Transition from covalent to noncovalent bonding between tetrel atoms

The strength and nature of the bonding between tetrel (T) atoms in R₂T⋯TR₂ is examined by quantum calculations. T atoms cover the range of Group 14 atoms from C to Pb, and substituents R include Cl, F, and NH₂. Systems vary from electrically neutral to both positive and negative overall charged radicals. There is a steady weakening progression in T-T bond strength as the tetrel atom grows larger, transitioning smoothly from a strong covalent to a much weaker noncovalent bond for the larger T atoms. The latter have some of the characteristics of a ditetrel bond, but there are also significant deviations from a classic bond of this type. The T₂Cl₄^- anions are more strongly bonded than the corresponding cations, which are in turn stronger than the neutrals.