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Database Analysis of Transition Metal Carbonyl Bond Lengths

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Database Analysis of Transition Metal Carbonyl Bond Lengths Organometallics 2007, 26, 2815-2823 2815 Database Analysis of Transition Metal Carbonyl Bond Lengths: Insight into the Periodicity of π Back-Bonding, σ Donation, and the Factors Affecting the Electronic Structure of the TM-CtO Moiety Rosalie K. Hocking*,§ and Trevor W. Hambley* Centre for HeaVy Metals Research, School of Chemistry, UniVersity of Sydney, Sydney, NSW 2006, Australia ReceiVed NoVember 23, 2006 Analysis of the relationships between TM-C and CtO distances of the more than 20 000 structures reported to the CSD has revealed new experimentally derived insights into the bonding in these systems. The databases of structures were investigated by a combination of DFT and statistical methods. The different abilities of transition metals to both accept and donate electrons are reflected in differences in their data sets. There are significant changes in gradient of the curves representing TM-C versus CtO scatter plots. One such change in gradient occurs at the bond length corresponding to CtO in its unbound state. There are also significant differences between the second and third transition series. The analysis provides a structural means of probing the distribution of electrons in a metal-carbonyl fragment and provides important insights into the periodicity of back-bonding, σ donation, and the ability of the carbonyl to stabilize different metal oxidation states. Introduction Transition metal (TM) carbonyl compounds form a class of metal complexes that is one of the most widely studied in When a ligand interacts with a metal, the amount of ligand chemistry, and of the crystal structures in the CSD, more than to metal donation and the metal to ligand back-donation 20 000 are transition metal carbonyls.12-20 These compounds determine many properties of both the metal center (e.g., have diverse applications ranging from catalysts to functional reduction or oxidation potential) and the ligand (e.g., lability). species in biochemistry.12,21 The stability of a transition metal Understanding these properties is important in understanding a carbonyl bond is a result of the synergistic effects of σ and π 1-4 diverse range of chemical properties. It is well understood bonding, as illustrated in Figure 1.22,23 σ and π bonding have that ligands that can behave as strong π acceptors have the opposite effects on carbonyl bond order, σ bonding increasing ability to stabilize low metal oxidation states, but determining the bond order and π bonding decreasing it. Thus, changes in quantitatively how metal oxidation state and ligand back- bonding can be detected by monitoring the carbonyl bond order, 5,6 bonding interplay has proved experimentally difficult. usually through the use of IR spectroscopy.24,25 - Experimental probes of metal ligand bonding in transition There are now so many crystallographically unique transition metals are usually based on either the metal center or some metal carbonyl observations that these pieces of information property of the ligand; the most unambiguous strategies experimentally probe both. Database analysis techniques can (9) Hocking, R. K.; Hambley, T. W. Chem. Commun. 2003, 13, 1516- provide significant insight into inorganic electronic structure 1517. and of a very different nature from that obtained using other (10) Davis, T. V.; Zaveer, M. S.; Zimmer, M. J. Chem. Educ. 2002, 79, 1278-1280. techniques, because they enable us to study global trends and (11) Hunter, A. D. Organometallics 1992, 11, 2251-2262. the relationship that different geometric fragments have to each (12) Spiro, T. G.; Zgierski, M. Z.; Kozlowski, P. M. Coord. Chem. ReV. other, rather than focusing on either the metal or the ligand, in 2001, 219-221, 923-936. a single molecule.7-11 (13) Vogel, K. M.; Kozlowski, P. M.; Zgierski, M. Z.; Spiro, T. G. J. Am. Chem. Soc. 1999, 121, 9915-9921. (14) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. * Author to whom correspondence should be addressed. Tel: 61-2-9351- AdVanced Inorganic Chemistry, 6th ed.; Wiley-Interscience: New York, 2830. Fax: 61-2-9351-3329. E-mail: [email protected]. 1999; p 637. § Current address: CSIRO Land and Water, Glen Osmond, South (15) Lupinetti, A. J.; Strauss, S. H. Prog. Inorg. Chem. 2001, 49,1-112. Australia. E-mail: [email protected]. (16) Willner, H.; Bodenbinder, M.; Broechler, R.; Hwang, G.; Rettig, (1) Serres, R. G.; Grapperhaus, C. A.; Bothe, E.; Eckhard, B.; Weyher- S. J.; Trotter, J.; von Ahsen, B.; Westphal, U.; Jonas, V.; Thiel, W.; Aubke, mueller, T.; Neese, F.; Wieghardt, K. J. Am. Chem. Soc. 2004, 126, 5138- F. J. Am. Chem. Soc. 2001, 123, 588-602. 