Hypoelectronic dirhenaboranes having eight to twelve vertices: internal versus surface rhenium-rhenium bonding.

Fehlner, Ghosh, and their co-workers have synthesized a series of dirhenaboranes Cp(2)Re(2)B(n-2)H(n-2) (n = 8, 9, 10, 11, 12) exhibiting unprecedented oblate (flattened) deltahedral structures. These structures have degree 6 and/or 7 rhenium vertices at the flattest regions on opposite sides of an axially compressed deltahedron thereby leading to Re═Re distances in the range 2.69 to 2.94 Å suggesting internal formal double bonds. These experimental oblate (flattened) deltahedral structures are shown by density functional theory to be the lowest energy structures for these dirhenaboranes. In some cases the energy differences between such oblate deltahedral structures and the next higher energy structures are quite considerable, that is, up to 25 kcal/mol for the nine-vertex Cp(2)Re(2)B(7)H(7) structures. The higher energy Cp(2)Re(2)B(n-2)H(n-2) structures are of the two types: (1) Most spherical (closo) deltahedra having unusually short 2.28 to 2.39 Å Re-Re edges with unusually high Wiberg bond indices suggesting formal multiple bonds on the deltahedral surface; (2) Deltahedra having one or two degree 3 vertices and 2.6 to 2.9 Å Re-Re edges. The latter deltahedra are derived from smaller deltahedra by capping Re(2)B faces with the degree 3 vertices.