Haptotropic Shifts in Cyclopentadienyl Organometallic Complexes

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Articles Haptotropic Shifts in Cyclopentadienyl Organometallic Complexes: Ring Folding vs Ring Slippage Luis F. Veiros† Centro de Quı´mica Estrutural, Instituto Superior Te´ cnico, 1049-001 Lisboa, Portugal Received July 10, 2000

The coordination geometry of shifted cyclopentadienyl ligands (Cp ) C5H5-) in organometallic complexes resulting from a two-electron reduction of η5-Cp parent species was investigated by means of molecular orbital calculations performed with the B3LYP HF/ DFT hybrid functional. Two types of complexes were studied: piano-stool mono(cyclopentadienyl) complexes, [(η5-Cp)M(CO)3], and bent metallocenes, [(η5-Cp)2M(CO)2]3+ with first (M ) Mn) and third (M ) Re) transition row metals, to study the influence of the coordination geometry as well as the metal size on the cyclopentadienyl haptotropic shift. The electronic structure of those species and of the reduced complexes, [(η-Cp)M(CO)3]2- and [(η-Cp)(η5Cp)M(CO)2]+, was analyzed with special emphasis on the (η-Cp)-M coordination. The reduction yielded haptotropic shifts in all cases, but the resulting (η-Cp)-M coordination geometry proved to depend on both the complex geometry and the metal size. For the bis(cyclopentadienyl) species folded η3-Cp compounds were obtained, with significant folding angles: ω ) 13° (M ) Mn) and 18° (M ) Re), while the reduced mono(cyclopentadienyl) complexes present slipped planar Cp ligands. The enhanced stability of the folded η3-Cp geometry for the bis(cyclopentadienyl) complexes is related with the mixing of a metal z2 type orbital into the reduced species highest occupied molecular orbital (HOMO), a Cp-M π* orbital that becomes occupied with the reduction. Introduction Haptotropic shifts are geometrical rearrangements of π ligand complexes produced by an increase on the metal electron count, as a result of ligand addition or electrochemical reduction.1 The relevance of those processes in the mechanism of associative substitution reactions was first pointed out by Basolo et al. with studies of phosphine addition to tricarbonylmanganese polyenyl complexes. An increase in the reaction rate with the ligand’s π system extension was observed, and the term indenyl effect was coined to describe it.2-4 Since then, haptotropic shifts have been shown to play an important role in the mechanism of a number of reactions with electronically saturated complexes, such as catalytic reactions.5-11 The instability of the lower †

E-mail: [email protected]. (1) O’Connor, J. M.; Casey, C. P. Chem. Rev. 1987, 87, 307. (2) Rerek, M. E.; Ji, L.-N.; Basolo, F. J. Chem. Soc., Chem. Commun. 1983, 1208. (3) Ji, L.-N.; Rerek, M. E.; Basolo, F. Organometallics 1984, 3, 740. (4) Rerek, M. E.; Basolo, F. Organometallics 1983, 2, 372. (5) Marder, T. B.; Roe, D. C.; Milstein, D. Organometallics 1988, 7, 1451. (6) Borrini, A.; Diversi, P.; Ingrosso, G.; Lucherini, A.; Serra, G. J. Mol. Catal. 1985, 30, 181. (7) Bo¨nnemann, H. Angew. Chem., Int. Ed. Engl. 1985, 24, 248. (8) Foo, T.; Bergman, R. G. Organometallics 1992, 11, 1801. (9) Schmid, M. A.; Alt, H. G.; Milius, W. J. Organomet. Chem. 1996, 514, 45. (10) Llinas, G. H.; Day, R. O.; Rausch, M. D.; Chien, J. C. W. Organometallics 1993, 12, 1283.

hapticity intermediates, species very difficult to isolate and characterize, led to a considerable amount of work aimed at the study of organometallic complexes with slipped or folded π ligands, to be used as models to help understand the mechanisms of those reactions. A wide variety of ligands and coordination modes have been covered, as shown by a Cambridge Structural Database (CSD)12 survey. Although indenyl (Ind ) C9H7-) is certainly the most studied ligand,13-36 larger ligands such as fluorenyl (11) Garrett, C. E.; Fu, G. C. J. Org. Chem. 1998, 63, 1370. (12) Allen, F. H.; Davies, J. E.; Galloy, J. J.; Johnson, O.; Kennard, O.; Macrae, C. F.; Watson, D. G. J. Chem. Inf. Comput. Sci. 1991, 31, 204. (13) Miller, G. A.; Therien, M. J.; Trogler. W. C. J. Organomet. Chem. 1990, 383, 271. (14) Ascenso, J. R.; Azevedo, C. G.; Gonç alves, I. S.; Herdtweck, E.; Moreno, D. S.; Pessanha, M.; Romaõ, C. C. Organometallics 1995, 14, 3901. (15) Calhorda, M. J.; Gamelas, C. A.; Gonç alves, I. S.; Romaõ, C. C.; Veiros, L. F. Eur. J. Inorg. Chem. 2000, 331. (16) Calhorda, M. J.; Gamelas, C. A.; Gonç alves, I. S.; Herdtweck, E.; Lopes, J. P.; Romaõ, C. C.; Veiros, L. F. Paper presented at the XIIth FECHEM, Prague, Czech Republic, 1997. (17) Calhorda, M. J.; Fe´lix, V.; Gamelas, C. A.; Gonç alves, I. S.; Romaõ, C. C.; Veiros, L. F. Paper presented at the XVII Reunioń del Grupo Especializado de Quı´mica Organometa´lica, Barcelona, Spain, 1998. (18) Lee, S.; Lovelace, S. R.; Cooper, N. J. Organometallics 1995, 14, 1974. (19) Amatore, C.; Ceccon, A.; Santi, S.; Verpeaux, J.-N. Chem. Eur. J. 1997, 3/2, 279. (20) Calhorda, M. J.; Gamelas, C. A.; Gonç alves, I. S.; Herdtweck, E.; Romaõ, C. C.; Veiros, L. F. Organometallics 1998, 17, 2597.

