Metal–metal bonding in deltahedral dimetallaboranes and trimetallaboranes: a density functional theory study
AbstractThe skeletal bonding topology as well as the Re=Re distances and Wiberg bond indices in the experimentally knownoblatoclosodirhenaboranes Cp*2Re2Bn−2Hn−2(Cp*=η5Me5C5,n=8–12) suggest formal Re=Re double bonds through the center of a flattened Re2Bn−2deltahedron. Removal of a boron vertex from theseoblatoclosostructures leads tooblatonidostructures such as Cp2W2B5H9and Cp2W2B6H10. Similar removal of two boron vertices from the Cp2Re2Bn−2Hn−2(n=8–12) structures generatesoblatoarachnostructures such as Cp2Re2B4H8and Cp2Re2B7H11. Higher energy Cp2Re2Bn−2Hn−2(Cp=η5-C5H5,n=8–12) structures exhibitclosodeltahedral structures similar to the deltahedral borane dianions BnHn2−. The rhenium atoms in these structures are located at adjacent vertices with ultrashort Re≣Re distances similar to the formal quadruple bond found in Re2Cl82−by X-ray crystallography. Such surface Re≣Re quadruple bonds are found in the lowest energy PnRe2Bn−2Hn−2structures (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)Bn−3Hn−3(n=5–12), related to the experimentally known Cp*3W3(H)B8H8, have central W3Bn−3deltahedra with imbedded bonded W3triangles. Similar structures are found for the isoelectronic trirhenaboranes Cp3Re3Bn−3Hn−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)B8H8all have the same central W3B8deltahedron and differ only by the location of the “extra” hydrogen atom. The isosceles W3triangles 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.