Resonance of an unshared electron pair between two atoms connected by a single bond.

The reported structure of the dimer of a compound of bicovalent tin indicates that the tin-tin bond is of a new type. It can be described as involving resonance between two structures in which there is transfer of an electron pair from one tin atom to the other. The tin atoms are connected by a single covalent bond (each also forms two covalent bonds with carbon atoms), and an unshared electron pair resonates between the fourth sp(3) orbitals of the two atoms. Similar structures probably occur in digermene and distannene.