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Ligand-Free Osmium Clusters Supported on MgO. A Density Functional Study Jesse F. Goellner,†,‡,§ Konstantin M. Neyman,† Markus Mayer,† Folke No¨rtemann,† Bruce C. Gates,‡,§ and Notker Ro¨sch*,† Institut fu¨ r Physikalische und Theoretische Chemie, Technische Universita¨ t Mu¨ nchen, 85747 Garching, Germany, and Institut fu¨ r Physikalische Chemie, Ludwig-Maximilians-Universita¨ t Mu¨ nchen, Butenandtstrasse 5-13 (Haus E), 81377 Munich, Germany Received September 17, 1999. In Final Form: December 1, 1999 The interactions of Os4, Os5, and Os5C clusters with various sites of a MgO(001) support were investigated theoretically with the aid of a scalar-relativistic density functional cluster model method. Adsorption geometries of C4v clusters centered above a magnesium cation and the Os atoms oriented either to the nearest surface oxygen anions (A) or between them (B) were considered. The influence of surface Vs and Vs2- defects on the adsorption of the clusters was also investigated. The calculated base Os-Os distances in supported Os5 and Os5C square-pyramidal clusters are at most 0.1 Å longer (2.5-2.6 Å) than the values calculated for the corresponding free osmium clusters but about 0.4 Å (or more) shorter than the values determined by EXAFS spectroscopy for MgO-powder-supported clusters formed by decarbonylation of [Os5C(CO)14]2- and shown to retain the Os5C frame. The experimental Os-Os distances characterizing the supported clusters are close to the experimental and calculated bond lengths for coordinatively saturated osmium carbonyl clusters; this result favors the suggestion that the supported clusters characterized by EXAFS spectroscopy were not entirely ligand-free. Calculated interaction strengths of the osmium clusters with the MgO(001) support range from nonbonding (defect-free site B when the basal Os atoms are aligned between the nearest O anions) to very weak (0.6 eV for Os5C at defect-free site A when the basal Os atoms are aligned with the nearest O anions) to weak (∼2 eV for pure Os clusters at defect-free site A) to rather strong (∼9 eV for Vs defect site A). The models reported here are inferred to be too simplified to capture all the pertinent structural details of MgO-powder-supported osmium clusters, but they are sufficient to indicate a significant role of defect sites in the adsorption of supported osmium clusters and, we infer, other transition metal clusters.
Introduction Metal clusters and particles on the surfaces of metal oxides are widely applied as catalysts, and metal-metal oxide interactions are significant in catalysts and microelectronic devices.1-7 Structural information characterizing metal-oxide interfaces has been obtained from extended X-ray absorption fine structure (EXAFS) spectroscopy for clusters of noble metals on metal oxide and zeolite supports; the results indicate metal-oxygen distances of typically 2.1-2.2 Å, often accompanied by longer metal-oxygen distances of about 2.5-2.7 Å.8 Similar distances have been found theoretically for Ni4, Pd4, Cu4, and Ag4 clusters on defect-free MgO(001)9-11 as well as * Corresponding author. † Technische Universita ¨ t Mu¨nchen. ‡ Ludwig-Maximilians-Universita ¨ t Mu¨nchen. § Permanent address: Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616. (1) Gates, B. C. Catalytic Chemistry; Wiley: New York, 1992. (2) Sachtler, W. M. H.; Zhang, Z. Adv. Catal. 1993, 39, 129. (3) Barthomeuf, D. Catal. Rev. 1996, 38, 521. (4) Campbell, C. T. Surf. Sci. Rep. 1997, 27, 1. (5) Lambert, R. M., Pacchioni, G., Eds. Chemisorption and Reactivity on Supported Clusters and Thin Films. Towards an Understanding of Microscopic Processes in Catalysis; NATO ASI Series E; Kluwer: Dordrecht, 1997; Vol. 331. (6) Henry, C. R. Surf. Sci. Rep. 1998, 31, 231. (7) Renaud, G. Surf. Sci. Rep. 1998, 32, 1. (8) Koningsberger, D. C.; Gates, B. C. Catal. Lett. 1992, 14, 271 and references therein. (9) Matveev, A. V.; Neyman, K. M.; Pacchioni, G.; Ro¨sch, N. Chem. Phys. Lett. 1999, 299, 603. (10) Neyman, K. M.; Vent, S.; Ro¨sch, N.; Pacchioni, G. Top. Catal. 1999, 9, 153.
for Ir4 clusters interacting with the six-rings facing faujasite zeolite supercages.12 Much remains to be learned about metal-metal oxide support interactions and the structures of supported metal clusters. Our goal was to use a density functional (DF) method to investigate theoretically the interactions of fourand five-atomic osmium clusters with MgO. These systems were chosen because small clusters on MgO formed by decarbonylation of supported [Os5C(CO)14]2-, where the Os content was 1 wt %, are among the most stable known supported metal clusters, with EXAFS coordination numbers13 indicating the retention of the metal frame, including the carbido C atom, after complete decarbonylation by treatment in He at 300 °C; thus, on the basis of the available structural information, these decarbonylated clusters are represented as Os5C. Furthermore, the structure of the common and most stable (001) surface of MgO, including defects, is well characterized.14 In the following, we address various questions crucial to the characterization of supported metal particles. (a) How are the structural parameters of the supported C4v clusters Os4, Os5, and Os5C on the most common sites (including defects sites) of MgO(001) modified relative to those of the corresponding free clusters and known osmium carbonyl clusters? (11) Ferrari, A. M.; Xiao, C.; Neyman, K. M.; Pacchioni, G.; Ro¨sch, N. Phys. Chem. Chem. Phys. 1999, 1, 4655. (12) Ferrari, A. M.; Neyman, K. M.; Mayer, M.; Staufer, M.; Gates, B. C.; Ro¨sch, N. J. Phys. Chem. B 1999, 103, 5311. (13) Panjabi, G.; Salvi, S.; Phillips, B.; Allard; L. F.; Gates, B. C. Manuscript in preparation. (14) Henrich, V. E.; Cox, P. A. The Surface Science of Metal Oxides; Cambridge University Press: Cambridge, U.K., 1994.
