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 ((3)sigma(-)) ground state. The quintet ((5)sigma(-)) state is 5.8-12.3 kcal/mol higher in energy and the singlet ((1)Delta) state is 13.1-15.3 kcal/mol above the triplet. The isoelectronic AeC molecules are also predicted to have a low-lying triplet ((3)sigma(-)) state but the quintet ((5)sigma(-)) state is only 2.2 kcal/mol (SrC) and 2.9 kcal/mol (CaC) above the triplet state. The triplet ((3)sigma(-)) and quintet ((5)sigma(-)) states of BaC are nearly isoenergetic. All systems have rather strong bonds. The calculated bond dissociation energies of the triplet ((3)sigma(-)) state are between D-e=38.3-41.7 kcal/mol for AeB(-) and D-e=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-1, ns(2)) and B- or C (P-3, 2 s(2)2p(pi)(1)2p(pi ')(1)). The eventually formed bonds in AeC are better described in terms of interactions between the ions Ae(+) (S-2, ns(1))+C- (S-4, 2 s(2)2p(pi)(1)2p(pi ')(1)2p(sigma)(1)). 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 sigma-bonding MO in the molecules, which feature valence orbitals with the order phi(1) (sigma-bonding)

Automated summary

This study reports the quantum chemical calculations of the diatomic molecules AeB(-) and AeC (Ae=Ca, Sr, Ba). The results show that AeB(-) has an electronic triplet ((3)sigma(-)) ground state, while AeC has a lower-lying triplet state. The bonding in AeC is stronger compared to AeB(-).