Large basis and weIl_correiated ab initio electronic structure cdlcuiations have been performed on the simple h&(I) salts, h&F2 and Mg2CI,. The electron withdrswing power of the halogens gives rise to significant metal-metal bonding and consequently these as yet unobservccl. univalent salts are calculated to be IO-12 kcal more stable than if the second mapnesium atom were absent from the molecule. With this stability, these species provide an energetically more accessible hIg atom for subsequent reaction than does solid magnesium where the atomization energy is 35 kcal. Thus, formal univalency of magnesium affords a route to activated magnesium.
Metal-metal
bonding among group IIa and IIb elements is uncommon enough that only mercury is generally classified as being able to exist in a univalent state as in the salt Hg&. Analogous zinc and cadmium compounds are unstable [ 11, and among the IIa elements there has been no isolation or characterization of any such sak In the simplest model of the electrank structures of group II ehments, this is not surprising. Each is characterized by a.n electron occupancy completed with two electrons filling an s orbital. These two electrons participate in chemical bonds, but for a pair of these atoms, such as Be2, Mg2 or Hgz, a destabilizing antibonding orbital is occupied along with a bonding orbital. The interatomic attraction is weak at best.
For there to be dimetal IIa molecules, the antibonding character of the metal-metal interaction must be mitigated. This can be accomplished directly by ionization that removes one or both antibonding electrons. In fact, a formally univalent magnesium moiety, Mg;' _ , accounts for the measured magnesium activity versus concentration in mixtures of elemental magnesium in the molten divalent salt [Z-4] _ Theoretical calculations
[S] have shown that there is a significantly attractive potential energy curve for the singly ionized species, Mgt, as well.
Ionic metal-halogen bonding is naturally sugges-