10.1021/la9912388 CCC: $19.00 © 2000 American Chemical Society Published on Web 01/28/2000
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(b) How does the MgO support influence the electronic properties of these supported clusters? (c) What are the adsorption bond strengths and preferred positions of Os4, Os5, and Os5C clusters on MgO(001)? (d) Do the results of the calculations characterizing these models provide insight into the structure and bonding of clusters formed by decarbonylation of [Os5C(CO)14]2supported on MgO powder and characterized by EXAFS spectroscopy?13 The paper is organized as follows. In the next section we summarize the computational method and models. In the third section, we present results calculated for Os4, Os5, and Os5C clusters on MgO(001). In the fourth section, we discuss these results along with relevant experimental results. Finally, we present some conclusions. Computational Method and Structural Models All-electron DF calculations were carried out with the help of a scalar-relativistic variant of the linear combination of Gaussian-type orbitals density functional method (LCGTO-DF),15,16 as implemented in the new code ParaGauss for employing parallel computers.17,18 Spinpolarization effects were included to account for the possible open-shell nature of the metal clusters. Large and flexible Gaussian-type basis sets were used to represent the Kohn-Sham orbitals. For Os, a (19s,14p, 10d,5f) basis set19 was augmented by two s (0.0132, 0.2064), three p (0.0247, 0.0619, 0.1564), two d (0.0475, 0.1187), and two f exponents (0.3684, 0.9210). The resulting basis set (21s,17p,12d,7f) was contracted to [9s,8p,5d,2f]. For C and O, basis sets (9s,5p,1d) f [5s,4p,1d] were adopted.20,21 For Mg, a basis set (15s,10p,1d) f [6s,5p,1d] was used. All contractions were of generalized form based on atomic eigenvectors of local density functional calculations (scalarrelativistic for Os). The auxiliary basis sets employed in the LCGTO-DF method were constructed in a standard fashion by properly scaling the s and p exponents of the orbital basis sets;15 for Os, only every second p-type exponent was used to construct d(r2)-type fitting functions. Furthermore, five p- and five d-type polarization exponents centered on each atom were added to the auxiliary basis set.22 The fitted charge density was used to calculate the classic Coulomb contribution to the electron-electron interaction.15 Exchange-correlation contributions to the effective one-electron potential and to the total energy were evaluated by numerical integration.23,24 (15) Dunlap, B. I.; Ro¨sch, N. Adv. Quantum Chem. 1990, 21, 317. (16) Ro¨sch, N.; Kru¨ger, S.; Mayer, M.; Nasluzov, V. A. In Recent Development and Applications of Modern Density Functional Theory. Theoretical and Computational Chemistry; Seminario, J. M., Ed.; Elsevier: Amsterdam, 1996; Vol. 4, p 497. (17) Belling, T.; Grauschopf, T.; Kru¨ger, S.; No¨rtemann, F.; Staufer, M.; Mayer, M.; Nasluzov, V. A.; Birkenheuer, U.; Ro¨sch, N. ParaGauss, version 1.9; Technische Universita¨t Mu¨nchen: Garching, Germany, 1998. (18) Belling, T.; Grauschopf, T.; Kru¨ger, S.; Mayer, M.; No¨rtemann, F.; Staufer, M.; Zenger, C.; Ro¨sch, N. In High Performance Scientific and Engineering Computing, Proceedings of the International FORTWIHR Conference on HPSEC; Bungartz, H.-J., Durst, F., Zenger, Chr., Eds.; Lecture Notes in Computational Science and Engineering 8; Springer: Heidelberg, 1999; p 439. (19) Gropen, O. J. Comput. Chem. 1987, 8, 982. (20) Van Duijneveldt F. B. IBM Res. Rep. 1971, No. RJ 945. (21) Neyman, K. M.; Strodel, P.; Ruzankin, S. P.; Schlensog, N.; Kno¨zinger, H.; Ro¨sch, N. Catal. Lett. 1995, 31, 273. (22) Chung, S. C.; Kru¨ger, S.; Pacchioni, G.; Ro¨sch, N. J. Chem. Phys. 1995, 102, 3695. (23) Pople, J. A.; Gill, P. M. W.; Johnson, B. G. Chem. Phys. Lett. 1992, 199, 557. (24) Gill, P. M. W.; Johnson, B. G.; Pople, J. A. Chem. Phys. Lett. 1993, 209, 506.
Figure 1. Orientations A and B of osmium clusters with respect to the MgO(001) support.
The geometries of the various molecular models of the adsorbate/adsorbent (cluster/support) combinations were optimized by using analytical energy gradients,25 with C4v symmetry constraints imposed. The calculations were carried out by self-consistently employing the gradientcorrected BP functional (Becke’s exchange functional26 in combination with Perdew’s correlation functional27). As a reference, we also optimized the geometry of the coordinatively saturated osmium carbonyl compound [Os3(CO)12] (with D3h symmetry constraint) using both the gradientcorrected BP functional and the local density approximation (LDA).28 Binding energies were not corrected for the basis set superposition error because estimates based on the counterpoise method29 indicate that such errors (about 0.1 eV) are only a small fraction of the computed binding energies for strongly bound systems. To represent the (001) face of MgO, we used a two-layer cluster model [Mg13O5]16+ (top, Mg9O4; bottom, Mg4O1) embedded in an array of 17 × 17 × 2-18 point charges (PCs) of (2.0 e to achieve overall electroneutrality of the total system, [Mg13O5]16+ plus [PC array]16- (Figures 1 and 2). The model cluster representing the support was chosen so that all anions of the clusters were surrounded by Mg cations; thus, unphysical distortion of the large, easily polarizable oxygen anions by neighboring positive PCs was avoided.30 The Mg-O distance was kept fixed at the bulk experimental value of 2.104 Å.31 Under a C4v symmetry constraint, two orientations of the four-atom base of the square-pyramidal osmium clusters with respect to the support are conceivable (Figure 1): the cluster aligned with four base Os atoms close to the nearest oxygen anions (site A) and the cluster oriented with the four base Os atoms between those anions (site B; cluster base rotated by 45° around the main axis relative (25) Nasluzov, V. A.; Ro¨sch, N. Chem. Phys. 1996, 210, 413. (26) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (27) Perdew, J. P. Phys. Rev. B 1986, 33, 8622; 1986, 34, 7406. (28) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200. (29) Boys, S. F.; Bernardi, F. Mol. Phys. 1970, 19, 553. (30) Yudanov, I. V.; Nasluzov, V. A.; Neyman, K. M.; Ro¨sch, N. Int. J. Quantum Chem. 1997, 65, 975. (31) Wycoff, R. W. G. Crystal Structures; Interscience: New York, 1963; Vol. 1.
