Molecular orbital calculations relevant to the hypercloso vs. iso-closo controversy in metallaboranes

Abstract

061ChemInform Abstract (theoretical analysis).

14‐Vertex Heteroboranes with 14 Skeletal Electron Pairs: An Experimental and Computational Study

“…Only one resonance signal attributed to C cage H protons was evident in the 1 H NMR spectrum of 5 (see the Supporting Information) but it fits best (Dd = 0.61 ppm) with one of the computed shifts for the 2,7,1,6 isomer (next smallest Dd = 4.90 ppm). Finally, the 2,7,1,6 isomer has the lowest computed free energy of all four isomers considered, being 4.9 kcal mol À1 more stable than the next most stable isomer (2,7,5,6). We therefore conclude that compound 5 is [2,7-Cp*-2,7,1,6-Ru 2 C 2 B 10 H 12 ].…”

“…DFT calculations strongly suggest that b = 1. R 2 is 0.9899 for the 2,7,1,6 isomer compared to 0.8611 for the next-best fit (2,7,6,11). Only one resonance signal attributed to C cage H protons was evident in the 1 H NMR spectrum of 5 (see the Supporting Information) but it fits best (Dd = 0.61 ppm) with one of the computed shifts for the 2,7,1,6 isomer (next smallest Dd = 4.90 ppm).…”

“…Thus linear regression of the computed and experimental 11 B NMR shifts yields an R 2 value for the 2,7,1,12 isomer of 0.9954, compared with R 2 = 0.8676 for the next-best isomer (2,7,3,12). In terms of 1 H NMR shifts, the difference in the sum (DSd) of the calculated and actual chemical shifts for the two C cage H resonance signals is only 1.21 ppm for the 2,7,1,12 isomer, compared with 5.01 ppm for the next-best isomer (2,7,4,12). Finally, of all of the ten isomers the lowest free energy computed (BP86-D3 with a correction for the THF solvent) was for the 2,7,1,12 isomer, which had a value 3.5 kcal mol À1 below that of the next most stable isomer (2,7,4,12).…”

Transition Metal Metallacarbaboranes

“…Metallacarbaboranes having four cage carbon atoms have been obtained via several routes, including metal vapor synthesis, 123 oxidative fusion o f dicarbon metallacarbaboranes, 18 ' 135 ' 41 ' 48 ' 57 metal insertion into tetracarbon carbaboranes, 13 ' 143 " 0 and, in one case, boron insertion into a metallacycle. 333 (Curiously, while the tetracarbon ni do -carbaborane C 4 B 2 H 6 and its derivatives have been known since the mid1960s, 334 no metallacarbaborane complexes o f this ligand have been reported.)…”

Geometry and chemical bonding in polyhedral boranes, metallaboranes, and dimetallaboranes: From closo to isocloso to oblatocloso polyhedra

“…However, as metallaborane chemistry was subsequently developed involving polyhedra other than the icosahedron, particularly by Kennedy and co-workers [4][5][6][7], a variety of deltahedral metallaborane structures were discovered based on deltahedra topologically distinct from the closo deltahedra. This led to the identification of a new class of less spherical deltahedra for metallaboranes and metallacarboranes called either isocloso [8] or hypercloso [9][10][11] deltahedra; the former terminology will be used in this paper. These isocloso metallaborane deltahedra are derived from the closo metal-free borane deltahedra by one or more diamond-square-diamond processes, typically generating a degree 6 vertex for the metal atom.…”