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14-September-2008 18:38:38 - Agostic interaction Agostic interaction is a term, in organometallic chemistry, for the interaction of a coordinately-unsaturated transition metal with a C-H bond, when the two electrons involved in the C-H bond enter the empty d orbital of a transition metal, resulting in a two electron three center bond. 1. Many catalytic transformations, e.g. oxidative addition and reductive elimination, are proposed to proceed via intermediates featuring agostic interactions. Agostic interactions are observed throughout organometallic chemistry in alkyl, alkylidene, and polyenyl ligands. Contents 1 History 2 Characteristics of agostic bonds 2.1 Strength of bond 3 Related bonding interactions 3.1 Anagostic bonds 4 Function 5 References 6 External links History The term agostic, derived from the Greek word for to hold close to oneself, was coined by Maurice Brookhart and Malcolm Green to describe this and many other interactions between a transition metal and a C-H bond.Often such agostic interactions involve alkyl or aryl groups that are held close to the metal center through an additional σ-bond.23. Short interactions between hydrocarbon substituents and coordinatively unsaturated metal complexes have been noted since the 1960s. For example, in tristriphenylphosphine ruthenium dichloride, a short interaction is observed between the RuII center and a hydrogen atom on the ortho position of one of the nine phenyl rings.4 Numerous borohydride complexes were also described crystallographically that were described using the two-electron three-center bonding model. MoPCy32CO3, featuring an agostic interaction. MoPCy32CO3, featuring an agostic interaction. The nature of the interaction was foreshadowed in main group chemistry in the structural chemistry of trimethylaluminium. Characteristics of agostic bonds Agostic interactions are best demonstrated by crystallography. Neutron diffraction data has shown that C-H and M-H bond distances are 5-20% longer than expected for isolated metal hydride and hydrocarbons. The distance between the metal and the hydrogen is typically 1.8 - 2.3 Ǻ, and the M-H-C angle falls in the range 90 - 140°. The presence of a 1H NMR signal that is shifted upfield from that of a normal aryl or alkane, often to the region normally assigned to hydride ligands. The coupling constant 1JCH is typically lowered to 70-100 Hz versus the 125 Hz expected for a normal sp3 carbon-hydrogen bond. Strength of bond On the basis of experimental and computational studies, the stabilization arising from an agostic interaction is estimated to be 10-15 kcal/mol. Thus, agostic interactions are stronger than most hydrogen bonds. Agostic bonds sometimes play a role in catalysis by increasing 'rigidity' in transition states. For instance, in Ziegler-Natta catalysis the highly electrophilic metal center has agostic interactions with the growing polymer chain. This increased rigidity influences the stereoselectivity of the polymerization process. Related bonding interactions The term agostic is reserved to describe two-electron, three-center bonding interactions between carbon, hydrogen, and a metal. Two-electron three-center bonding is clearly implicated in the complexation of H2, e.g., in WCO3PCy32H2, which is closely related to the agostic complex shown in the figure.5 Silane binds to metal centers often via agostic-like, three-centered Si-H-M interactions. Because these interactions do not include carbon, however, they are not classified as agostic. Anagostic bonds Certain M---H-C interactions are not classified as agostic but are described by the term anagostic. Anagostic interactions are more electrostatic in character. In terms of structures of anagostic interactions, the M---H distances and M-H-C angles fall into the ranges 2.3-2.9 Ǻ and 110-170°, respectively.26 Function Agostic interactions serve a key function in olefin polymerization and stereochemistry, as well as migratory insertion. References ^ Agostic Interaction. IUPAC Compendium of Chemical Terminology. 21 Jan 2008. http://www.iupac.org/goldbook/AT06984.pdf ^ a b Maurice Brookhart, Malcolm L. H. Green, Carbon-hydrogen-transition metal bonds. Journal of Organometallic Chemistry, 2501, 395-408 1983. doi:10.1016/0022-328X8385065-7. ^ Maurice Brookhart, Malcolm L. H. Green, and Gerard Parkin Agostic interactions in transition metal compounds Proceeding of the U.S. National Academy of Sciences 2007, volume 104, 6908-6914.doi:10.1073/pnas.0610747104 ^ Sam J. La Placa and James A. Ibers A Five-Coordinated d6 Complex: Structure of Dichlorotristriphenylphosphineruthenium II Inorganic Chemistry, 1965, volume 4, pp 778 - 783. doi:10.1021/ic50028a002 ^ Kubas, G. J., Metal Dihydrogen and σ-Bond Complexes, Kluwer Academic/Plenum Publishers: New York, 2001 ^ Braga D, Grepioni F, Tedesco E, Biradha K., Desiraju G. R. 1997. Organometallics 16, 1846-1856 External links International Union of Pure and Applied Chemistry. agostic. Compendium of Chemical Terminology Internet ion. http://www.ilpi.com/organomet/agostic.html v d e Chemical bonds Strong Covalent bonds Antibonding Sigma bonds: 3c-2e bent bond · 3c-4e Hydrogen bond, Dihydrogen bond, Agostic interaction · 4c-2e Pi bonds: π backbonding · Conjugation · Hyperconjugation · Aromaticity · Metal aromaticity Delta bond: Quadruple bond · Quintuple bond · Sextuple bond Coordinate covalent bond · Hapticity Ionic bonds Cation-pi interaction · Salt bridge Metallic bonds Metal aromaticity Weak Hydrogen bond Dihydrogen bond · Dihydrogen complex · Low-barrier hydrogen bond · Symmetric hydrogen bond · Hydrophile Other noncovalent van der Waals force · Mechanical bond · Halogen bond · Aurophilicity · Intercalation · Stacking · Entropic force · Chemical polarity other Disulfide bond · Peptide bond · Phosphodiester bond Note: the weakest strong bonds are not necessarily stronger than the strongest weak bonds Retrieved from http://en..org/wiki/Agostic_interaction Categories: Chemical bonding | Organometallic chemistry Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page Languages Deutsch Français 中文 This page was last modified on 2 September 2008, at 13:27
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