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Goellner et al. Table 1. Calculated Parameters of Free and MgO-Supported Square-Planar Os4 Clustersa d(Os-Os) d(Os-O) z(Os-MgO) BE q(Os4) ∆ configurationb assigned state
free
site A
site A(Vs)
site B(Vs)
2.27
2.33 2.25 2.20 1.73 -1.48 0.43 e2 3A 2
2.27 2.03 1.96 8.70 -0.18 0.21 e2 3A 2
2.27 2.68 2.21 2.85 0.00 0.26 a12 1A 1
0.00 0.41 e4 1A 1
a Distance d and height z of the cluster plane above the top MgO crystal layer in angstroms, angles in degrees, Mulliken charges q in electrons, binding energy BE of Os4 to MgO and HOMO-LUMO splitting ∆ (within the spin manifold of the LUMO) in electronvolts. No adsorption was found on the regular site B. b Highest occupied or partially filled Kohn-Sham levels for closed-shell and openshell configurations, respectively.
Figure 2. Cluster models Os4/Mg13O5 (a, b), Os5/Mg13O5 (c), and Os5C/Mg13O5 (d) in orientation A. Panels a, c, and d show regular sites of MgO(001), and sketch b shows a Vs defect.
the orientation at site A). Previous investigations showed that orientation A is preferred under a C4v symmetry constraint;9,10 however, a geometry optimization at lower symmetry may lead to distortions of the metal cluster frame.11 Nevertheless, a restricted structure optimization should provide sufficient information for our goals, since moderate expected structure alterations due to symmetry reductions cannot change the conclusions of the present model study. The influence of the point defects on the metal cluster adsorption was examined by considering surface Vs centers. They were created by simply removing the central Mg atom from the MgO surface layer (Figure 2b), either as a neutral Mg species (Vs defect; [Mg12O5]16+) or as a Mg2+ ion (Vs2- defect; [Mg12O5]14+); the coordinates of the ions and PCs of the rest of the support were kept fixed at their bulk terminated values. Neglecting the support relaxation in response to creating Vs and Vs2- vacancies can be justified by results of our previous investigations of metal adsorption on various Fs centers of MgO(001),32 which showed that support relaxation induced by removal of a large oxygen anion caused no qualitative differences, either in the bonding mechanism or in the calculated trends of other parameters, compared with defects described by unrelaxed models of the support with a bulk terminated geometry. A similar effect with only limited quantitative changes of the calculated results as a consequence of relaxation near Vs centers has also been demonstrated.33 Results Interaction of a Planar Os4 Cluster with Regular and Defect Sites of MgO(001). The interaction of a square-planar Os4 cluster with the ideal MgO(001) surface (regular sites) and that incorporating Vs defects was investigated in orientations A and B under C4v symmetry constraints (Table 1, Figure 2). (32) Matveev, A. V.; Neyman, K. M.; Yudanov, I. V.; Ro¨sch, N. Surf. Sci. 1999, 426, 123. (33) Ferrari, A. M.; Pacchioni, G. J. Phys. Chem. 1995, 99, 17010.
Os4 binds weakly at the regular site A, with an energy of 1.73 eV, or 0.43 eV per atom at the optimized Os-O distance of 2.25 Å. These results may be compared to the observables calculated for an Os atom adsorbed at an O center of a MgO(001) surface: an equilibrium distance of 2.09 Å and a binding energy of 0.79 eV that is not very strong with respect to other single atoms.34 No adsorbatesupport binding was found at the regular site B. The OsOs bonds of the adsorbed Os4 cluster, 2.33 Å in length, are slightly elongated, by 0.06 Å, with respect to the free species. This elongation is in the range of adsorptioninduced bond elongation (0.03-0.10 Å) calculated for M4/ MgO clusters (M ) Cu, Ni, Ag, Pd) bound to the support with interaction strengths from 0.8 (Ag4) to 2.0 eV (Ni4).9 The Mulliken charge of adsorbed Os4 is -0.37 e (Table 1). (We stress that the Mulliken population analysis affords only a qualitative characterization of the charge distribution when such flexible basis sets as that of the present investigation are employed.) Upon adsorption of Os4, the HOMO-LUMO gap (taken within the spin manifold of the LUMO) increased negligibly by 0.02 to 0.43 eV, accompanied by a change of state from 1A1 to 3A2. The latter counterintuitive effect of a spin increase upon adsorption has also been found for Pd4/MgO in C4v symmetry.9 When there is a Vs defect in the MgO surface at site A, the cluster-support interaction is substantially stronger, 8.70 eV; thus, the presence of the defect significantly strengthens the interaction by 1.74 eV per metal atom. Concomitantly, the Os-O distance decreases by 0.22 Å in comparison with 2.03 Å for Os4/A and the Os-Os bond distance shrinks to restore the free cluster value of 2.27 Å (Table 1); the state at site A(Vs) is the same as at the regular site A, 3A2. Likewise, for the B orientation the binding interaction between Os4 and the surface becomes energetically favorable at a Vs center; the total binding energy, 2.85 eV, is even higher (by 1.12 eV) than that at the regular site A. However, the Os4 cluster evidently is less affected by adsorption at a defect site B(Vs) than by adsorption at defect-free site A. At defect site B(Vs), neither a change of state nor an Os-Os bond elongation relative to the gas-phase values was calculated for Os4 (Table 1). In the latter case, the distance from the Os4 plane to the top crystal layer of the MgO surface (2.21 Å) is somewhat longer than at the site A(Vs) and very similar to the value for the cluster adsorbed at the regular site A. The shortest Os-O distance calculated for the Os4/B(Vs) complex, 2.68 Å, is close to the longer of two metal-O distances (34) Yudanov, I.; Pacchioni, G.; Neyman, K. M.; Ro¨sch, N. J. Phys. Chem. B 1997, 101, 2786.
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Table 2. Calculated Parameters of Free and MgO-Supported C4v Os5 Clustersa free d(Osb-Osb) d(Osb-Osa) d(Osb-O) z(Osb-MgO) ∠(Osb-Osa-Osb) face ∠(Osb-Osa-Osb) cross solid BE q(Osb) q(Osa) q(Os5) ∆ configurationb assigned state
2.35 2.50 56.0 83.2 -0.01 0.05 0 0.19 b22 1A 1
site A
site A(Vs)
site B(Vs)
2.36 2.51 2.34 2.30 56.0 83.3 2.89 -0.25 -0.45 -0.70 0.26 a11b11e2 5B 2
2.31 2.59 2.05 2.00 52.8 78.0 9.08 0.00 -0.22 -0.23 0.21 b21a11e2 5B 1
2.30 2.55 2.72 2.28 53.6 79.2 2.72 -0.06 0.02 -0.21 0.44 b11a11 3B 1
a Distance d and height z of the cluster plane above the top MgO crystal layer in angstroms, angles in degrees, Mulliken charges q in electrons, binding energy BE of Os5 to MgO and HOMO-LUMO splitting ∆ (within the spin manifold of the LUMO) in electronvolts. Osb and Osa are the base and apex atoms of the square pyramid, respectively. No adsorption was found on the regular site B. b Highest occupied or partially filled Kohn-Sham levels for closedshell and open-shell configurations, respectively.
