Metal–metal bonding in deltahedral dimetallaboranes and trimetallaboranes: a density functional theory study

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Abstract The skeletal bonding topology as well as the Re=Re distances and Wiberg bond indices in the experimentally known oblatocloso dirhenaboranes Cp*2Re2B n−2H n−2 (Cp*=η5Me5C5, n=8–12) suggest formal Re=Re double bonds through the center of a flattened Re2B n−2 deltahedron. Removal of a boron vertex from these oblatocloso structures leads to oblatonido structures such as Cp2W2B5H9 and Cp2W2B6H10. Similar removal of two boron vertices from the Cp2Re2B n−2H n−2 (n=8–12) structures generates oblatoarachno structures such as Cp2Re2B4H8 and Cp2Re2B7H11. Higher energy Cp2Re2B n−2H n−2 (Cp=η5-C5H5, n=8–12) structures exhibit closo deltahedral structures similar to the deltahedral borane dianions B n H n 2−. The rhenium atoms in these structures are located at adjacent vertices with ultrashort Re≣Re distances similar to the formal quadruple bond found in Re2Cl8 2− by X-ray crystallography. Such surface Re≣Re quadruple bonds are found in the lowest energy PnRe2B n−2H n−2 structures (Pn=η5,η5-pentalene) in which the pentalene ligand forces the rhenium atoms to occupy adjacent deltahedral vertices. The low-energy structures of the tritungstaboranes Cp3W3(H)B n−3H n−3 (n=5–12), related to the experimentally known Cp*3W3(H)B8H8, have central W3B n−3 deltahedra with imbedded bonded W3 triangles. Similar structures are found for the isoelectronic trirhenaboranes Cp3Re3B n−3H n−3. The metal atoms are located at degree 6 and 7 vertices in regions of relatively low surface curvature whereas the boron atoms are located at degree 3–5 vertices in regions of relatively high surface curvature. The five lowest-energy structures for the 11-vertex tritungstaborane Cp3W3(H)B8H8 all have the same central W3B8 deltahedron and differ only by the location of the “extra” hydrogen atom. The isosceles W3 triangles in these structures have two long ~3.0 Å W–W edges through the inside of the deltahedron with the third shorter W–W edge of ~2.7 to ~2.8 Å corresponding to a surface deltahedral edge.