Polyhedral dinickelaboranes as analogues of the dicarbaboranes

Abstract

The lowest energy structures of the dinickelaboranes Cp 2 Ni 2 B n-2 H n-2 (n = 8 to 12) are found by density functional theory to be the most spherical closo deltahedra similar to those of the isoelectronic dicarbaboranes C 2 B n-2 H n. The nickel atoms in such dinickelaboranes do not prefer higher degree vertices in contrast to metallaboranes containing earlier transition metals. Similarly, Cp 2 Ni 2 B n-2 H n-2 structures having adjacent nickel atoms are of higher energy than isomeric structures having non-adjacent nickel atoms. Thus the brown icosahedral ortho Cp 2 Ni 2 B 10 H 10 isomer with adjacent nickel atoms obtained by Hawthorne and co-workers from B 10 H 10 2and Cp 2 Ni, [Cp 3 Ni 2 ][BF 4 ], or Cp 2 Ni 2 (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 11 B NMR for the Cp 2 Ni 2 B 8 H 8 isomer isolated in small quantities by Grimes and co-workers from a B 5 H 8-/C 5 H 5-/NiBr 2 reaction mixture is found to be the lowest energy isomer by ~6 kcal/mol. This latter reaction also gives small quantities of a yellow Cp 2 Ni 2 B 10 H 10 isomer suggested by its 11 B NMR spectrum to be the lowest energy meta isomer with non-adjacent non-antipodal nickel atoms.