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(C13H9-), cyclopenta[def]phenanthrenyl (C15H9-),22,37-43 and 1-hydronaphthalene (C10H9-)44,45 have also been addressed. On the other hand, examples of wellcharacterized complexes with shifted cyclopentadienyl (Cp ) C5H5-) are relatively scarce, η3-Cp species being detected in solution46 or in the solid state.47-49 A great number of theoretical studies on haptotropic shifts have appeared since the early work by Hoffmann et al.50 Most dealt with cyclopentadienyl and indenyl complexes, studying the ligand bonding in different coordination geometries51-56 or specifically addressing (21) Kowalewski, R. M.; Rheingold, A. L.; Trogler, W. C.; Basolo, F. J. Am. Chem. Soc. 1986, 108, 2460. (22) Zhou, Z.; Jablonski, C.; Brisdon, J. J. Organomet. Chem. 1993, 461, 215. (23) Biagioni, R. N.; Luna, A. D.; Murphy, J. L. J. Organomet. Chem. 1994, 476, 183. (24) Brown, D. A.; Fitzpatrick, N. J.; Glass, W. K.; Ahmed, H. A.; Cunningham, D.; McArdle, P. J. Organomet. Chem. 1993, 455, 157. (25) Pevear, K. A.; Banaszak Holl, M. M.; Carpenter, G. B.; Rieger, A. L.; Rieger, P. H.; Sweigart, D. A. Organometallics 1995, 14, 512. (26) Merola, J. S.; Kacmarcik, R. T.; Van Engen, D. J. Am. Chem. Soc. 1986, 108, 329. (27) Forschner, T. C.; Cutler, A. R.; Kullnig, R. K. Organometallics 1987, 6, 889. (28) Poli, R.; Mattamana, S. P.; Falvello, L. R. Gazz. Chim. Ital. 1992, 122, 315. (29) Ascenso, J. R.; Azevedo, C. G.; Gonç alves, I. S.; Herdtweck, E.; Moreno, D. S.; Romaõ, C. C.; Zu¨hlke, J. Organometallics 1994, 13, 429. (30) Gonç alves, I. S.; Romaõ, C. C. J. Organomet. Chem. 1995, 486, 155. (31) Faller, J. W.; Crabtree, R. H.; Habib, A. Organometallics 1985, 4, 929. (32) Faller, J. W.; Chen, C.-C.; Mattina, M. J.; Jakubowski, A. J. Organomet. Chem. 1973, 52, 361. (33) Huber, T. A.; Be´langer-Garie´py, F.; Zargarian, D. Organometallics 1995, 14, 4997. (34) Huber, T. A.; Bayrakdarian, M.; Dion, S.; Dubuc, I.; Be´langerGarie´py, F.; Zargarian, D. Organometallics 1997, 16, 5811. (35) Crabtree, R. H.; Panell, C. P. Organometallics 1984, 3, 1727. (36) Marder, T. B.; Calabrese, J. C.; Roe, D. C.; Tulip, T. H. Organometallics 1987, 6, 2012. (37) Biagioni, R. N.; Lorkovic, I. M.; Skelton, J.; Hartung, J. B. Organometallics 1990, 9, 547. (38) Rerek, M. E.; Basolo, F. Organometallics 1984, 3, 647. (39) Decken, A.; Britten, J. F.; McGlinchey, M. J. J. Am. Chem. Soc. 1993, 115, 7275. (40) Decken, A.; Rigby, S. S.; Girard, L.; Bain, A. D.; McGlinchey, M. J. Organometallics 1997, 16, 1308. (41) Diamond, G. M.; Green, M. L. H.; Mountford, P.; Popham, N. A.; Chernega, A. N. J. Chem. Soc., Chem. Commun. 1994, 103. (42) Bochmann, M.; Lancaster, S. J.; Hursthouse, M. B.; Mazid, M. Organometallics 1993, 12, 4718. (43) Calhorda, M. J.; Gonç alves, I. S.; Herdtweck, E.; Romaõ, C. C.; Royo, B.; Veiros, L. F. Organometallics 1999, 18, 3956. (44) Georg, A.; Kreiter, C. G. Eur. J. Inorg. Chem. 1999, 651. (45) Son, S. U.; Paik, S.-J.; Lee, I. S.; Lee, Y.-A.; Chung, Y. K.; Seok, W. K.; Lee, H. N. Organometallics 1999, 18, 4114. (46) Simanko, W.; Sapunov, V. N.; Schmid, R.; Kirchner, K.; Wherland, S. Organometallics 1998, 17, 2391. (47) Huttner, G.; Brintzinger, H. H.; Bell, L. G.; Frieddrich, P.; Bejenke, V.; Neugebauer, D. J. Organomet. Chem. 1978, 141, 329. (48) Van Raaij, E. U.; Brintzinger, H. H.; Zsolnai, L.; Huttner, G. Z. Anorg. Allg. Chem. 1989, 577, 217. (49) Smith, M. E.; Anderson, R. A. J. Am. Chem. Soc. 1996, 118, 11119. (50) Albright, T. A.; Hofmann, P.; Hoffmann, R.; Lillya, C. P.; Dobosh, P. A. J. Am. Chem. Soc. 1983, 105, 3396. (51) Kakkar, A. K.; Taylor, N. J.; Calabrese, J. C.; Nugent, W. A.; Roe, D. C.; Connaway, E. A.; Marder, T. B. J. Chem. Soc., Chem. Commun. 1989, 990. (52) O’Hare, D.; Green, J. C.; Marder, T. B.; Collins, S.; Stringer, G.; Kakkar, A. K.; Kaltsoyannis, N.; Kuhn, A.; Lewis, R.; Mehnert, C.; Scott, P.; Kurmoo, M.; Pugh, S. Organometallics 1992, 11, 48. (53) Crossley, N. S.; Green, J. C.; Nagy, A.; Stringer, G. J. Chem. Soc., Dalton Trans. 1989, 2139. (54) Frankcom, T. M.; Green, J. C.; Nagy, A.; Kakkar, A. K.; Marder, T. B. Organometallics 1993, 12, 3688. (55) Green, J. C.; Parkin, R. P. G.; Yang, X.; Haaland, A.; Scherer, W.; Tapifolsky, M. J. Chem. Soc., Dalton Trans. 1997, 3219. (56) Field, C. N.; Green, J. C.; Moody, A. G. J.; Siggel, M. R. F. Chem. Phys. 1996, 206, 211.

