The synaptic order: a key concept to understand multicenter bonding

The concept of multicenter bond is paradoxically counter intuitive in chemistry because most of chemistry and molecular physics is thought in terms of two body interactions. Moreover there is a technical difficulty to account for multicenter bonding in the framework of standard approaches such as the MO theory. The topological approach of the chemical bonding enables a position space partition of the electron density in terms of basins of attractor whose chemical significance is given by their location with respect to the nuclei. There are core and valence basins. Core basins correspond to the inner atomic shell density whereas valence basin density is organized around and between the core basins. The valence basins are characterized by the number of core basin with which they have a boundary. This number is called the synaptic order. There are therefore monosynaptic basins corresponding to electron lone pairs, disynaptic basins to conventional two-center bonds, trisynaptic basins to 3c-2e bonds, etc. The usefulness of the concept is illustrated by examples belonging to both molecular and solid state chemistry.