Spherical Closo Deltahedra with Surface Metal-Metal Multiple Bonding versus Oblate Deltahedra with Internal Metal-Metal Bonding in Dichromadicarbaborane Structures: The Nature of Stone's Icosahedral Dichromadicarbaborane.

The dichromadicarbaboranes Cp2Cr2C2B n-4H n-2 ( n = 8-12) related to the icosahedral structure reported in 1983 by Stone and co-workers and formulated by them as Cp2Cr2C2B8H10 have been investigated using density functional theory. In most cases, the lowest-energy structures are flattened oblatocloso structures with degree 6 and 7 chromium vertices similar to the lowest-energy and experimental structures of the isoelectronic dirhenaboranes Cp2Re2B n-2H n-2. However, most isomeric spherical closo deltahedral structures with surface Cr≣Cr quadruple bonds as well as isocloso structures with surface metal-metal Cr≡Cr triple bonds lie at accessible energies, typically lower than those in the corresponding dirhenaborane systems. However, for the 11-vertex Cp2Cr2C2B7H9 system, the most spherical closo/ isocloso deltahedral structure with a degree 6 metal vertex and degree 4 carbon vertices as well as a surface M≡M triple bond lies energetically below the lowest-energy oblatocloso structure. Calculations of the Cr-Cr distances in an icosahedral Cp2Cr2C2B8H10 structure and in a dihydrogenated icosahedral Cp2Cr2(μ-H)2C2B8H10 structure suggest the latter structure for "Cp2Cr2C2B8H10" reported by Stone and co-workers.