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the indenyl effect,15-17,20,57,58 but fluorenyl and cyclopenta[def]phenanthrenyl have also been studied from the theoretical point of view.39,40,43 This work was prompted by recent results on the haptotropic shifts of tricarbonylmanganese complexes, [(η5-X)Mn(CO)3], with a series of π ligands, X, induced by a two-electron reduction59 or by phosphine addition.60 In fact, the geometries obtained for the shifted cyclopentadienyl complexes, [(η1-Cp)Mn(CO)3]2- and [(η2-Cp)Mn(CO)3(PH3)], respectively, present nearly planar Cp ligands, the change on the cyclopentadienyl hapticity being accomplished by ring slippage rather than by ring folding. An increased difficulty in obtaining folded η3Cp complexes, when compared with the indenyl analogues, may be explained, in principle, by the gain in the benzene aromaticity associated with a folded η3-Ind coordination. Nevertheless, there are some examples, although rare, of fully structurally characterized complexes in which the η3 coordination of cyclopentadienyl is achieved through ring folding. The most striking cases are Brintzinger’s bent metallocenes, [(η3-Cp)(η5-Cp)W(CO)2]47 and its chromium pentamethylcyclopentadienyl analogue,48 both with clearly folded Cp rings, with folding angles of ω ) 20 and 17°, respectively. Thus, the electronic reasons behind the coordination geometry of shifted Cp ligands (planar vs folded) are here investigated by means of ab initio61 and DFT62 calculations, complemented with the orbital analysis provided by the extended Hu¨ckel method.63,64 The comparative study of a two-electron reduction on Basolo’s piano-stool complexes, [(η5-X)M(CO)3], and Brintzinger’s bis-Cp analogues, [(η5-Cp)2M(CO)2]3+, M ) Mn, Re, allows the discussion of the influence of both metal size and complex geometry on the Cp haptotropic shift. Results and Discussion The electronic factors associated with a cyclopentadienyl haptotropic shift must be traced to the bonding of this ligand in an η5 coordination mode. Thus, although this has been well-known for quite some time,66 a qualitative analysis of the (η5-Cp)-M bond is reviewed in Figure 1, in a schematic way based on orbital symmetry considerations, for a typical example, [(η5Cp)Mn(CO)3]. A previously used57-59 intuitive notation is adopted, in which “s” and “a” mean symmetric and antisymmetric with respect to the plane of Cs symmetry. The overall Cp-M bond is based in three orbital interactions, one σ and two π. The σ component is a three-orbital interaction and results from the combination of metal z and z2 orbitals, empty and filled, respectively, with the ligand’s all symmetrical π orbital, (57) Bonifaci, C.; Ceccon, A.; Santi, S.; Mealli, C.; Zoellner, R. W. Inorg. Chim. Acta 1995, 240, 541. (58) Calhorda, M. J.; Veiros, L. F. Coord. Chem. Rev. 1999, 185186, 37. (59) Veiros, L. F. J. Organomet. Chem. 1999, 587, 221. (60) Veiros, L. F. Organometallics 2000, 19, 3127. (61) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; Wiley: New York, 1986. (62) Parr, R. G.; Yang, W. Density Functional Theory of Atoms and Molecules; Oxford University Press: New York, 1989. (63) Hoffmann, R. J. Chem. Phys. 1963, 39, 1397. (64) Hoffmann, R.; Lipscomb, W. N. J. Chem. Phys. 1962, 36, 2179. (65) Lucas, C. R.; Green, M.; Forder, R. A.; Prout, K. J. Chem. Soc., Chem. Commun. 1973, 97. (66) Albright, T. A.; Burdett, J. K.; Whangbo, M. H. Orbital Interactions in Chemistry; Wiley: New York, 1985.

Haptotropic Shifts in Organometallic Complexes

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Figure 1. Simplified schematic MO diagram for the interaction between (η5-Cp)- and [Mn(CO)3]+.

the more stable of the π set, named 1πs in Figure 1. Three molecular orbitals arise from this interaction, a filled bonding orbital, σ, being mostly composed by the ligand’s 1πs orbital, its antibonding counterpart, σ* (not represented in Figure 1), which is empty and mainly metal z in character, and, finally, a filled nonbonding orbital, corresponding to metal z2. The two π components of the bonding result from the combination of the empty metal d orbitals pointing toward the ligand, xz and yz, in Figure 1, with the ligand’s π orbitals having the right symmetry: the Cp e1′′ set, 2πs and 1πa, respectively, in Figure 1 notation. From these interactions result the complex frontier orbitals, the two highest occupied molecular orbitals (HOMO), πs and πa, and the two lowest unoccupied molecular orbitals (LUMO), πs* and πa*. The net result is the establishment of three (η5-Cp)-M bonds, which formally correspond to three CpfM two-electron donations. Two extreme possibilities may be envisioned for the coordination geometry of a shifted cyclopentadienyl (Scheme 1). In one case the haptotropic shift corresponds to the folding of the C5 ring, two carbon atoms being pushed away from the metal, beyond bonding distances (C4 and C5, in Scheme 1), the other three M-Cx bond lengths remaining practically unchanged (x ) 1-3). Besides the differences in the five M-C distances, this Cp coordination geometry is characterized by significant values of the folding angle, ω. This is the angle between the plane of C1, C2, and C3 and the mean plane of C1, C3, C4, and C5, being equivalent to Ω, defined to characterize η3-indenyl complexes.31 The resulting coordination geometry, named here as folded

