Subvalent Organometallic Compounds of the Alkaline Earth Metals in Low Oxidation States

Abstract Alkaline earth metals (Ae) are regarded as redox‐inactive, and their chemistry is dominated by the oxidation state +2. Nevertheless, in recent years several compound classes with alkaline earth metals in low oxidation states were investigated. Various concepts proved to be valid for the stabilization of such compounds and are discussed in this review. In the solid state, subvalency can be achieved by offering a matrix that takes over the excess electrons as, for example, in subnitrides. This fact leads to normal‐valent alkaline earth metals with electrons free to move between alkaline‐earth‐metal‐containing cages with or without a metal matrix. Another concept focuses on the synthesis of [Ae 2 ] 2+ cations with adequate substituents. The homodinuclear Ae–Ae bonds exhibit binding energies that should allow the synthesis of molecules such as R–Ae–Ae–R. The synthesis of magnesium derivatives succeeded by use of extremely bulky bidentate ligands with a delocalized anionic charge. The heavier alkaline earth metal derivatives are investigated by quantum chemical methods. Another possibility takes advantage of the fact that the first and second ionization potentials of the alkaline earth metals are clearly separated. Therefore, an arene with an extended π‐system having an energy level between the two ionization potentials should be able to overtake only one electron, which leads to Ae + cations. Sophisticated procedures allowed the synthesis of a calcium(I) derivative, [(thf) 3 Ca(μ‐η 6 ,η 6 ‐C 6 H 3 ‐1,3,5‐Ph 3 )Ca(thf) 3 ], and such structures are investigated by quantum chemical methods also for the other alkaline earth metals.