Agostic and Hydrogen-Bonding X–H…M Interactions Involving a d8 Metal Center: Recent Advances Towards Their Understanding

The binding of d8 transition metal ions to X–H bonds (X = non-metal) has been subject of intense research in the last two decades. Two different types of orbital interactions can stabilize X–H…M bonds: (1) charge transfer from a filled orbital of the metal into the empty σ*-antibonding orbital of the X–H bond; (2) charge transfer from the filled σ-bonding orbital of the X–H bond into an empty orbital of the metal. The first type corresponds to a hydrogen bond, whereas the second is commonly designated as an agostic bond. The present article analyses experimental and theoretical approaches to the characterization of these two interaction types in d8 transition metal complexes, points out some assignment errors that occurred in the past, and summarizes recent advances towards the understanding of the structure, dynamics, and physical origin of these weak interactions.