η3-Cp (A in Scheme 1), is the one found in Brintzinger’s metallocenes, [(η3-Cp′)(η5-Cp′)M(CO)2] (M ) W, Cp′ ) C5H547 and M ) Cr, Cp′ ) C5Me548). On the other hand, the haptotropic shift may be achieved through ring slippage, the Cp ring keeping its planarity, with negligible folding angles, ω. A slipped η-Cp complex results, in which the Cp final hapticity depends on the slippage geometric details. Thus, a hapticity between η1 and η3, depending on the slippage degree, is obtained if the geometrical rearrangement brings a Cp carbon atom closer to the metal (C2 in Scheme 1). This coordination geometry is characterized by one shorter (x ) 2), two intermediate (x ) 1, 3), and two longer (x ) 4, 5) M-Cx distances, being represented by B in Scheme 1. This is the geometry previously optimized for [(η1-3-Cp)Mn(CO)3]2-, resulting from the reduction of the parent neutral η5-Cp complex.59 However, there is another possibility for the hapticity of a planar slipped Cp. Ring slippage will yield a η2-Cp group if a C-C bond approaches the metal, resulting in two shorter (x ) 4, 5) and three longer (x ) 1-3) M-Cx distances: C in Scheme 1. Although they are rare, slipped planar η2Cp groups have been found before, for example in the X-ray structure of tris(cyclopentadienyl)titanium65 or in the calculated geometry of [(η2-Cp)Mn(CO)3(PH3)], the intermediate of the phosphine addition to the tricarbonylmanganese cyclopentadienyl complex.60 Figure 1 may be used to help in finding the electronic factors determinant to a Cp haptotropic shift and to understand the coordination geometry of the shifted ligand. In fact, those geometric rearrangements are normally associated with an increase of two electrons in the metal center, as a consequence of ligand addition or simple reduction of the η5-Cp parent species. The two extra electrons will occupy the LUMO of the reactant, which, in the case of an electronic saturated complex, is a Cp-M antibonding orbital, πs* or πa* (see Figure 1). The relief of the antibonding character of this orbital is the driving force for the haptotropic shift geometrical distortion.58-60 Figure 2 shows a simplified schematic view of the frontier orbitals of an example model complex, [(η-Cp)Mn(CO)3]2-, resulting from a two-electron reduction of the parent neutral piano-stool complex. Three coordination geometries are discussed: folded η3-Cp, planar η1-3Cp, and planar η2-Cp (A-C in Scheme 1, respectively). Thus, in the case of a ring folding haptotropic shift a stabilization of πs* is achieved as two of the Cp carbon

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Figure 3. Optimized structures of [(η5-Cp)M(CO)3] for M ) Mn (top) and Re (bottom) with the more relevant geometrical parameters (distances in Å). The experimental values are presented in italics.

Figure 2. Schematic representation of the (η-Cp)-M π interactions (πs and πa), for different shifted Cp coordination geometries: (A) folded η3-Cp; (B) planar η1-3-Cp; (C) planar η2-Cp.

atoms are pushed away from the metal, diminishing that orbital antibonding character. Some energetic costs are, however, associated with this type of rearrangement. On one hand, the folding of the ligand implies the breaking of its π system aromaticity, and on the other, there is a weakening of the M-Cp bonding interactions with the augmentation of two M-C distances, reflected by a destabilization of the πs bonding orbital. In the case of a ring slippage yielding coordination geometry B the fragment’s local symmetry is maintained, resulting in nearly degenerate πs and πa orbitals and their antibonding counterparts, in a way similar to what happens in the η5-Cp complex (Figure 1). In other words, the stabilization of the πs* and πa* orbitals is paralleled by a destabilization of corresponding bonding orbitals, πs and πa, resulting in the weakening of the M-Cp bond. The ligand’s aromaticity is kept, and the short energy gap between πs* and πa* can lead to paramagnetic species with a spin triplet ground state. An increased slippage degree toward an η1 coordination will further destabilize the M-Cp bond, leading to a

smaller energy gap between the bonding and the antibonding π orbitals. In the limit of an ideal η1-Cp, the overall metal to cyclopentadienyl bond will be based essentially on the σ interaction (see Figure 1). Finally, for the third coordination geometry, C, a different bonding scheme is obtained from this simple symmetry-based orbital analysis. In this case, ring slippage places a C-C bond over the metal, and consequently, one of the complex π interactions is lost (πs), since a practically zero overlap is produced between the interacting fragment orbitals, metal xz and Cp 2πs, which remain essentially nonbonding. The ligand’s aromaticity is maintained, and there are no occupied antibonding orbitals. On the other hand, the overall M-Cp bond is considerably weakened, as one of the interactions is lost, and an electron-deficient metal center results. The final coordination geometry of the Cp ligand, following a haptotropic shift in a given complex, will be the result of a balance between the electronic factors discussed above and the different van der Waals interligand interactions. The performance of the theoretical model used in this work (see Computational Details) was tested through the comparison between the optimized structures and the experimental X-ray values of the mono(cyclopentadienyl) piano-stool complexes [(η5-Cp)M(CO)3], M ) Mn,67 Re.68 The calculated geometries and the more relevant geometrical parameters are presented in Figure 3. For the manganese complex the mean and (67) Cowie, J.; Hamilton, E. J. M.; Laurie, J. C. V.; Welch, A. J. J. Organomet. Chem. 1990, 394, 1. (68) Fitzpatrick, P. J.; LePage, Y.; Butler, I. S. Acta Crystallogr. 1981, B37, 1052.


maximum deviations between the calculated and the experimental distances are 0.028 and 0.049 Å, respectively, and a 1° difference is found for the folding angle, ω. The bonding in the rhenium complex is not so well described, with 0.031 and 0.069 Å mean and maximum deviations, respectively, and a 4° difference for the folding angle. The larger differences are found in the obtained Re-C(Cp) distances (see Figure 3), probably due to the simple treatment of the relativistic corrections included in the basis set (see Computational Details). Those results, although not perfect, are more than appropriate for the structural discussion here intended, thus confirming the validity of the theoretical method employed. In the calculated geometries of both [(η5-Cp)M(CO)3] complexes (M ) Mn, Re) the Cp bonding is typical of an η5 coordination, with the five M-C(Cp) distances within a narrow range (2.181-2.183 Å for Mn and 2.348-2.353 Å for Re) and small values for the folding angle (ω ) 1° (Mn) and 0° (Re)). The small degree of distortion of the Cp coordination geometry in those complexes can also be seen in the regularity of the C-C bond lengths in the C5 ring: 1.418-1.432 Å for M ) Mn and 1.423-1.437 Å for M ) Re. Other conclusions can be drawn from the comparison between the obtained structures of both complexes (M ) Mn, Re). A larger metal size implies longer bond distances to the metal for the rhenium complex, the mean increase being 0.07 and 0.13 Å for the M-C(Cp) and M-C(CO) bonds, respectively. On the other hand, a small but consistent increase in the C-O bond distance is found in both the theoretical (0.005 Å) and the experimental (0.016 Å) structures going from the Mn to the Re complex. This suggests a stronger metal to carbonyl back-donation in the case of the rhenium complex, which is corroborated by the calculated Mulliken charges of the two metals in the [(η5-Cp)M(CO)3] complexes (-0.308 for Mn and 0.031 for Re), as well as by the extended Hu¨ckel C-O overlap populations on model complexes, 1.21 and 1.17 for Mn and Re, respectively, showing a more electron deficient metal and a weaker C-O bond for the rhenium species. The calculated ν(CO) frequencies (1978 and 2034 cm-1 for M ) Mn, and 1955 and 2027 cm-1 for M ) Re) also point toward an increased back-donation on the rhenium complex and compared very well with the experimental values: 1944, 2027 cm-1 (Mn)69 and 1926, 2025 cm-1 (Re).70 The optimized geometries of the complexes [(η-Cp)M(CO)3]2-, resulting from a two-electron reduction of the parent η5 species, are presented in Figure 4 with the more relevant geometrical parameters. Both metals present slipped planar cyclopentadienyl ligands with negligible folding angles ω ) 2° (Mn) and 1° (Re). For the manganese complex an η1-Cp is observed, with one short (Mn-C2 ) 2.362 Å) and four longer Mn-C(Cp) distances (>3 Å). This corresponds to coordination geometry B (see Scheme 1) and presents a triplet spin state (represented in Figure 4) 8.7 kcal/mol more stable than the corresponding singlet, in which a planar η2Cp is observed (C in Scheme 1). The reverse situation is found for M ) Re, with singlet C, with an η2-Cp (69) Parker, D. J. J. Chem. Soc., Dalton Trans. 1974, 155. (70) Mink, J.; Bencze, E. Unpublished results.

