Analogies between the chemical bonding topologies in metal–olefin complexes and in metallaboranes: the role of three-center two-electron bonding

Three-center two-electron (3c-2e) interactions are found both in the C2M σ-bonding in the Dewar–Chatt model of the olefin–metal bond and in the B2H and B3 bonding in borane structures based on deltahedra or deltahedral fragments. Incorporation of metals as vertices of boron deltahedra frequently changes their shapes to provide degree 6 and even degree 7 vertices for the metal atoms. In addition, metal incorporation frequently modifies the core+surface skeletal bonding of the original BnHn2− to pure surface bonding consisting mainly or entirely of 3c-2e bonds. In such structures the resulting 3c-2e B2M bonds are related to the C2M σ-bonding in the Dewar–Chatt model. The skeletal bonding of deltahedral metallaboranes containing two or more metal vertices can also include 2c-2e metal–metal bonding through the center of the deltahedron.