Polyhedral dinickelaboranes as analogues of the dicarbaboranes

Abstract

The lowest energy structures of the dinickelaboranes Cp2Ni2Bn−2Hn−2 (n=8–12) are found by density functional theory to be the most spherical closo deltahedra similar to those of the isoelectronic dicarbaboranes C2Bn−2Hn. The nickel atoms in such dinickelaboranes do not prefer higher degree vertices in contrast to metallaboranes containing earlier transition metals. Similarly, Cp2Ni2Bn−2Hn−2 structures having adjacent nickel atoms are of higher energy than isomeric structures having non-adjacent nickel atoms. Thus the brown icosahedral ortho Cp2Ni2B10H10 isomer with adjacent nickel atoms obtained by Hawthorne and co-workers from B10H102− and Cp2Ni, [Cp3Ni2][BF4], or Cp2Ni2(CO)2 and structurally characterized by X-ray crystallography is not the lowest energy isomer but instead a kinetically favored product. However, the structure with non-adjacent nickel atoms suggested by 11B NMR for the Cp2Ni2B8H8 isomer isolated in small quantities by Grimes and co-workers from a B5H8−/C5H5−/NiBr2 reaction mixture is found to be the lowest energy isomer by ∼6kcal/mol. This latter reaction also gives small quantities of a yellow Cp2Ni2B10H10 isomer suggested by its 11B NMR spectrum to be the lowest energy meta isomer with non-adjacent non-antipodal nickel atoms.