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Figure 4. Optimized geometries of [(η1-Cp)Mn(CO)3]2(top) and [(η2-Cp)Re(CO)3]2- (bottom) with the more relevant geometrical parameters (distances in Å).

(represented in Figure 4) 8.8 kcal/mol more stable than triplet B, in which the Cp presents an η1 coordination. The η2 coordination in [(η2-Cp)Re(CO)3]2- is established through two M-C(Cp) short distances, 2.481 and 2.672 Å, the remaining three carbon atoms being pushed beyond bonding distances (>3.3 Å) by the ring slippage. These results are in good agreement with the DFT energies obtained by Harris et al.71 for the reactive intermediates [(η5-Cp)M(CO)2] (M ) Mn, Re), resulting from the UV photolysis of a CO ligand of the parent tricarbonyl complexes; the triplet is more stable than the singlet state for the manganese species, while the reverse was found for M ) Re. This corroborates the known72 higher tendency of 3d metals, when compared with 5d elements, to adopt a higher spin configuration due to a greater spin pairing energy. It is worth noting that these results can be explained with Figure 2 qualitative orbital analysis for the bonding of a shifted Cp. In fact, the maintenance of the fragment’s local symmetry for geometry B leads to a triplet ground state for the first-transition-row metal Mn. On the other hand, the preference for a lower spin state of the heavier metal, Re, results in coordination geometry C in the reduced complex, overtaking the loss of one M-Cp interaction (πs). The attribution of an η1 hapticity for cyclopentadienyl in the reduced complex is not straightforward from the Figure 4 optimized geometry. In fact, an η3-Cp as well as an η1-Cp coordination may be associated with geometry B. However, for complex [(η1-Cp)Mn(CO)3]2- both the M-C(Cp) distances, already mentioned, and the C-C distances (see Chart 1) point toward a η1-cyclopentadienyl. In an η3-Cp group a CdC double bond is formed between the two uncoordinated carbon atoms (71) Yang, H.; Asplund, M. C.; Kotz, K. T.; Wilkens, M. J.; Frei, H. Harris, C. B. J. Am. Chem. Soc. 1998, 120, 10154. (72) Poli, R. Chem. Rev. 1996, 96, 2135.

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Chart 1

(C4 and C5); thus, a shorter C-C bond should be found in the C5 ring (left side of Chart 1). On the other hand, in an η1-Cp two double bonds are formed, C1dC5 and C3dC4, which should correspond to shorter distances in the Cp ring (right side of Chart 1). This is the case for the manganese reduced complex, in which those two bond distances (1.40 Å) are shorter than the other three (1.42-1.43 Å), suggesting an η1 coordination mode. The carbonyl bonding is another interesting aspect that can be noted from the structures calculated for the reduced complexes. An increase in the C-O bond lengths is found with the reduction, from 1.16 to 1.19 Å for M ) Mn and from 1.16 to 1.20 Å for M ) Re. This reflects the increase in the metal to carbonyl backdonation resulting from the augmentation of the metal electron density and its releasing to the CO π acceptors. This is further shown by a decrease in the M-C(CO) bond lengths from 1.92 to 1.87-1.90 Å for the rhenium species, where the stereochemical effects are less important. The calculated ν(CO) frequencies (1752, 1832 cm-1 for M ) Mn and 1744, 1817 cm-1 for M ) Re) point toward the same conclusion, being significantly smaller than the parent complex frequencies (see above). The results confirm the structure of [(η1-Cp)Mn(CO)3]2-, obtained in a previous work,59 in which a less accurate theoretical model was used and an uncompleted search of the triplet state potential energy surface was performed. The η2-Cp coordination geometry present in the rhenium species is similar to what was found for the phosphine addition intermediate [(η2-Cp)Mn(CO)3(PH3)].60 In this case the presence of the phosphine breaks the metallic fragment symmetry, leading to an increased energy gap between πs* and πa* and, thus, to an η2-Cp and corresponding singlet ground state (see Figure 2 and the discussion above). All the structures obtained for the reduction products of the [(η5-Cp)M(CO)3] complexes present planar Cp ligands. This leads to a simple question: why do the haptotropic shifts not proceed through ring folding in those species? This is especially puzzling, since the equivalent complexes with larger π ligands, such as indenyl and indacene, were shown to produce the ring folded reduced complexes [(η3-X)Mn(CO)3]2-.59 The better way to answer that question is to seek under what conditions the Cp ring folding is found, bringing us back to Brintzinger’s bent metallocenes.47,48 Thus, calculations were performed on the group 7 analogues of the former species, [Cp2M(CO)2]3+, M ) Mn, Re, and its twoelectron reduction products, to comparatively study the electronic factors involved in the coordination geometry of a shifted Cp ligand: namely, the ring folding known to occur in the bis-Cp complexes and the ring slippage obtained for the mono-Cp complexes. The optimized geometries for the [Cp2M(CO)2]3+ complexes are represented in Figure 5, with the more relevant structural parameters. Although no symmetry constraints were imposed on the calculations, a pseudoC2v symmetry is observed in the resulting structures, with an equivalence between each pair of ligands: that