determined by EXAFS spectroscopy for numerous transition metal clusters supported on metal oxides (about 2.7 Å)35 and 0.43 Å longer than the value for adsorption at the regular site A. Adsorption of Os4 at B(Vs) causes a decrease of the HOMO-LUMO gap by 0.15 eV, whereas this gap shrinks by 0.2 eV for Os4 at site A(Vs). Comparing the calculated results for the whole series of supported Os4 clusters, we conclude that square-planar osmium species adsorbed at a Vs center are almost neutral (or very slightly negatively charged), whereas a noticeable negative charge results at site A. Both on regular and Vs sites, square-planar Os4 clusters strongly prefer orientation A, which allows the most efficient Os-O interactions. Interaction of a Square Pyramidal Os5 Cluster with Regular and Defect Sites of MgO(001). To Complement the calculations for Os4 clusters, calculations were carried out to represent the interaction of a squarepyramidal Os5 cluster with the regular MgO(001) surface (Figure 2c) and that with Vs defects, in both orientations A and B. Table 2 is a summary of parameters characterizing the structures optimized in C4v symmetry. On the ideal MgO(001) surface, the osmium cluster again was not found to bind in orientation B. In orientation A, the binding energy of Os5 (2.89 eV) is notably larger than for Os4, by 0.29 eV per Os-O bond. The distance between the basal plane of four Os atoms and the (001) surface plane, 2.30 Å, is 0.1 Å longer than that characterizing supported Os4. The geometry of the square-pyramidal cluster Os5 barely changes as a result of adsorption at the ideal site A; we calculated elongations of the Os-Os bonds and alterations of valence angles in the adsorbed Os5 that are negligibly small with respect to those of the gas-phase species. The HOMO-LUMO gap increases slightly by 0.07 eV compared with gas-phase Os5. All these characteristics manifest a rather weak metal cluster-surface interaction. The Mulliken charges are indicative of a moderate electron charge transfer from the support to Os5; the charge of the Os5 cluster is q ) -0.70 e (Table 1). The electronic state of Os5 supported on regular MgO(001), 5B2, shows spin decoupling with respect to the free cluster, similar to what has been calculated for Os4. When one Os atom is added (35) Koningsberger, D. C. In Synchrotron Techniques in Interfacial Electrochemistry; Melendres, C. A., Tadjeddine, A., Eds.; Kluwer: Dordrecht, 1994; p 181 and references therein.
as an apex atom to Os4/MgO(A), two more unpaired electrons are found. As for Os4, the interaction of Os5 with the MgO surface was calculated to be significantly stronger at a Vs defect than on the ideal surface. The shortest distance from a basal Os center to a surface O center (Osb-O) is 2.05 Å, matching the short metal-oxygen distances determined by EXAFS spectroscopy for many metal oxide supported metal clusters.35 Compared with the cluster Os5 at the ideal site A and with the same orientation, introduction of a defect site (Vs) decreases the distance between the Os4 basal plane of Os5 and the MgO surface plane (by 0.30 Å to 2.00 Å) and increases the binding energy by 6.19 eV to 9.08 eV. This cluster adsorption energy translates into four strong Osb-O bonds of 2.27 eV each. As for the Os4 cluster, we find that the basal intermetal distance OsbOsb expands when the gas-phase cluster is adsorbed at ideal site A, but the change is only slight for Os5. Furthermore, this distance contracts noticeably (by 0.05 Å) in Os5 when it interacts with the defect site A(Vs). On the other hand, the distance from a basal to the apical metal center (Osb-Osa) expands markedly in Os5/A(Vs), by 0.08 Å compared with Os5/A. The spin state was assigned to be a quintet, as for adsorption at the regular site A. The situation of Os5 at defect site B(Vs) is comparable to that of Os4 at that site. The structure of the Os5 cluster is similar to that at defect site A(Vs) (Table 2). However, the long Osb-O distance of 2.72 Å reflects the significantly smaller adsorption energy of 2.72 eV, comparable in strength to that at the ideal site A. Thus, again, orientation A is clearly preferred over B at defect site A(Vs). Interaction of an Os5C Cluster with Regular and Defect Sites of MgO(001). Free and supported Os5C clusters were considered in a square-bipyramidal structure (Figure 2d and Table 3). This is the Os5C core structure observed by X-ray diffraction crystallography for the compounds [Os5C(CO)15] and [Os5C(CO)14]2- (the latter in a salt).32 This geometry was chosen for the ligand-free Os5C moieties and its surface complexes because [Os5C(CO)14]2- is the majority surface species prior to the decarbonylation treatment, which yields the MgO-powdersupported moieties shown by EXAFS coordination numbers to retain the Os5C frame.13 The Os5C species was assumed to be adsorbed with the C atom of the Os5C frame directed toward the MgO surface. Such an orientation is in agreement with the NMR data characterizing Os5C clusters supported on powder MgO, which imply that the C atom is between the metal and the support surface.13 Furthermore, this “C-down” orientation (Figure 2d) probably allows for stronger interactions between four Os atoms of the cluster and O anions of the MgO surface than the alternative “C-up” orientation (not shown); furthermore, in this way the smaller apex C atom fits better sterically into the V vacancy than the larger apex Os atom. The unsupported cluster Os5C exhibits four unpaired electrons and a notable HOMO-LUMO gap of 0.54 eV. Not unexpectedly, the interaction of Os5C with the regular MgO(001) surface was found to be either rather weak (0.59 eV, at site A) or nonbonding (site B). At regular site A, the distance of 3.26 Å between the plane of the four Os atoms of the Os5C bipyramid base and the (001) plane results in a rather long nearest-neighbor Os-O distance of 3.28 Å. The base distance Osb-Osb, 2.43 Å, is only 0.02 Å longer than that in gas-phase Os5C, but it is 0.06 Å longer than that in Os5. Adsorption of Os5C leaves the valence angles essentially unaffected, but the C atom moves slightly toward the Osb base plane, and the Osb-C
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Table 3. Calculated and Experimental Parameters of Free and MgO-Supported C4v Os5C Clustersa free d(Osb-Osb) d(Osb-Osa) d(Osb-C) d(Osa-C) d(Osb-O) z(Osb-MgO) z(Osb-C) ∠(Osb-Osa-Osb) face ∠(Osb-Osa-Osb) cross solid ∠(Osb-C-Osb) face ∠(Osb-C-Osb) cross solid BE q(Osb) q(Osa) q(C) q(Os5C) ∆ configurationc assigned state
2.41 2.49 2.05 2.94
site A
site A(Vs)
site B(Vs2-)
-1.13 58.0
2.43 2.50 2.01 2.86 3.28 3.26 -1.04 58.2
2.52 2.63 1.91 2.62 2.14 2.12 -0.68 57.2
