In search of a fundamental understanding of C-H & C-C agostic bonds

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The discovery of the 'agostic' bond, an interaction between an electronically unsaturated transition metal centre and a saturated ligand, has radically altered the way in which organometallic chemists think about structure, mechanism and catalysis. The structural consequences of the agostic 'interaction' are clear: the ligand and the coordination sphere distort so that a C-H bond can approach the metal centre. The underlying origins of the interaction are, however, rather less obvious, and at least four distinct electronic mechanisms have been proposed to explain the distortions. The earliest, and most widely accepted, of these is the donation of two electrons from the C-H bond to the metal, but this simple model fails to rationalise many of the more subtle aspects of agostic bonding. As a result, some authors have proposed that delocalisation of the M-C, rather than C-H, bonding electrons is the key, while others have invoked charge transfer from metal to the antibonding orbital on the C-H group. Clearly, then, the term 'agostic interaction' is a very general one that encompasses a broad range of electronic mechanisms, and it is possible that the relative importance of each of these varies from case to case.During the course of a long-standing collaboration with the experimental group of Michel Etienne in Toulouse, we have characterised a cyclopropyl complex that represents, in our view, the first example of a C-C agostic interaction in an unconstrained system. In qualitative terms, there is no reason why a saturated C-C bond should not participate in the same types of interactions as a C-H bond. The C-H agostic alternative is, however, preferred in the vast majority of cases for steric and electronic reasons, and the only previously reported examples of C-C agostics occur where there is no viable CH agostic alternative. In contrast, potentially agostic C-H bonds are readily accessible in the cyclopropyl ligand, suggesting a genuine electronic preference for a C-C agostic structure in this case.The discovery of this fundamentally new bonding mode immediately raises questions about the precise nature of the C-C agostic interaction. Are its electronic origins the same as the more common C-H agostics, or is the balance between the different stabilising mechanisms fundamentally different? In this proposal, we aim to explore these questions using state-of-the art computational techniques that allow us to place the detailed distribution of electrons under the microscope. Through an examination of the electronic structure, and also, where available, careful comparison with experiment, we aim to unify the different explanations of the term 'agostic', and offer an intimate understanding of the fundamental nature of these interactions.