Quest of Quadruple Bonding Between Two Main-Group Atoms in AeB and AeC (Ae=Ca, Sr, Ba) and the Role of d Orbitals of Heavier Alkaline-Earth Atoms in Covalent Interactions.

Quantum chemical calculations using ab initio methods at the MRCI+Q(6,8)/def2-QZVPP and CCSD(T)/def2-QZVPP levels as well as density functional theory are reported for the diatomic molecules AeB and isoelectronic AeC (Ae=Ca, Sr, Ba). The boride anions AeB have an electronic triplet ( Σ ) ground state. The quintet ( Σ ) state is 5.8-12.3 kcal/mol higher in energy and the singlet ( Δ) state is 13.1-15.3 kcal/mol above the triplet. The isoelectronic AeC molecules are also predicted to have a low-lying triplet ( Σ ) state but the quintet ( Σ ) state is only 2.2 kcal/mol (SrC) and 2.9 kcal/mol (CaC) above the triplet state. The triplet ( Σ ) and quintet ( Σ ) states of BaC are nearly isoenergetic. All systems have rather strong bonds. The calculated bond dissociation energies of the triplet ( Σ ) state are between D =38.3-41.7 kcal/mol for AeB and D =49.4-57.5 kcal/mol for AeC. The barium species have always the strongest bonds whereas the calcium and strontium compounds have similar BDEs. The bonding analysis indicates that there is little charge migration in AeB in the direction Ae→B where the alkaline earth atoms carry positive charges between 0.09 e-0.22 e. The positive charges at the Ae atoms are much larger in AeC where the charge migration Ae→C is between 0.90 e-0.91 e. A detailed analysis of the interatomic interactions with the EDA-NOCV method shows that all diatomic species AeB and AeC are built from dative interactions between Ae ( S, ns ) and B or C ( P, 2 s 2p 2p ). The eventually formed bonds in AeC are better described in terms of interactions between the ions Ae ( S, ns )+C ( S, 2 s 2p 2p 2p ). Inspection of the orbital interactions suggests that the alkaline earth atoms Ca, Sr, Ba use mainly their (n-1)d AOs besides the (n)s AOs for the covalent bonds. This creates a second energetically low-lying σ-bonding MO in the molecules, which feature valence orbitals with the order ϕ (σ-bonding)<ϕ (σ-bonding)<ϕ (degenerate π-bonding). All four occupied valence MOs of AeB and AeC are bonding orbitals. Since the degenerate π orbitals ϕ are only singly occupied, the formal bond order is three.