2.39 2.52 2.04 3.01 3.48 3.14 -1.14 56.6
86.6
87.0
85.2
84.2
72.1
74.4
82.6
71.7
112.7
117.5
138.0
111.8
0.02 0.05 -0.13 0 0.54 a11b21e2 5B 1
0.59 -0.24 0.04 -0.26 -1.17 0.65 a11b11e2 5B 2
4.84 -0.03 -0.38 0.44 -0.08 0.15 b21a11e2 5B 1
1.81 -0.47 -0.16 0.23 -1.80 0.30 b21e2a11 5B 1
exptlb 2.89 1.98 2.47
a Distance d and height z in angstroms (Os above the top MgO b crystal layer, C below the cluster base plane), angles in degrees, Mulliken charges q in electrons, binding energy BE of Os5C to MgO and HOMO-LUMO splitting ∆ (within the spin manifold of the LUMO) in electronvolts. Osb and Osa are the base and apex atoms of the square pyramid, respectively. No adsorption was found on the regular site B. Experimental X-ray diffraction data for [Os5C(CO)14]2- are the following: d(Osb-Osb) ) 2.88 ( 0.07 Å, d(Osb-Osa) ) 2.86 ( 0.05 Å, d(Osb-C) ) 2.045 ( 0.013 Å, d(OsaC) ) 2.22 Å, z(Osb-C) ) -0.21 Å, ∠(Osb-Osa-Osb) face ) 61.6 ( 0.4°, ∠(Osb-Osa-Osb) cross solid ) 90.7 ( 0.3°, ∠(Osb-C-Osb) face ) 89.4 ( 3.0°, ∠(Osb-C-Osb) cross solid ) 168.0 ( 0.8°; ref 36. b EXAFS results for powder sample; ref 13. c Highest occupied or partially filled Kohn-Sham levels for closed-shell and openshell configurations, respectively.
distance decreases slightly from 2.05 to 2.01 Å. The latter value is in good agreement with the result determined by EXAFS spectroscopy, 1.98 Å (Table 3) for the cluster formed by decarbonylating [Os5C(CO)14]2- on powdered MgO,13 because the uncertainty associated with limitations of the EXAFS technique for such a distance is (0.02 Å.35 A partial electron transfer from the support to Os5C can be deduced from the Mulliken charge, -1.17 e (but see reservations stated above); both the carbido C atom and the base Os atoms were found to be slightly negatively charged. In the neutral, gas-phase cluster Os5C, the carbido carbon center carries a small negative charge, -0.13 e. The HOMO-LUMO gap of Os5C increases upon adsorption by 0.11 eV, but the spin state is not changed upon adsorption. However, the state of Os5C changes from 5 B1 to 5B2 upon adsorption from the gas phase to the defectfree site in orientation A. As for Os4 and Os5, the binding interaction of Os5C with the MgO(001) surface at site A(Vs) is noticeably greater, 4.84 eV, than the value characterizing its interaction with the defect-free surface in the same A orientation (Table 3). This interaction with site A(Vs) is also manifested by substantially shorter adsorbate-support bonds than that on the defect-free surface. In Os5C/A(Vs) the shortest Osb-O distance, 2.14 Å, is less than that of Os5C at site A by 1.14 Å, this distance being in the same range as the shorter metal-oxygen distances determined by EXAFS spectroscopy for metal oxide supported transition metal clusters.8,35 As a result of adsorption, the C atom of Os5C/ A(Vs) is pushed considerably closer to the metal frame
than in the case of Os5C/A, with the corresponding distances of C to the basal Os4 plane being reduced by 0.36 Å to 0.68 Å (Table 3). Concomitantly, the osmium cluster frame is expanded noticeably in the presence of the defect site; the intermetal distances are greater by 0.09 (Osb-Osb) and 0.13 Å (Osb-Osa) than for the cluster adsorbed on defect-free site A. A Mulliken analysis yields a minimal electron transfer from the support to Os5C at site A(Vs). Although the HOMO-LUMO gap of Os5C at site A(Vs), 0.15 eV, is significantly smaller, by 0.39 eV, than in the gas phase, both systems exhibit the same electronic state, 5B1. Although the formation energy of a Vs2- defect is considerably greater (by 11-15 eV for various cluster models at the Hartree-Fock level33) than that of a Vs defect, a test calculation of an adsorption complex of Os5C with such a defect was undertaken with the B orientation assumed. The results of Table 3 show that introduction of a Vs2- defect at site B allows a weak binding interaction of Os5C with MgO(001). A significant charge, -1.80 e, is transferred from the support to the adsorbed Os5C at B(Vs2-). Os5C at each of the two adsorption sites characterized by notable binding energies, A(Vs) and B(Vs2-), incorporates a C atom that bears a positive Mulliken charge (Table 3) consistent with the carbido C atom being formally an electron donor in organometallic clusters with an Os5C core.36 Bond distances in Os5C at B(Vs2-) are much closer to the values calculated for free Os5C than to those determined by EXAFS data for decarbonylated [Os5C(CO)14]2- supported on MgO powder.13 The latter Os-Os distances are hardly different from the X-ray distances characterizing coordinatively saturated carbonyl compounds with an Os5C core (Table 3). On the other hand, none of the calculated Osb-O distances fits the EXAFS value of 2.47 Å with the expected accuracy of DF calculations. In the following section we discuss possible rationalizations for this apparent discrepancy. Discussion Importance of Defects in Bonding of Osmium Clusters to MgO. The calculations show that the binding energy of each of the osmium clusters, Os4, Os5, and Os5C, increased when V defects were introduced into the MgO(001) surface. The ideal MgO(001) surface has an intrinsically low reactivity,14 and we infer that defects are generally important in stabilizing the adsorption of metal clusters (and likely of other species) on MgO powder. Transmission electron micrographs38 reported for osmium clusters approximated as 10-atom clusters on MgO powder confirm the importance of defects in the adsorption; the clusters were observed to be adsorbed preferentially along surface steps and ledges. However, transmission electron micrographs characterizing [Os5C(CO)14]2- and the decarbonylated Os5C on MgO powder indicate an apparently random distribution, which suggests that more than one kind of defect may be important in bonding of metal clusters to MgO.13 Furthermore, the preparation conditions may affect the locations of the surface-bound clusters. Possible Significance of Os-O Distances. Consistent with the results of previous investigations,9,10,34,39,40 (36) Johnson, B. F. G.; Lewis, J.; Nelson, W. J. H.; Nicholls, J. N.; Puga, J.; Raithby, P. R.; Rosales, M. J.; Schro¨der, M.; Vargas, M. D. J. Chem. Soc., Dalton Trans. 1983, 2447. (37) Vaarkamp, M. Catal. Today 1998, 29, 271. (38) Long, N. J.; Gates, B. C.; Kelley, M. J.; Lamb, H. H. In Physics and Chemistry of Small Clusters; Jena, P. B., Rao, K., Khanna, S. N., Eds.; Plenum: New York, 1987; p 831. (39) Yudanov, I. V.; Vent, S.; Neyman, K. M.; Pacchioni, G.; Ro¨sch, N. Chem. Phys. Lett. 1997, 275, 245. (40) Neyman, K. M.; Ro¨sch, N.; Pacchioni, G. Appl. Catal. A 2000, 191, 3.
