From closo to isocloso structures and beyond in cobaltaboranes with 9 to 12 vertices.

Density functional theory (DFT) studies predict the dianions CpCoB(n-1)H(n-1)(2-) (n = 9, 10, 11, 12; Cp = eta(5)-C(5)H(5)) to have structures based on the most spherical deltahedra found in the isoelectronic boranes B(n)H(n)(2-). In the CpCoB(8)H(8)(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 approximately 1 kcal/mol). For the CpCoB(n-1)H(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-1)H(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 CpCoB(8)H(8) the closo tricapped trigonal prismatic structure with the cobalt atom at a degree 5 vertex is energetically preferred by approximately 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 CpCoB(8)H(8)(2+) and CpCoB(9)H(9)(2+) are highly oblate (flattened) deltahedra with the cobalt atom at a degree 7 vertex. A complicated potential energy surface was found for CpCoB(10)H(10)(2+) including non-deltahedral structures with a single quadrilateral or pentagonal face. The predicted lowest energy structures for both CpCoB(11)H(11) and CpCoB(11)H(11)(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.