High-level quantum-chemical computations (G4MP2) are carried out in the study of complexes featuring tetrel bonding between the carbon atom in the carbenoid CB11H11 - obtained by hydride removal in the C-H bond of the known closo-monocarbadodecaborate anion CB11H12() and acting as Lewis acid (LA) - and Lewis bases (LB) of different type; the electron donor groups in the tetrel bond feature carbon, nitrogen, oxygen, fluorine, silicon, phosphorus, sulphur and chlorine atomic centers in neutral molecules as well as anions H(-), OH(-) and F(-). The empty radial 2pr vacant orbital on the carbon center in CB11H11 , which corresponds to the LUMO, acts as a Lewis acid or electron attractor, as shown by the molecular electrostatic potential (MEP) and electron localization funcion (ELF). The thermochemistry and topological analysis of the complexes {CB11H11:LB} are comprehensively analyzed, and classified according to sharing or closed-shell interactions. ELF analysis shows that the tetrel C···X bond ranges from very polarised bonds, as in H11B11C:F() to very weak interactions as in H11B11C···FH and H11B11C···O=C=O.

MP2/aug-cc-pVTZ calculations were carried out for the ZFH₃-B complexes (Z = C, Si, Ge, Sn and Pb; B = C₂H₂, C₂H₄, C₆H₆ and C₅H₅^-; relativistic effects were taken into account for Ge, Sn and Pb elements). These calculations are supported by other approaches; the decomposition of the energy of interaction, Quantum Theory of Atoms in Molecules (QTAIM) and Natural Bond Orbital (NBO) method. The results show that tetrel bonds with π-electrons as Lewis bases are classified as Z…C links between single centers (C is an atom of the π-electron system) or as Z…π interactions where F-Z bond is directed to the mid-point (or nearly so) of the CC bond of the Lewis base. The analogues systems with Z…C/π interactions were found in the Cambridge Structural Database (CSD). It was found that the strength of interaction increases with the increase of the atomic number of the tetrel element and that for heavier tetrel elements the ZFH₃ tetrahedral structure is more deformed towards the structure with the planar ZH₃ fragment. The results of calculations show that the tetrel bond is sometimes accompanied by the Z-H…C hydrogen bond or even sometimes the ZFH₃-B complexes are linked only by the hydrogen bond interaction.