Osmium Clusters on MgO
cluster orientation A with the Os atoms directed toward the O anions of MgO was found to be preferred over bridge adsorption (site B) for all three clusters, Os4, Os5, and Os5C. The Os-O distances of the clusters adsorbed on MgO(001) with significant binding energies (greater than about 2 eV) roughly match the short (∼2.1 Å) and long (∼2.7 Å) distances measured by EXAFS spectroscopy for numerous metal oxide supported noble metal clusters.8 The shorter value was calculated for site A and the longer for site B. Since both metal-oxygen distances are typically observed for a given sample by EXAFS spectroscopy and since these data represent averages over the sample, we suggest that configurations comparable to both A and B may exist in MgO-powder-supported metal clusters. The present theoretical results, combined with those calculated for Ir4 clusters in faujasites,12 rationalize the measured metal-oxygen distances as representative of bonding interactions (∼2.1 Å, close to the sum of the atomic radii, r(Os) ) 1.35 Å41 and r(O) ) 0.73 Å42) or weak interactions (∼2.7 Å), which may in part be determined by a geometric mismatch (a nonpseudomorphic fit) between the metal cluster and the metal oxide or zeolite support. We note, however, that alternative explanations have been offered for the simultaneous existence of both long and short metal-oxygen distances,8,43,44 including more complex MgO surface models. Vaarkamp et al.43 and Miller et al.44 explained the long metal-oxygen distance by suggesting the existence of hydrogen atoms between the metal and oxygen atoms. Such longer distances have recently been calculated for the interaction of a Re(CO)3 species with a hydroxyl covered MgO corner defect site.45 Short and long metal-oxygen distances can be hypothesized for a metal cluster bonded to a hydroxylated MgO surface, with one distance being associated with oxygen centers of OH groups and the other with O atoms in the MgO surface lattice. Effect of Adding One Os Atom to the Cluster: Comparison of Os4 and Os5. Comparison of the results for adsorbed Os4 and Os5 provides insight into the influence on the metal-support interaction resulting from addition of one Os atom on top of the four-atom raft (Tables 1 and 2). Both Os4 and Os5 were found to be slightly distorted by adsorption; at the regular site A, the adsorption-induced elongation of the Osb-Osb bond of Os5 is 0.05 Å less than in Os4, but at A(Vs) the bond length change is 0.04 Å greater in Os5 than in Os4. The Os-O bonds in the Os5 systems are noticeably longer than those in the corresponding Os4 systems, and the increase of the cluster binding energy to MgO resulting from the introduction of a Vs defect is 0.8 eV greater for Os4 than for Os5. The adsorption energy of Os5 is greater than that of Os4 (by 1.16, 0.38, and 0.03 eV at sites A, A(Vs), and B(Vs), respectively). (Since four Os atoms in each adsorbed cluster, Os4 and Os5, are similarly directed at the MgO surface and form the same number of metal-surface bonds, the adsorption energy does not need to be normalized to the number of Os atoms for a comparison of the binding of the two clusters.) (41) Wells, A. F. Structural Inorganic Chemistry; Cambridge University Press: Cambridge, U.K., 1984. (42) Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry: Principles of Structure and Reactivity, 3rd ed.; Harper Collins: New York, 1993. (43) Vaarkamp, M.; Modica, F. S.; Miller, J. T.; Koningsberger, D. C. J. Catal. 1993, 144, 611. (44) Miller, J. T.; Meyers, B. L.; Modica, F. S.; Lane, G. S.; Vaarkamp, M.; Koningsberger, D. C. J. Catal. 1993, 143, 395. (45) Hu, A.; Neyman, K. M.; Staufer, M.; Belling, T.; Gates, B. C.; Ro¨sch, N. J. Am. Chem. Soc. 1999, 121, 4522.