5153. (17) Aubke, F.; Wang, C. Coord. Chem. ReV. 1994, 137, 483-524. (2) Szilagyi, R. K.; Lim, B. S.; Glaser, T.; Holm, R. H.; Hedman, B.; (18) Szilagyi, R. K.; Frenking, G. Organometallics 1997, 16, 4807- Hodgson, K. O.; Solomon, E. I. J. Am. Chem. Soc. 2003, 125, 9158-9169. 4815. (3) Lever, A. B. P.; Gorelsky, S. I. Struct. Bonding 2004, 107,77- (19) Frenking, G. J. Organomet. Chem. 2001, 635,9-23. 114. (20) Lupinetti, A. J.; Frenking, G.; Strauss, S. H. Angew. Chem., Int. (4) Enemark, J. H.; Feltham, R. D. Coord. Chem. ReV. 1974, 13, 339- Ed. 1998, 37, 2113-2116. 406. (21) Spiro, T. G.; Wasbotten, I. H. J. Inorg. Biochem. 2005, 99,34-44. (5) Aliaga-Alcalde, N.; Debeer George, S.; Mienert, B.; Bill, E.; (22) Dewar, M. J. S. Bull. Soc. Chim. Fr. 1951, 18, C71-C79. Wieghardt, K.; Neese, F. Angew. Chem., Int. Ed. 2005, 44, 2908-2912. (23) Chatt, J.; Duncanson, L. A. J. Chem. Soc. 1953, 2939-2947. (6) Walker, F. A. Inorg. Chem. 2003, 42, 4526-4544. (24) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry, (7) Hocking, R. K. Ph.D. Thesis, The University of Sydney: Sydney, 5th ed.; John Wiley and Sons: New York, 1988, p 1034. 2003. (25) Gruhn, N. E.; Lichtenberger, D. L. Inorganic Electronic Structure (8) Hocking, R. K.; Hambley, T. W. Dalton Trans. 2005, 5, 969-978. and Spectroscopy; Wiley-Interscience: 1999; Vol. 2, pp 533-574. 10.1021/om061072n CCC: $37.00 © 2007 American Chemical Society Publication on Web 04/17/2007 2816 Organometallics, Vol. 26, No. 11, 2007 Hocking and Hambley Calculations were performed with the 6-311G* basis set on the Fe atom and 6-31G* on the carbonyl group, as this level of theory/ basis set combination has been shown by others to give a well- converged solution.39-43 The Fe-C bond lengths were set at different values, and the remainder of the molecule was geometry optimized. Thorough studies using similar types of calculations have been reported extensively by Frenking and co-workers.18,20,44-46 B. Figure 1. Schematic illustration of the effects of σ donation and 2- π back-donation in transition metal carbonyl compounds. Starting structures for the compounds [M(CO)4] , [M(CO)5], and 2+ 47,48 [M(CO)6] , where M ) Fe/Ru/Os, were geometry optimized can be combined in a meaningful way to provide further using the basis set SDD and the hybrid functional B3LYP.49-51 information about bonding. In this work we examine the nature The SDD basis set is standard in the Gaussian98 package and has of the correlation between CtO and TM-C bond length and been used by other authors.52,53 It treats Ru and Os as 18 valence its periodicity. The relationships will be discussed in terms of electron systems with a pseudopotential and employs a [6s,5p,4d/ inorganic electronic structure, the energetics of bonding, and 5d] contracted Gaussian for the valence electrons.54 The remaining the changes in the distribution of electrons throughout the TM- atoms in the SDD treatment are assigned a Dunning/Huzinaga55 CtO fragment as its electron population changes. valence double-ú basis set (O[6s,4p] and C[4s,2p]). A double-ú basis set was thought to suffice for this work, as the interest was Experimental Methods in geometry. All geometries were optimized with the default criteria in Gaussian98. Search Criteria and Data Retrieval. All reported unique terminal TM-carbonyl fragments from crystal structures with Calculations Performed Using ADF. Gradient-corrected cal- R-factors < 7.5% were catalogued according to the metal, and the culations were performed using the exchange functional of Becke50 TM-C and CtO bond lengths were placed in arrays using CSD and the correlation functional of Perdew50,56 (BP86). The frozen software.26,27 Where sufficient data existed, subsets with different core approximation57 was used for the 1s-4d orbitals of Os, the coordination numbers, oxidation states, and trans-ligands were 1s-3d orbitals of Ru, the 1s-2p orbitals of Fe, and the 1s orbitals analyzed. of O and C. Scalar relativistic corrections that used the zero-order To identify any trends, three different types of analysis were regular approximation (ZORA)58-60 were applied to all ADF done. First, principal component analysis (PCA) was performed calculations. For valence orbitals, Slater-type orbital (STO) basis 28,29 using the student version of Matlab. The resulting line and sets of triple-ú quality were employed with polarization functions gradient were then transposed to the average of the data set using on the ligand atoms (2p for H, 3d for all others) and additional Microsoft Excel.30 As the second means of analysis, the weighted means for the data set were taken; xj(TM-C|CtO) and xj(Ct | - O TM C). In the first instance the data sets were divided into (37) Braga, D.; Grepioni, F.; Orpen, A. G. Organometallics 1993, 12, subsets with different TM-C bond lengths using increments of 1481-1483. ∼0.01 Å.
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