Figure 5. Optimized structures of [(η5-Cp)2M(CO)2]3+ for M ) Mn (top) and Re (bottom) with the more relevant geometrical parameters (distances in Å).

is, the two Cp and the two carbonyl ligands. For this reason, only half of the structural parameters are presented in Figure 5, the second half being identical. The obtained geometries correspond to typical12 bent metallocene complexes with the Cp ligands coordinating in an η5 mode, as shown by the small folding angles, ω ) 0° (Mn) and 2° (Re), as well as by narrow ranges of the M-C(Cp) bond lengths, 2.220-2.310 Å and 2.2962.407 Å for the manganese and the rhenium species, respectively. The major difference between the structures in Figure 5 is the relative conformation of the two Cp rings: perfectly eclipsed for the Re complex and a 17° rotation for the Mn species. This is caused by the different interligand repulsions in the two complexes. Thus, the bond distances in the metal coordination sphere are shorter for the manganese complex, Mn-Cp (centroid) ) 1.92 vs 2.03 Å for M ) Re and Mn-C(CO) ) 1.920 vs 2.025 Å for M ) Re, and the X-M-X angles are narrower, 133 and 134° for X ) Cp and M ) Mn and Re, respectively, and 85° and 87° for X ) CO, in the same order. In other words, the stereochemical repulsion overtakes the electronic factors in the Mn complex, preventing the Cp rings from adopting an eclipsed conformation. On the other hand, for the Re species the interligand repulsion, although present as stated by more distorted Cp coordination geometries (compare the folding angles, for example), is considerably diminished, the final conformation being dictated by the electronic factors. The “rule” saying that a decrease in the stereochemical repulsion and an increase in the metalligand interaction strength is expected on going down a transition group72 is found in these complexes, as it was before in the mono-Cp species. The optimized geometries for the Re reduced complex, [Cp2Re(CO)2]+, are presented in Figure 6 with the more relevant geometrical parameters. Two limiting Cp conformations were optimized, starting from a parent


Figure 6. Optimized structures of [(η3-Cp)(η5-Cp)Re(CO)2]+ with an alternated (top) and an eclipsed (bottom) conformation of the Cp rings. The more relevant geometrical parameters are presented (distances in Å).

complex with eclipsed and alternated rings, respectively. The most striking feature of the structures in Figure 6 is the clearly folded η3-Cp coordination mode obtained for one of the Cp ligands in each conformer (coordination mode A in Scheme 1), as established by significant folding angles (16 and 18°) and by two long M-C(Cp) bond distances (>3 Å), well over the values found for the coordinating carbon atoms, 2.206-2.498 Å. The C-C bond distances within the Cp rings are also indicative of an η3-Cp group (see Chart 1), with the bond lengths between uncoordinated carbon atoms, 1.352 and 1.358 Å, much shorter than the rest (>1.4 Å), revealing some CdC double-bond character. The second Cp ligand presents an η5-Cp coordination mode for both conformers, with small folding angles (0 and 4°) and a narrow range for the M-C(Cp) bond distances (2.281-2.415 Å). An analysis of the C-O bonding reveals an increase in the metal to carbonyl back-donation with the reduction, similar to what was found for the piano-stool complexes. This is shown by the shortening of the ReC(CO) distances from 2.025 Å in the parent complex to 1.930 and 1.942 Å in the reduced species and by the corresponding augmentation of the C-O bond lengths from 1.131 to 1.149-1.152 Å. A closer analysis of the values obtained for two conformers of the [(η3-Cp)(η5Cp)Re(CO)2]+ complex shows, however, that a more effective back-donation is achieved for an alternated conformation with slightly shorter Re-C(CO) distances (1.930 vs 1.942 Å) and longer C-O bond lengths (1.152 vs 1.149 Å). These differences, although small, are significant, being the result of an improved overlap between the donor metal d orbitals and the carbonyl π* acceptor, as shown previously for [(η3-Cp)Mo(CO)2(CH3CN)3]+.20,58 This is further corroborated by the calculated ν(CO) frequencies, dropping from 2161/2178 cm-1 in the parent [(η5-Cp)2Re(CO)2]3+ to 2024/2065 and 2009/


Figure 7. Optimized structures of [(η2-Cp)(η5-Cp)Mn(CO)2]+ (top) and [(η3-Cp)(η5-Cp)Mn(CO)2]+ (bottom) with the more relevant geometrical parameters (distances in Å).

2050 cm-1 for the eclipsed and alternated conformations of the reduced complex, respectively. Despite the enhanced Re-CO bond, the alternated conformation is not the most stable one. The eclipsed conformation is slightly more stable (by 3.1 kcal/mol) being also the one experimentally found for the tungsten analogue.47 This, and the similarity between the folding angles, 18° for the calculated geometry of the Re complex and 20° for the experimental structure of the W complex, gives strong support to the optimized geometry of [(η3-Cp)(η5-Cp)Re(CO)2]+, especially as different metal atoms and complex charges are involved. The preferred eclipsed conformation is the result of a stronger (η3-Cp)-M interaction, which prevails over a slightly weaker M-CO bond and an increased repulsion between Cp rings. This effect, already discussed for the parent [(η5-Cp)2Re(CO)2]3+ complex, is shown by the comparison of the η5 coordinated Cp ligand distortion in the structures in Figure 6, with folding angles of 0 and 4° for the alternated and the eclipsed conformations, respectively. In fact, the latter is the largest folding angle obtained in this work for a η5-coordinated Cp ligand. The optimized geometries of the manganese reduced bis-Cp complexes, [(η-Cp)(η5-Cp)Mn(CO)2]+, are presented in Figure 7, with the more relevant structural parameters. The two conformations already discussed for M ) Re were attempted, following the same calculation procedure: that is, starting from the alternated and the eclipsed conformations of the parent bis(η5-Cp) species. The obtained complexes, although energetically equivalent (only 0.9 kcal/mol apart), correspond both to alternated conformations of the two Cp ligands, but with different coordination geometries for the shifted Cp ring. In one case a shift toward the carbonyl ligands occurs, yielding a folded η3-Cp with coordination geometry A, and an overall geometry equivalent to the alternated conformer of the rhenium complex. In the other, the