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Effect of a Carbon Atom: Comparison of Os5C and Os5. The effect of adding a sub-basal C atom to the Os5 square pyramid can be deduced from a comparison of Tables 2 and 3. Os5C is slightly more perturbed by adsorption than Os5, particularly on defects, as shown by the larger elongations of the Osb-Osb and Osb-Osa bonds of Os5C relative to those of Os5. The difference is more pronounced at site A(Vs), which exhibits a substantial binding with Os5C (4.84 eV), than at the ideal site A, where the bonding is quite weak (0.59 eV). Incorporation of a Vs defect at site A increases the binding of Os5C by about 4 eV, compared with an increase by about 6 eV for the adsorption of Os5. The presence of the C atom reduces the binding energy of the cluster Os5C to the support (relative to the corresponding adsorption complexes of Os5) by 2.30 eV at site A and by 4.24 eV at site A(Vs). Os5C appears to be somewhat less reactive than Os5, as indicated by its slightly greater HOMO-LUMO gap, both in the gas phase and (with less difference) when adsorbed at defect-free site A. The nearest Os-O distances of the adsorbed Os5C are longer than those of the corresponding Os5 systems, by 0.9 Å at the ideal site A and by 0.1 Å at site A(Vs). The perpendicular distances between the planes of four Osb atoms and the MgO surface planes are longer for adsorbed Os5C than for the corresponding Os5 systems. Site A(Vs) is the only one investigated here for which both Os5C and Os5 are significantly bound to MgO. Comparison of Calculated Results Characterizing Os5C/MgO and EXAFS Results Characterizing Decarbonylated [Os5C(CO)14]2-/MgO. Table 3 includes a comparison of the present cluster DF results with the EXAFS parameters determined for MgOpowder-supported clusters formed by decarbonylation of [Os5C(CO)14]2-/MgO by treatment in He at 300 °C.13 The EXAFS Os-Os bond distances of these supported clusters, 2.89 Å, are longer than those of bulk osmium, 2.70 Å,41 essentially matching those of [Os5C(CO)14]2- in a crystalline salt (see footnote of Table 3).13 Only one (average) Os-Os first-shell distance can be distinguished in the EXAFS data of a sample represented as Os5C/MgO, with the same Os5C core as in [Os5C(CO)14]2-, because the OsbOsb (2.88 ( 0.07 Å) and Osb-Osa distances (2.86 ( 0.05 Å) are the same within the capability of EXAFS to resolve shells of the same backscatterer. However, as a consequence of the C4v symmetry imposed on the Os5C clusters in our calculations, the Osb-Osb and Osb-Osa distances are symmetry-inequivalent and distinguishable computationally. The distances Osb-Osb and Osb-Osa of all the MgOsupported Os5C systems calculated in this work are considerably shorter (by 0.3-0.5 Å) than the Os-Os distance determined by EXAFS spectroscopy for the decarbonylated MgO-powder-supported sample, 2.89 Å. The magnitude of these deviations exceeds any error associated with the common accuracy of DF methods for such bond lengths, which are conservatively estimated to about 0.15 Å, even for 5d metal compounds treated at the scalar relativistic level.45 Thus, an explanation of the difference between the experimental and calculated distances is needed. From X-ray diffraction it is known that [Os5C(CO)15] and [Os5C(CO)14]2- exhibit C2v or C2 symmetry.36 Infrared spectra of [Os5C(CO)14]2- on MgO powder imply a similarly reduced symmetry prior to decarbonylation.13 Thus, the clusters adsorbed on MgO may also be distorted to a symmetry lower than C4v. The small HOMO-LUMO gaps,
2742
Langmuir, Vol. 16, No. 6, 2000
which decrease with increasing cluster-support interaction, imply that the driving force for symmetry-lowering may increase with the strength of the binding. Therefore, the supported Os5C systems with a quintet spin state may undergo a Jahn-Teller distortion to lower than C4v symmetry. The Os5C cluster could be distorted from the nido-square-bipyramid structure of [Os5C(CO)14]2- and [Os5C(CO)15] to the arachno-pentagonal-bipyramidal (bridged butterfly) structure of [Os5C(CO)16].36 The carbido C atom is inside the metal frame of the arachno pentagonal bipyramid; since EXAFS contributions are radially averaged, this technique cannot distinguish between carbido centers that are internal and those that are external to the metal frame. Nevertheless, we do not consider the possible oversymmetrization of the present models as sufficient cause of the discrepancies between the calculation results and the structural parameters determined by EXAFS spectroscopy (Table 3).13 Explicit attempts (with C4v symmetry constraint) to find a minimum for the C atom of the unsupported cluster Os5C inside the Os5 framework failed; such configurations were found to be about 10 eV higher than the calculated equilibrium structure with the carbido center below the Osb plane. In comparison with the EXAFS Os-Os distance for the MgO-powder-supported sample formed by decarbonylation of [Os5C(CO)14]2-, the calculated Osb-Osb and OsbOsa distances are shorter by 0.48 and 0.40 Å in the gasphase Os5C, by 0.46 and 0.39 Å on the defect-free site A, by 0.37 and 0.26 on A(Vs), and by 0.50 and 0.37 Å on B(Vs2-). The system Os5C on A(Vs), in which the Os-Os distances come the closest to the EXAFS Os-Os distance (while still being significantly too short), is also the system with the strongest binding energy to the support. This result is expected because the perturbation of the Os5C structure as a result of binding to MgO depends on the strength of the cluster-surface bonding interaction. The lack of agreement between the calculated Os-Os distances and those determined by EXAFS spectroscopy makes it in principle conceivable that an adsorption site with a stronger binding energy for the Os5C cluster than those investigated here might exist, which would imply a more significant perturbation of the Os5C frame (see below). Two points are raised by this discussion. First, the energy required to distort the free Os5C cluster in C4v symmetry from its calculated geometry to a structure that is compatible with the EXAFS data was estimated to be as much as 6 eV. For this estimate we employed the averaged EXAFS Os-Os distance, 2.89 Å, and we chose an Os-C distance of 2.04 Å, which places the C atom in the center of the square-pyramidal base plane of Os5C. However, this Os-C distance is longer than the EXAFS value of 1.98 Å.13 This energy cost of about 6 eV would have to be compensated by the interaction with the support. This value is significantly larger than the value of 1.6 eV required to adjust the cluster structure from the gas-phase equilibrium to the geometry optimized for the cluster Os5C adsorbed at site A(Vs). Second, the accuracy of the present DF method for the determination of bond distances can be illustrated for the osmium carbonyl compound [Os3(CO)12], which has been characterized by X-ray crystallography.46 The averaged experimental OsOs distance of 2.877 Å compares well with the value 2.87 Å optimized with the LDA level; with the gradientcorrected BP energy functional employed here, a longer distance of 2.96 Å is obtained. For the equatorial Os-C distances the values are 1.912 Å (exptl), 1.89 Å (LDA), 1.91 Å (BP); the corresponding values for the axial Os-C (46) Churchill, M. R.; DeBoer, B. G. Inorg. Chem. 1977, 16, 878.
