Ab initio calculations on the Li-4(2+) and Na4(2+) T(d) structures show that two valence electrons are capable of binding four alkali metal centers. These structures have positive binding energies (E(b/n) = 4.62 (Li-4(2+), QCISD(T)(full)/6-311G(2d)//MP2(full)/6-311G(2d); 4.74 MP4SDQ(full)/6-31+G(3d)//MP2(full)6-311G(2d)) and 1.66 kcal/mol (Na4(2+), QCISD(T)(full)/6-31G(d)//MP2(full)/6-31G(d)). However, they are thermodynamically unstable with respect to the dissociations into different two sets of cations, i.e. into M3+ (D3h) and M+ and into 2M2+. The decomposition barriers into M3+ and M+ are 3.4 kcal/mol for Li-4(2+) (QCISD(T)(full)/6-311G(2d)//MP2(full)/6-311G(2d)+ZPE(MP2/6-31G(2d)) and 2.3 kcal/mol for Na4(2+) (MP4SDTQ(fc)/6-31G(2df)/6-31G(d)//MP2(full)/6-31G(d) + ZPE(HF/6-31G(d))), respectively. Four-center, two-electron (4c-2e) bonding suffices to stabilize an otherwise unsupported molecule. Alternative geometries of the M4(2+) clusters (e.g., linear (D(infinity h)), Y-type (D3h), square (D3h), and rhombus (D2h)) are not minima. In contrast to H-4(2+), the relative stabilities of the Li-4(2+) forms strongly disagree with the expectations based on the charge alternation model. Effective 4c-2e bonding in Li-4(2+) (T(d)) depends on the 2p lithium orbitals. When these orbitals are excluded from the basis set, Li-4(2+) behaves somewhat similarly to H-4(2+); in particular, the T(d) form is not a minimum. Topological analysis (AIM) of the electron charge density shows the existence of a nonnuclear maximum (attractor, in this case pseudoatom) in the center of the Li-4(2+) tetrahedron. There is no direct Li-Li bonding. The electronic structure analysis shows that Li-4(2+) (T(d)) is not very stable; this is consistent with the low dissociation barrier.