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shift pushes the Cp ligand away from the carbonyls, just as in the eclipsed [(η3-Cp)(η5-Cp)Re(CO)2]+ complex, but a practically planar (ω ) 3°) η2-Cp in coordination geometry C is obtained. This is a consequence of the increased importance of the interligand repulsion for first-transition-row metal complexes, in comparison with its third-row analogues. Thus, if the shift brings the two Cp ligands closer, a planar geometry is adopted by the shifted Cp, to diminish the steric repulsion between the rings already observed for the eclipsed conformation of the reduced Re bis-Cp complex. The preference for coordination geometry C over B results from the symmetry breaking introduced by the presence of the other Cp ligand in the metallic fragment, [CpMn(CO)2]2+, and the consequent enlarged energy gap between the complex frontier orbitals, πs* and πa*, similar to what was found for [(η2-Cp)Mn(CO)3(PH3)]60 and discussed above for the mono-Cp complexes. It should be noted that calculations on the [Cp2Mn(CO)2]+ complexes triplet state have shown these to be less stable (by 5.2 kcal/mol) than the spin singlet species presented in Figure 7. The geometric parameters obtained for the [(η-Cp)(η5-Cp)Mn(CO)2]+ complexes (see Figure 7) are similar to the ones found for the other species here studied with the equivalent Cp coordination geometries. Thus, the η2-Cp coordination is characterized by two short (2.180 and 2.181 Å) and three long (2.944-3.363 Å) M-C(Cp) bond distances. The folded η3-Cp presents a significant folding angle, ω ) 13°; of the five M-C(Cp) lengths, three are within the usual values (2.111-2.407 Å) and the other two are over bonding distances (>3 Å). The formation of the CdC double bond between the uncoordinated carbon atoms is also shown by the corresponding bond length, 1.355 Å, well below the remaining C-C distances (>1.4 Å). The second Cp ligand in each of the complexes in Figure 7 has an η5-Cp coordination mode, with small folding angles (1 and 2°) and a narrow range for metal to carbon bond lengths, 2.157-2.257 Å. The reduction produces an enhanced metal to carbonyl back-donation, just as in the previously discussed examples, reflected in a decrease in the Mn-C(CO) distances, from 1.920 Å in the parent complex to 1.8321.833 Å in the reduced species, and in the corresponding increase in the C-O bond lengths, from 1.129 to 1.1441.146 Å. The calculated ν(CO) frequencies lead to the same conclusion, dropping from 2176/2179 cm-1 in the parent complex to 2057/2073 and 2047/2061 cm-1 for the η2-Cp and the η3-Cp complexes, respectively. The geometry calculated for [(η3-Cp)(η5-Cp)Mn(CO)2]+ with a folded Cp ligand (bottom of Figure 7) is equivalent to the experimental structure of its Cr analogue with permethylcyclopentadienyl (Cp*),48 with the same relative conformation of the two Cp rings and similar folding angles, ω ) 13° for the manganese complex and 17° for the chromium species, thus providing a good degree of confidence in the optimized structure. The presence of Cp* in the Cr complex results in a sterically more crowded metal coordination sphere, justifying a 4° larger folding angle on this species. On the other hand, since Cp* is more sterically demanding than Cp, an increased steric repulsion between rings would arise if the two rings would approach each other during the shift. This breaks the equivalence found for the two Mn

Veiros

Figure 8. 3D representations of the HOMO (πs*) and the corresponding metal d orbital hybridization for (a) [(η3-Cp)Mn(CO)3]2- and (b) [(η3-Cp)(η5-Cp)Mn(CO)2]+.

complexes of Figure 7, favoring the observed geometry, in which the shift brings the two rings apart and a folded η3-Cp results. The results discussed so far show that a two-electron reduction of electronic saturated complexes yields haptotropic shifts of the Cp ligand. Although spin state and steric effects play a role in determining the details of the coordination geometry of the shifted ligand, especially for a 3d metal, folded η3-Cp groups are only obtained for the bent metallocenes [Cp2M(CO)2]+. Planar coordination geometries of that ligand are always found for the mono-Cp piano-stool complexes [CpM(CO)3]2-, regardless of the metal size (M ) Mn, Re). This indicates that the preference for a ring folding haptotropic shift over a ring slippage shift depends on the complex geometry rather than on the transition-metal size. The electronic factors associated with a ring folding haptotropic shift leading to coordination geometry A can be traced to the reduced complex HOMO (πs*), since this is the orbital that becomes occupied in the reduction, the releasing of its antibonding character being the driving force for the shift. Representations of that orbital can be found in Figure 8, for two model complexes with a folded η3-Cp: [(η3-Cp)Mn(CO)3]2- and [(η3-Cp)(η5-Cp)Mn(CO)2]+. In both cases M-Cp π antibonding character is present, being diminished by the augmentation of two M-C(Cp) bond distances, in accordance with the simplified qualitative bond analysis discussed above (see Figure 2). However, some differences can be found with a closer look at the nature of the metal d orbital involved in each complex HOMO. Thus, for the mono-Cp complex both lobes of the metal d orbital pointing toward the Cp ligand are equivalent, while in the metallocene the


Figure 9. 3D representation of the [(η3-Cp)Mn(CO)3]2- “z2” molecular orbital.

