Unusual quadruple bonds featuring collective interaction-type σ bonds between first octal-row atoms in the...

Quantum chemical calculations are reported for the complexes of alkaline earth metals AeOLi₂ (Ae = Be-Ba) at the BP86-D3(BJ)/def2-QZVPP and CCSD(T)/def2-QZVPPQZVPP levels. The nature of the Ae-OLi₂ bond has been analyzed with a variety of methods. The AeOLi₂ molecules exhibit an unprecedented σ donor bond Ae→OLi₂ where the (n)s² lone-pair electrons of the Ae atom are donated to vacant O-Li₂ antibonding orbitals having the largest coefficient at lithium. This is a covalent bond where the accumulation of the associated electronic charge is located at two positions above and below the Ae-OLi₂ axis. The bifurcated component of orbital interactions is structurally related to the recently proposed collective bonding model, but exhibits a completely different type of bonding. The most stable isomer of AeOLi₂ has a C _2v geometry and a singlet (¹A₁) electronic ground state. The bond dissociation energy (BDE) of the Ae-OLi₂ bonds exhibits a zig-zag trend from BeOLi₂ to BaOLi₂, with BeOLi₂ having the largest BDE (D _e = 73.0 kcal mol^-1) and MgOLi₂ possessing the lowest BDE (D _e = 42.3 kcal mol^-1) at the CCSD(T) level. The calculation of the atomic partial charges by the Hirshfeld and Voronoi methods suggests that Be and Mg carry small negative charges in the lighter molecules whereas the heavier atoms Ca-Ba have small positive charges. In contrast, the NBO and QTAIM methods give positive charges for all Ae atoms that are larger for Ca-Ba than that calculated by the Hirshfeld and Voronoi approaches. The molecules AeOLi₂ have large dipole moments where the negative end is at the Ae atom with the polarity Ae→OLi₂. The largest dipole moments are predicted for the lighter species BeOLi₂ and MgOLi₂ and the smallest value is calculated for BaOLi₂. The calculation of the vibrational spectra shows a significant red-shift toward lower wave numbers for the Ae-OLi₂ stretching mode in comparison to diatomic AeO. Besides the Ae→OLi₂ σ-donor bonds there are also three dative bonds due to Ae←OLi₂ backdonation which consist of one σ bond and two π bonds. The appearance of strong Ae→OLi₂ σ donation leads to quadruple bonds AeOLi₂ in all systems AeOLi₂, even for the lightest species with Ae = Be, Mg. The valence orbitals of Ca, Sr, and Ba, which are involved in the dative interactions, are the (n)s and (n-1)d AOs whereas Be and Mg use their (n)s and (n)p AOs. The EDA-NOCV results are supported by the AdNDP calculations which give four 2c-2e bonding orbitals. Three bonding orbitals have occupation numbers ∼2. One σ orbital has smaller occupation numbers between 1.32 and 1.73 due to the delocalization to the lithium atoms. The analysis of the electronic structure with the ELF method suggests multicenter bonds with mainly trisynaptic and tetrasynaptic basins, which also support the results of the EDA-NOCV calculations.