Tetrel bonds, penta- and hexa-coordinated tin and lead centres

A tetrel bond - an interaction between a Group 14 element acting as Lewis acid centre and an electron-donating moiety - is analysed for ZF(4) (Z = Sn, Pb) complexes with NH3 and HCN species. MP2/aug-cc-pVTZ calculations were performed and supported by results of the quantum theory of atoms in molecules. The results of calculations show that the tetrel centre may be considered as a pentavalent one if ZF(4) interacts with one NH3 or HCN ligand or even as a hexavalent centre if it interacts with two ligands; thus the hypervalency phenomenon is discussed for the complexes analysed here. The theoretical analysis is supported by a discussion of the crystal structures containing the SnF4 fragment; these structures are characterized by a hexa-coordinated tin centre.