lobe pointing toward the three coordinating carbon atoms is considerably diminished. This results in a smaller M-Cp overlap and, hence, in a reduced antibonding character with the consequent stabilization. If one takes the zz axis as the bonding axis of the shifted Cp in each geometry, for discussion sake, the mixing of a z2 type orbital provides the metal orbital hybridization present in the [(η3-Cp)(η5-Cp)Mn(CO)2]+ HOMO (see Figure 8). This is shown by the participation of metal z2 in the πs* molecular orbital of the two complexes, which represents 2% of that orbital electronic density in the case of the mono-Cp species and rises to 11% in the metallocene. This same orbital mixing explains the alternating conformation of the indenyl ligands in the 20-electron complex [(Ind)2Ni].58 An analysis of the piano-stool complex molecular orbitals provides the explanation for the absence of the stabilizing z2 mixing in the those complexes πs* orbital. In fact, given the “cylindrical” symmetry of the metallic fragment, [Mn(CO)3]-,66 and the metal electronic configuration (d6), the z2 orbital is essentially devoted to back-donation from the metal to the carbonyl ligand inphase combination of π* orbitals with adequate symmetry (see Figure 9). On the other hand, the symmetry breaking imposed by the second Cp ligand in the bisCp species metallic fragment, [CpMn(CO)2]2+, and the metal electronic configuration (d2) leave an available and empty z2 orbital that can be mixed into the complex HOMO, πs*. This fine-tuning proved to be crucial in obtaining a folded η3 coordination geometry for cyclopentadienyl, although not for [(η3-X)Mn(CO)3]2- with larger π ligands, such as indenyl and indacene,59 showing that the (η3-X)-M bond strength in the studied complexes is considerably weaker for X ) Cp than for its indenyl analogues, similar to what was previously found for molybdenum and tungsten species.15,58 It is not surprising, though, that the only complexes presenting clearly folded η3-Cp with determined X-ray structures are Brintzinger’s group 6 bent metallocenes, [(η3Cp′)(η5-Cp′)M(CO)2] (M ) W, Cp′ ) C5H547 and M ) Cr, Cp′ ) C5Me548), in which an extra stabilization of the complex HOMO (πs*) is achieved through the mixing of the metal z2 orbital. Conclusions Haptotropic shifts resulting from a two-electron reduction of the piano-stool cyclopentadienyl complexes [CpM(CO)3] were shown to proceed through ring slippage, yielding planar coordination geometries for the shifted Cp ligand, for both M ) Mn and M ) Re. Spin configuration plays an important role in determining the final complex geometry for the lighter transition


element, originating an η1-Cp complex, with a triplet ground state. This spin state effect is absent in the rhenium analogue, [(η2-Cp)Re(CO)3]2-, which has a singlet ground state and a η2-Cp ligand. The equivalent geometrical distortion in the bent metallocenes [Cp2M(CO)2]3+ produced ring folded η3-Cp complexes, interligand repulsive interactions being important in the establishment of the final Cp coordination geometry for the first transition row element, Mn. A stabilization of the bis-Cp reduced complex HOMO (πs*) is achieved through the mixing of a metal z2 orbital pointing toward the shifted Cp; this proved to be crucial in determining the preference of the bis-Cp species for a ring folding haptotropic shift. Computational Details The geometry optimizations were accomplished by means of ab initio and DFT calculations performed with the Gaussian 98 program.73 The B3LYP hybrid functional was used in all optimizations. That functional includes a mixture of HartreeFock61 exchange with DFT62 exchange correlation, given by Becke’s three-parameter functional74 with the Lee, Yang, and Parr correlation functional, which includes both local and nonlocal terms.75,76 All the optimized geometries are the result of full optimizations without any symmetry constraints. Extensive potential energy searches were performed for each molecule with a LanL2DZ77,78 exploratory basis set. The structures obtained with this basis set were reoptimized with an f-polarization function79 added to the transition elements (Mn and Re) and a 6-31G**80 basis set for the remaining atoms (C, H, and O). The same general features were found on the geometries obtained with the smaller basis and in the final optimized structures, for all species studied. Spin contamination was carefully monitored for all the unrestricted calculations performed for the triplet species, and the values of 〈S2〉 indicate minor spin contamination: 2.0057 and 2.0000 for the Mn and Re mono-Cp complexes and 2.0057 and 2.0069 for the two conformations of the Mn bis-Cp complex. All the stationary points were confirmed by frequency calculations, and the energies were zero point corrected. The frequencies presented along the text were scaled by a 0.9613 factor.81 (73) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; HeadGordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, Revision A.7; Gaussian, Inc., Pittsburgh, PA, 1998. (74) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (75) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (76) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Chem. Phys. Lett. 1989, 157, 200. (77) Dunning, T. H., Jr.; Hay, P. J. In Modern Theoretical Chemistry; Schaefer, H. F., III, Ed.; Plenum: New York, 1976; Vol. 3, p 1. (78) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (b) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (c) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 2299. (79) Ehlers, A. W.; Bo¨hme, M.; Dapprich, S.; Gobbi, A.; Ho¨llwarth, A.; Jonas, V.; Ko¨hler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking, G. Chem. Phys. Lett. 1993, 208, 111. (80) (a) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54, 724. (b) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257. (c) Hariharan, P. C.; Pople, J. A. Mol. Phys. 1974, 27, 209. (d) Gordon, M. S. Chem. Phys. Lett. 1980, 76, 163. (e) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. (81) Foresman, J. B.; Frisch, Æ. Exploring Chemistry with Electronic Structure Methods, 2nd ed.; Gaussian: Pittsburgh, PA, 1996.

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The extended Hu¨ckel calculations63,64 were done with the CACAO program,82 and modified Hij values were used.83 The basis set for the metal atoms consisted of ns, np, and (n - 1)d orbitals. The s and p orbitals were described by single Slatertype wave functions, and the d orbitals were taken as contracted linear combinations of two Slater-type wave functions. The parameters used for the transition elements were as follows (Hii (eV) and ζ): for Mn, 4s, -9.880, 1.800; 4p -5.450, 1.800; 3d, -12.530, 5.150, 1.900 (ζ2), 0.5311 (C1), 0.6479 (C2); for Re, 6s, -9.360, 2.398; 6p, -5.960, 2.372; 5d, -12.660, 5.343, 2.277 (ζ2), 0.6378 (C1), 0.5658 (C2). Standard parameters were used for other atoms. Calculations were performed on models based on the optimized geometries with (82) Mealli, C.; Proserpio, D. M. J. Chem. Educ. 1990, 67, 39. (83) Ammeter, J. H.; Bu¨rgi, H.-J.; Thibeault, J. C.; Hoffmann, R. J. Am. Chem. Soc. 1978, 100, 3686.

Veiros idealized maximum symmetry and the following distances (Å) and angles (deg): M-Cp(centroid) ) 1.77 (M ) Mn) and 1.95 (M ) Re), M-C(CO) ) 1.80, C-O ) 1.15, C-C ) 1.40, C-H ) 1.08; Cp-M-CO ) 120 ([CpM(CO)3] complexes), CO-MCO ) 80, Cp-M-Cp ) 140 ([Cp2M(CO)2] complexes), ω ) 30°.

Acknowledgment. Praxis XXI is acknowledged for partial funding of this work. Supporting Information Available: Tables of atomic coordinates for all of the optimized structures. This material is available free of charge via the Internet at http://pubs.acs.org. OM000589A

Haptotropic Shifts in Cyclopentadienyl Organometallic Complexes

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