Impossible made possible Sub-valence in electropositive s-block-metals

The preparation of subvalent electropositive metal compounds succeeds in general by means of three different concepts: i) Stabilization can be achieved by delocalization of electrons in metallic matrices. A formal subvalence results from the total formula, whereas on closer examination of the bonding situation an expected "normal" valence of the metal atoms according to the octet rule can be concluded. ii) According the rules of determination of the oxidation state a formal subvalence arises from the formation of homonuclear element-element bonds or metal clusters. However, in the case M-2(2+) units a normal valence is realized (which is well-known in the chemistry of mercury as Hg-2(2+), e.g. colomel Hg2Cl2). iii) The stabilization of subvolent metals with the aid of expanded pi*-systems of aren ligands succeeds when the energy lies between the two first ionization energies of the alkaline earth metal.