Quadruple Bonding of Alkaline Earth Atoms in AeCLi₄ (Ae = Be - Ba) Complexes

The results of quantum chemical calculations of the complexes AeCLi₄ (Ae = Be - Ba) are reported at the BP86-D3(BJ)/def2-QZVPP and CCSD(T)/def2-QZVPP level. The calculated equilibrium geometries with Ae = Be, Mg have a trigonal bipyramidal geometry (C_3v symmetry) as the energetically lowest-lying form. A slightly higher-lying isomer has a square pyramidal geometry (C_4v symmetry), which is only < 1 kcal/mol less stable than the C_3v form. In contrast, only the square pyramidal structure is an energy minimum of the heavier homologues with Ae = Ca, Sr, Ba. The calculated bond dissociation energies of the Ae-CLi₄ bond are very high. The strongest bond is computed for the Be-CLi₄ bond (D_e = 82.9 kcal/mol at CCSD(T)/def2-QZVPP). The weakest bond is calculated for the Mg-CLi₄ bond (D_e = 40.6 kcal/mol). The heavier homologues have values between D_e = 63.1 kcal/mol (Sr-CLi₄) and D_e = 72.0 kcal/mol (Ba-CLi₄). Inspection of the occupied valence orbitals and the AdNDP results suggests that there are four Ae-CLi₄ bonds in the complexes. This is supported by the EDA-NOCV analysis, which reveals that there is a dominant Ae → CLi₄ σ-donation, which is enhanced by weaker Ae ← CLi₄ σ-backdonation and degenerate Ae ← CLi₄ π-backdonation. The best signature of the chemical bonds is Ae  CLi₄. The lighter atoms, Be, Mg, use their (n)s and (n)p AOs for the covalent bonds, whereas the heavier atoms, Ca, Sr. Ba, employ their (n)s and (n-1)d AOs for the covalent interactions. The NBO method does not provide a reasonable account of the covalent bonds, because it does not consider the (n)p and (n-1)d AOs of Ae atoms as genuine valence orbitals.