The concept of hypermetalation, characterized by molecules with unprecedented stoichiometries, is extendedto the magnesium-oxygen combinations, Mg20, Mg30, and Mg40. Their equilibrium geometries and fundamental frequencies were calculated at the HF/6-31G* and correlatedMP2 (full)/6-31G* levels. Extensive searches of possible structures and electronic states were carried out. The global minima are as follows: linear Mg20 (Z)„*, both singlet 12g+ and the triplet

32u “states), planar Mg30 (Dih,^ f), and Mg40 forms slightly distorted from the square planar arrangement (Z) M,'A] and planar MgOMg3 (C „‘A^ structures have nearly the same energy). These Mg20, Mg30, and Mg40 species are stable with regard to all possible decomposition pathways. Representative dissociation energies are the following: Mg20, 74.5 kcal/mol into MgO+ Mg at QCISD (T)/6-311+ G (3df)+ ZPE; Mg30, 26.7 kcal/mol into Mg20+ Mg at QCISD (T)/6-311+ G*+ ZPE; and Mg40, 7.9 kcal/mol into Mg30+ Mg at MP4SDTQ/6-311+ G*+ ZPE. Magnesium-magnesium bonding contributes significantly to the stability of Mg3Qand Mg40.