From Brookhart, Green, and Parkin Introduction to Agostic Interactions From Brookhart, Green, and Parkin “Agostic Interactions in Transition Metal Compounds” Proc. Natl. Acad. Sci. 2007, 104 (7), 6908-6914 Created by Thomas Brown, SUNY Oswego (thomas.brown@oswego.edu), Emma Downs, Fitchburg State University (edowns1@fitchburgstate.edu), Tanya Gupta, South Dakota State University (tanya.gupta@sdstate.edu), Rob Scarrow, Haverford College (rscarrow@haverford.edu), and Shirley Lin, US Naval Academy (lin@usna.edu) and posted on VIPEr (www.ionicviper.org) on June 3, 2017. Copyright Thomas Brown, Emma Downs, Tanya Gupta, Rob Scarrow and Shirley Lin 2017. This work is licensed under the Creative Commons Attribution-NonCommerical-ShareAlike 3.0 Unported License. To view a copy of this license visit http://creativecommons.org/about/license/.

Learning Slide Students will be able to... Define an agostic interaction and relate it to other types of bonding. Provide examples of how the presence of an agostic interaction can be determined experimentally and through computational methods

Definition of Agostic Interaction The word agostic is derived from the Greek for “to hold onto one’s self”. The traditional definition of an agostic interaction refers to the intramolecular interaction between a C-H bond and a transition metal center, forming a 3- center, 2-electron bond. The definition was eventually expanded to include all metals and the term agostic interaction has even been applied to silicon. Whether the interaction of a heteroatom-hydrogen bond with a metal or metalloid can be considered an agostic interaction is the source of some controversy.

β-Agostic interaction in (Me2PCH2CH2PMe2)TiEtCl3 Dawoodi, Green, Mtetwa & Prout J Chem Soc Chem Commun, 1982, 802. Cf. 3-electron-2-center bonding in diborane:

Agostic Versus Anagostic [Instructor should download Figure 6 as Powerpoint slide from PNAS 2007 paper (free access)] This figure presents the structural and spectroscopic differences between agostic and anagostic interactions.

Methods for characterizing agostic interactions Dependent on observing interactions between the Lewis acidic metal center and a nearby C-H bond. X-ray/neutron diffraction Short M-H bond length (1.8 -2.3 Ǻ) Less than the sum of the van der Waals radii Small M-H-C bond angles (90-140°) NMR Low 1JCH value Upfield shift of hydrogen atom compared to uncoordinated group IR

Other methods for characterizing agostic interactions Dependent on observing interactions between the Lewis acidic metal center and a nearby C-H bond. IR Lower C-H stretching frequency Kinetic isotope effects (compare C-H vs C-D) Computational / Modeling Atoms in Molecules (AIM) Natural Bond Orbitals (NBO) Electron localization function (ELF)

Importance of Agostic Interactions [Instructor should download Scheme 6 as Powerpoint slide from PNAS 2007 paper (free access)]. This scheme shows a β-agostic interaction leading to β- hydride elimination. Many other examples exist of how agostic interactions are important but undergraduate students will likely be the most familiar with β-hydride elimination.