Transition-Metal Complexes with (C-C)→M Algostic Interactions

The coordination of sigma bonding electron density to a Lewis acid was for some time largely the domain of boron hydride chemistry. Subsequently it was found that C-H bonds could undergo intramolecular coordination to metal centers, forming what are now known as agostic bondsand agostic complexes. Thereafter, even the H-H bond in molecular hydrogen was found to be capable of coordination, leading to what are commonly referred to as sigma complexes. Eventually, coordination by much more shielded C-C bonds was established, and is of particular significance with regard to to the interest in C-C bond activation. This review focuses on the key developments in this field, including the nature of the bonding interactions in these and some related species, as revealed by spectroscopic, structural, and theoretical studies, their involvement in important chemical reactions such as alkene polymerizations and metathesis, and comparisons with the other types of electron-deficient bridge bonding.