Beyond the Wade-Mingos Rules: Deviations from Sphericality in Metallaborane Structures

Deviation from the most spherical closo deltahedral geometry can occur in metallaboranes and metallacarbaboranes because of the energetic preference of transition metal moieties for higher degree vertices than boron and/or carbon. The isocloso structures with a metal at a degree 6 vertex and two fewer skeletal electrons than the corresponding closo structures represent a minor deviation from such sphericity and are particularly favorable for 10-vertex systems. Spherical supraicosahedral deltahedra with 14 to 16 vertices, also known as Frank-Kasper deltahedra, necessarily have several degree 6 vertices and accordingly can have isocloso skeletal electron counts. More substantial deviations from sphericality occurs in the oblate ellipsoidal structures of the dirhenaboranes Cp2Re2Bn-2Hn-2 (8 <= n <= 12) with the rhenium atoms antipodally situated at degree 6 or 7 vertices of relatively low local curvature and the boron atoms situation at degree 4 and 5 vertices of relatively high local curvature. Species isoelectronic with the Cp2Re2Bn-2Hn-2 derivatives can also have closo or isocloso structures with surface metal-metal multiple bonds. The experimentally known icosahedral Cp2Cr2C2B8H10 was believed to be such a species with a surface Cr(sic)Cr quadruple bond. However, theoretical prediction of chromium-chromium distances suggest that a Cp2Cr2(-mu-H)(2)C2B8H10 structure with a surface Cr equivalent to Cr triple bond bridged by two "extra" hydrogen atoms is more likely. The tri- and tetrametallaboranes Cp*W-3(3)(H)B8H8 and Cp*4Mo4B7H7 of the groups 6 metals molybdenum and tungsten provide examples of non-spherical 11-vertex deltahedra with metal atoms located at degree 6 and 7 vertices and boron atoms located at degree 4 and 5 vertices. The topology of the lowest energy Cp*4Mo4Bn-4Hn-4 (n = 10, 11, 12) deltahedra can be generated by capping n - 8 vertices of a central Mo4B4 cube and then drawing diagonals across the remaining uncapped faces.