Alkaline Earth Organometallics ☆

For over 100 years, the well-known and synthetically important Grignard reagents (RMgX) have dominated research in organoalkaline earth chemistry. Nevertheless, both the smallest (Be) and largest (Ca–Ba) members of the Group 2 metals have distinctive organometallic compounds with structures and reactivity that have no counterparts in the organomagnesium halides. The Group 2 metals are divalent, and their isolated compounds almost always display the + 2 oxidation state. They also feature strongly polar bonding in their compounds. The change in size from Be 2+ to Ba 2+ is nearly sixfold, however, and a correspondingly large range of coordination numbers (from 2 to 9) and structural types (from monomers to coordination polymers) are found in organoalkaline earth complexes. In recent years, sterically bulky ligands have been used increasingly to form complexes that have lower coordination numbers, greater solubility, and altered reactivity compared with compounds containing less bulky ligands. Computational investigations have also played a progressively more important role in describing the metal–ligand interactions in organoalkaline earth chemistry, and have been used to elucidate bonding arrangements (including fluxional processes) and mechanisms of catalysis. Although a large part of non-magnesium Group 2 chemistry has focused on cyclopentadienyl derivatives, non-cyclopentadienyl species (e.g., alkyls, acetylides, allyls, dienes, and other π-bound species) are now well represented with the heavier alkaline earth elements. Organoalkaline earth complexes have expanded from their traditional role in organic synthesis (the Grignard reagents and dialkylmagnesiums) to include applications in materials science such as the chemical vapor deposition of metals and metal oxides and the polymerization of polar monomers.