Quadruple Bonding of Alkaline Earth Atoms in AeCLi ₄ (Ae = Be − Ba) Complexes

The results of quantum chemical calculations of the complexes AeCLi 4 (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 4 v symmetry), which is only < 1 kcal/mol less stable than the C 3 v 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‐CLi 4 bond are very high. The strongest bond is computed for the Be‐CLi 4 bond ( D e = 82.9 kcal/mol at CCSD(T)/def2‐QZVPP). The weakest bond is calculated for the Mg‐CLi 4 bond ( D e = 40.6 kcal/mol). The heavier homologues have values between D e = 63.1 kcal/mol (Sr‐CLi 4 ) and D e = 72.0 kcal/mol (Ba‐CLi 4 ). Inspection of the occupied valence orbitals and the AdNDP results suggests that there are four Ae‐CLi 4 bonds in the complexes. This is supported by the EDA‐NOCV analysis, which reveals that there is a dominant Ae → CLi 4 σ‐donation, which is enhanced by weaker Ae ← CLi 4 σ‐backdonation and degenerate Ae ← CLi 4 π‐backdonation. The best signature of the chemical bonds is Ae CLi 4 . 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.