.36
-
C
1
(7” on
0
{
on 4n
-0.30 3.16 3.14
-0.22 3.14 3.08
-0.34 3.17 3.17
- 0.32 3.16 3.16
C
{
Aon Aqz
-0.37 0.12
-0.17 0.03
-0.40 0.30
- 0.32 0.27
0
1
Aq, Aiq,
-0.01 0.21
-0.03 0.15
0.00 0.24
-0.01 0.23
Q
“Values (in a.u.) of energies including Epstein-Nesbet second order corrections obtained for: RuCO “I- (R=3.55; r=2.26) and 5Z- (R=4.05; r=2.26); OsCO %- (R=3.40; r=2.26) and 5I(R=3.50; r=2.26). SCF energies are E (RuCO 3X-) = -37.395; E (OsCO3C-)= -36.033. bEnergy differences in eV between the geometry equilibrium and the point at RMc= 20.0 a.u.
surfaces to dissociate chemisorbed molecules more or less easily according to the position of M in the Periodic Table (see refs. 25-28). Predominant Aa or predominant An transfers correspond to dissociative or molecular adsorption respectively, and a good balance between Ao and AZ occurs with metals that are at the borderline of the two processes. This is the case for the present series of elements: Fe is dissociative, Ru and still more OS are molecular. In spite of the foregoing, there is a difference between the RuCO and OsCO molecules in the nature of their dissociation products in the 3C- ground state where Ru is obtained in the d8 configuration and OS in the d7s1 configuration ( d7s ’ configurations are found for M in all “C - states). This change of configuration is not obvious from the population analysis of the equilibrium wavefunctions of MC0 molecules, because the contribution of s atomic orbitals to
221
the gross atomic population of M is always close to unity (0.8 in Ru and 1.2 in OS), but it becomes apparent if we consider the nature of the main configuration in the CI expansion of the multi-reference wavefunction, namely 93.7% of Ru(d8) and 0.4% of Ru (d7s1) for the 3C- state of RuCO, and 73.5% of Os(d7s’) and 18.6% of Os(d6s2) for the 3C- state of OsCO. We may attribute the fact that metal retrieves the same type of configuration, d7s ‘, in FeCO and OsCO, to the relativistic stabilization of s orbitals and destabilization of d orbitals previously mentioned. ACKNOWLEDGEMENTS
The authors wish to express their thanks to Professors C. Barbier (Lyon) and J.P. Flament (Palaiseau) for helpful discussions.
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