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-bo nding in the Dewar-Chatt model of the olefin-metal bond and in the B2H and B-3 bonding in borane structures based on deltahedra or deltahedral fragmen ts. Incorporation of metals as vertices of boron deltahedra frequently chan ges their shapes to provide degree 6 and even degree 7 vertices for the met al atoms. In addition, metal incorporation frequently modifies the core + s urface skeletal bonding of the original BnHn2- to pure surface bonding cons isting mainly or entirely of 3c-2e bonds. In such structures the resulting 3c-2e B2M bonds are related to the C2M sigma -bonding in the Dewar-Chatt mo del. The skeletal bonding of deltahedral metallaboranes containing two or m ore metal vertices can also include 2c-2e metal-metal bonding through the c enter of the deltahedron. (C) 2001 Elsevier Science B.V. All rights reserve d.