Goellner et al.
distances are 1.946, 1.94, and 1.96 Å, respectively. These results for the Os-Os distance are in line with the expectation that bond lengths calculated at the BP level usually are slightly longer than experimental values.47 If we transfer this propensity of the BP calculations to the Os-Os distances calculated for the free and supported Os5C clusters, we expect them to be longer than those found by experiment. This inference is at variance with the considerable and atypical underestimation implied by the EXAFS distances for the clusters formed by decarbonylation of [Os5C(CO)14]2- on powder MgO.13 The major reason for the lack of agreement between the theoretical results and the EXAFS distances is most likely that the supported clusters were not free of ligands; rather, hydrogen, hydrocarbon, water, carbon, or other low-molecular-weight species might have formed during the complete decarbonylation (as demonstrated by infrared spectroscopy13) and might have become bound to Os5C on the MgO. Such ligands consist of low-Z scatterers that are difficult (C) or impossible (H) to identify by metaledge EXAFS spectroscopy; especially when a nonuniform distribution of ligand spheres consisting of these species is present in the sample, the metal-backscatterer contributions characterizing these species are difficult to separate from the metal-support oxygen contributions. Thus, we conclude that representation of the sample made by decarbonylation of [Os5C(CO)14]2-/MgO as simply bare Os5C supported on MgO is an oversimplification that significantly misrepresents the chemistry of the supported clusters. Further experiments are needed to elucidate the possible light-atom ligand spheres of Os5C bound to MgO powder. Here, we provide some arguments that indicate the plausibility of ligands with low-Z atoms attached to the MgO-powder-supported Os5C clusters. First, the Os-Os distance determined by EXAFS matches Os-Os distances determined by X-ray diffraction crystallography for coordinatively saturated clusters with the Os5C frame, as opposed to the coordinatively unsaturated samples investigated in this work. Furthermore, the surface chemistry involved in the synthesis of the MgO-powdersupported samples likely creates light molecules or atoms, such as hydrocarbons, water, and hydrogen,48 that could bind to the Os5C clusters, which are highly reactive because of their coordinative unsaturation. The samples were formed by complete decarbonylation of [Os5C(CO)14]2supported on hydroxylated MgO powder surfaces by 300 °C treatment in He.13 The chemistry of the decarbonylation process is not well understood, but it likely involves C-O bond-breaking reactions comparable to those occurring in Fischer-Tropsch synthesis catalyzed by metals supported on metal oxides, leading to the formation of groups that could be strong ligands, such as C, H, hydrocarbons, and water.49 The inference of the presence of ligands on the decarbonylated clusters is consistent with a recent DF investigation12 of faujasite-supported Ir4, according to which bonding of a single C or H atom to the Ir4 cluster brings the calculated Ir-Ir distance nearly into agreement with the EXAFS value for the cluster formed by decarbonylation of supported [Ir4(CO)12], and consistent with the suggestion of why the chemisorption capacity of the clusters represented as Ir4 on γ-Al2O3 is less than that of supported iridium clusters prepared at temperatures (47) Go¨rling, A.; Trickey, S. B.; Gisdakis, P.; Ro¨sch, N. In Topics in Organometallic Chemistry; Brown, J., Hofmann, P., Eds.; Springer: Heidelberg, 1999; Vol. 4, p 109. (48) Scott, S. L.; Basset, J. M. J. Am. Chem. Soc. 1994, 116, 12069. (49) Smith, A. K.; Theolier, A.; Basset, J.-M.; Ugo, R.; Commereuc, D.; Chauvin, Y. J. Am. Chem. Soc. 1978, 100, 2590.
Osmium Clusters on MgO
higher than those of the decarbonylation.50 Infared and NMR spectroscopies (the techniques that Panjabi et al.13 used in concert with EXAFS spectroscopy) are not sensitive enough to detect such ligands in such small amounts (e.g., one hydrocarbon ligand per Os5C, with the Os content of the sample being only 1 wt %). Thus, the experimental results are consistent with the inference that the supported Os5C, while fully decarbonylated, was not entirely ligandfree. Thus, although the results of the present work indicate that the preferred surface sites for osmium cluster adsorption are defect sites and although more sophisticated models of the support may have to be invoked to provide sufficiently detailed information about the metal cluster-support interface, we conclude that the overriding issue to account for the disagreement between theoretical metal-metal bond lengths and those determined by EXAFS spectroscopy is likely the existence of light atom ligands attached to the MgO-powder-supported clusters. Such light ligands might not saturate the clusters, and they might be more labile than the MgO surface ligands and capable of undergoing facile ligand exchange, consistent with the observation of toluene hydrogenation catalyzed by MgO-powder-supported clusters made by decarbonylation of MgO-powder-supported [Os5C(CO)14]2-.13 Conclusions The calculations representing Os4, Os5, and Os5C on MgO show that Os5 and Os4 have very similar binding energies on the same adsorption sites of the MgO(001) surface (except for the regular site A). Both Os4 and Os5 are more strongly bound to the ideal MgO(001) surface than Os5C. All three osmium model clusters are bound more strongly at surface point Vs defect sites of MgO(001) than on the ideal surface. Adsorption at such surface defects perturbs the Os5C cluster most and the Os5 clusters least. Consistent with the results of previous computational studies of other MgO-supported transition metal clusters,9,10,34,39,40 orientation A with the osmium atoms directed toward the surface oxygen atoms is found to be preferred over orientation B with the osmium atoms directed toward O-O bridge sites. The results suggest that Os5C may exhibit a lower symmetry than C4v, which was assumed in this work for computational efficiency. (50) Alexeev, O.; Gates, B. C. J. Catal. 1998, 176, 310.
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The coordination numbers determined by EXAFS spectroscopy for MgO-powder-supported clusters formed by decarbonylation of [Os5C(CO)14]2- on MgO powder agree with those of Os5C assumed in this investigation, but the Os-Os bond distances are calculated to be about 0.3-0.5 Å shorter than the values measured by EXAFS spectroscopy. For reasonably strong bonds such as found here, this difference is outside the bounds of the generally accepted inaccuracy of DF methods.47 Thus, the results of this work, combined with those of a recent investigation12 of metal clusters supported by a zeolite, indicate that the MgO-powder-supported Os5C clusters (and, we suggest, other supported metal clusters formed by decarbonylation of metal carbonyl cluster precursors) are likely not entirely ligand-free. The most plausible model of the MgO-powdersupported clusters includes a distribution of light-atom ligands formed during decarbonylation (consistent with the Fischer-Tropsch-like chemistry) and virtually undetectable with EXAFS and other spectroscopic techniques. The theoretical results also indicate that models of metal clusters on the ideal MgO(001) surface are too simple for accurate representation of clusters on nonideal MgO surfaces. More realistic adsorption site models than those investigated here may be needed to account for structural features such as kinks and steps and surface hydroxyl groups. Thus, these results demonstrate how theoretical investigations of judiciously chosen models complement experimental investigations of supported metal clusters and provide insight about the metal-support interface and the ligand environment of the clusters. Acknowledgment. We thank E. Ivanova and M. Staufer for assistance during the preparation of the manuscript. B.C.G. thanks the Alexander von Humboldt Foundation. J.F.G. is grateful to Professor H. Kno¨zinger for the hospitality during his stay in Munich. B.C.G. and J.F.G. thank the U.S. Department of Energy, Office of Energy Research, Office of Basic Energy Sciences, Division of Chemical Sciences, Contract FG02-87ER13790. This work was supported by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie. LA9912388
