Computational Studies of Metal-Ligand Bond Enthalpies across the Transition Metal Series Jamal Uddin, Christine M. Morales, James H. Maynard, and Clark R. Landis* Department of Chemistry, UniVersity of Wisconsin-Madison, 1101 UniVersity AVenue, Madison, Wisconsin 53706 ReceiVed April 5, 2006 Relative to the p-block of the periodic table, data for transition metal-ligand bond dissociation enthalpies are less comprehensive. Recent developments in computational methods make systematic assessment of trends in metal-ligand bond enthalpies across the transition series a relatively rapid and accurate exercise. We report a systematic study of metal-ligand bond enthalpies for saturated transition metal complexes that encompasses the entire d-block of the periodic table and a wide assortment of ligands. The saturated complexes have the form MH n -L such that closed-shell molecules are formed with the maximum number of two-center, two-electron (2c/2e) bonds under the constraint that the metal electron count does not exceed 12. Bond enthalpies for MH n -L molecules with higher electron counts (14 and 16 electrons) are assessed for some group 10 and 11 metals. The primary methods are density functional theory (DFT) using the hybrid B3LYP density functional and CCSD(T) ab initio computations. Bond enthalpies are reported as the first bond dissociation enthalpies for neutral and cationic complexes of the type MH n -R (R ) H, CH 3 ,C 2 H 5 , CH(CH 3 ) 2 , C(CH 3 ) 3 , CH 2 F, C 2 H, C 2 H 3 , NH 2 , OH, F, and BH 2 ) for all transition elements. Electronic structure analysis of the complexes features natural bond orbital (NBO) analysis of bond polarity. 1. Introduction Bond enthalpies are valuable quantities to chemists. In part, the value of bond enthalpies derives from the conciseness with which they express the results of thermochemical experiments. For example, understanding of product distributions in free radical additions of HBr to alkenes relies on knowledge of bond enthalpies. Thermodynamics favor the formation of the more stable alkyl radical, leading to the more substituted (Markovni- kov) product. 1 More generally, reaction enthalpies can quickly be estimated as the sum of bond enthalpy contributions from bonds that are formed and broken. 2 Such bond additivity estimates are broadly applicable, while more sophisticated group additivity estimates are also available for some classes of organic reactions. 3 This knowledge base aids chemists in developing new reaction processes and determining reaction mechanisms. Main-group bond enthalpies follow well-defined trends. Underlying these trends is the transferability of 2c/2e bond units. For example, the C-H bonds of methane and cyclohexane have many similar features. More detailed examination of the deviations from perfect transferability has led to some of the most fundamental and enduring concepts of modern chemistry. The concept of electronegativity arose from analysis of trends in homo- and heteronuclear bond enthalpies for simple diatom- ics. 4 Similarly the concepts of hybridization and resonance provide bases for understanding other trends in bond enthalpies along with other physical properties. Although “typical” bond enthalpies are well known for main- group atom pairs, the more delocalized bonding typical of organometallic compounds raises new questions. How do metal-ligand bond enthalpies depend on the metal and the “auxiliary” ligands? Do the familiar concepts of bond ionicity, hybridization, and resonance stabilization translate into useful predictors for the thermodynamics of metal-ligand bonding? Homolytic transition metal-ligand bond enthalpies span a wide range, perhaps best illustrated in a critical review by Martinho Simoes and Beauchamp. 5,6 Much of this variation can be attributed to the coordination environment around the metal. Detailed understanding of the factors governing these quantities may allow chemists to “tune” the thermodynamics and kinetics of organometallic reactions. Organometallic catalysts are widely used for industrially useful organic transformations such as hydrocarbon function- alization and carbon-carbon bond formation. 7 Modification of substrate-catalyst interactions can lead to improvement in the selectivity, catalytic turnover, and rates of these synthetic “toolkit” reactions. For example, Shilov’s 8 groundbreaking work in the activation of hydrocarbons was motivated by the assumption that metal-carbon and metal-hydrogen bond enthalpies are reasonably similar. More recently, Marks and co- workers’ 9 application of relative M-N and M-C bond enthal- pies to the development of catalytic hydroamination reactions using organolanthanide complexes represents a landmark in (1) (a) Markownikoff, V. V. Ann. Chem. 1870, 153, 256. (b) McMurray, J. E. Organic Chemistry, 6 ed.; Brooks Cole: Monterey, CA, 2003. (2) Pauling, L. C.; Yost, D. M. Proc. Natl. Acad. Sci. U.S.A. 1932, 18, 414-416. (3) (a) Cohen, N.; Benson, S. W. Chem. ReV. 1993, 93, 2419-2438. (b) Bader, R. F. W.; Bayles, D. J. Phys. Chem. A 2000, 104, 5579-5589. (4) Pauling, L. Nature of the Chemical Bond, 3rd ed.; Cornell University Press: Ithaca, NY, 1960; Chapter 3. (5) Connor, J. A. In Inorganic Chemistry Metal Carbonyl Chemistry; Boschke, F. L., Dewar, M. J. S., Hafner, K., Heilbronner, E., Ito, S., Lehn, J.-M., Niedenzu, K., Scha ¨fer, K., Wittig, G., Eds.; Springer-Verlag: New York, 1977; Vol. 71. (6) Martinho Simoes, J. A.; Beauchamp, J. L. Chem. ReV. 1990, 90, 629- 688. (7) Crabtree, R. H. J. Organomet. Chem. 2004, 689, 4083-4091. (8) Shilov, A. E. In Alkane ActiVation Processes by Cyclopentadienyl Complexes of Rhodium, Iridium, and Related Species; Hill; C. L., Ed.; Wiley: New York, 1989; p 372. (9) Hong, S.; Marks, T. J. Acc. Chem. Res. 2004, 37, 673-686. 5566 Organometallics 2006, 25, 5566-5581 10.1021/om0603058 CCC: $33.50 © 2006 American Chemical Society Publication on Web 10/06/2006