Transition metal boryl complexes

The article represents a review of the chemistry of transition metal complexes containing the boryl (-BX2) ligand based on the chemical literature published up to and including the calendar year 2006. A brief compendium of papers published in the year 2007 is also included. Notwithstanding the ever-growing volume of literature detailing useful organic transformations in which these complexes have been implicated (e.g. C-H activation, hydroboration, diboration), the primary focus of this article is a review of issues of structure and bonding in boryl systems. Thus, the evidence for boryl ligands acting - to a greater or lesser extent - as sigma donors and/or pi acceptors is examined at length, as are issues relating to secondary interactions (e.g with hydride co-ligands) within the metal coordination sphere, and the consequent relationship to borane sigma complexes.