Polyhedral cobaltadiselenaboranes: nido structures without bridging hydrogens

Abstract

The lowest energy CpCoSe₂B_n−3H_n−3 structures (8 ≤ n ≤ 12) have central CoSe₂B_n−3 nido polyhedra derived from a closo or isocloso deltahedron by removal of a vertex. This leads to an “open” non-triangular face having four to six edges. The two selenium vertices in such structures are located on this open face reflecting the general preference of selenium for low degree vertices. The lowest energy 12-vertex CpCoSe₂B₉H₉ structure has been synthesized and characterized structurally by X-ray crystallography. This structure can be derived from the regular icosahedron by removing two edges attached to the same vertex thereby leading to a bent pentagonal face containing both selenium atoms. For the 11-vertex CpCoSe₂B₈H₈ system, all of the lowest energy structures within 15 kcal mol⁻¹ are derived from the regular icosahedron by removal of a vertex. The two lowest energy 10-vertex CpCoSe₂B₇H₇ structures have a framework similar to that of the known B₁₀H₁₄ with a bent hexagonal hole containing both selenium atoms. The seven lowest energy 9-vertex CpCoSe₂B₆H₆ structures are derived from the bicapped tetragonal antiprism by removing either a degree 5 or degree 4 vertex giving pentagonal and tetragonal open faces, respectively. The latter process gives the capped tetragonal antiprism. Similarly, the lowest energy 8-vertex CpCoSe₂B₅H₅ structures are derived from the tricapped trigonal prism by removing a degree 5 vertex to give a pentagonal open face.