From closo to isocloso Structures and Beyond in Cobaltaboranes with 9 to 12 Vertices

Density functional theory (DFT) studies predict the dianions CpCoB n−1H n−1 2− (n = 9, 10, 11, 12; Cp = η5-C5H5) to have structures based on the most spherical deltahedra found in the isoelectronic boranes B n H n 2−. In the CpCoB8H8 2− dianion the non-equivalent structures with the cobalt atom at a degree 4 vertex and at a degree 5 vertex are essentially degenerate in terms of energy (within ∼1 kcal/mol). For the CpCoB n−1H n−1 2− dianions (n = 10, 11, 12) the cobalt atom prefers energetically the vertices of the lowest possible degree (four for n = 10 and 11, five for n = 12). Structures for the neutral species CpCoB n−1H n−1 (n = 10, 11, 12) based on isocloso deltahedra with the cobalt atom at a degree 6 vertex are preferred energetically by 9, 19, and 53 kcal/mol, respectively, over alternative structures. However, for CpCoB8H8 the closo tricapped trigonal prismatic structure with the cobalt atom at a degree 5 vertex is energetically preferred by ∼9 kcal/mol over the isocloso deltahedral structure with the cobalt atom at a degree 6 vertex. The lowest energy structures predicted for the dications CpCoB8H8 2+ and CpCoB9H9 2+ are highly oblate (flattened) deltahedra with the cobalt atom at a degree 7 vertex. A complicated potential energy surface was found for CpCoB10H10 2+ including non-deltahedral structures with a single quadrilateral or pentagonal face. The predicted lowest energy structures for both CpCoB11H11 and CpCoB11H11 2+ are based on the same 12-vertex deltahedron with three degree 6, six degree 5, and three degree 4 vertices, and thus topologically different from the regular icosahedron normally found in boron chemistry.