Electronic structures of two kinds of oligoborane radical anions: mixed-valence description of a reduced p-phenylenediborane and spin density distributions in distorted octahedral clusters [B(6)Hal(6)](center dot-), Hal = Cl, Br, I

Density-functional theory (DFT) calculations have been carried out for the radical anions 1,4[Mes(2)B-(C6H4)-BMes(2)](center dot-) (1) (Mes, mesityl), and for [B6Cl6](center dot-) (2a), [B6Br6](center dot-) (2b) and [B6I6](center dot-) (2c). Previous EPR experiments had indicated significant spin delocalisation based on H-1, B-10 and B-11 hyperfine structure (1) and on g factor anisotropy (2a - c). Starting from optimized geometries the calculations confirmed this notion and yielded quantitative information on the spin distribution. While 1 with about 1/3 of spin density on each of the B atoms and on the organic bridge can be well-formulated as a delocalised (Class III) diboron(III,II) mixed-valent species with a p-phenylene spacer, similarly as the p-phenylenediamine radical cations, the paramagnetic anions 2 are trigonally Jahn-Teller distorted octahedral clusters with variable contributions from the halide substituents to the accommodation of the unpaired electron. While the chloride substituents in 2a participate rather little at the spin distribution, the iodide 2c is calculated with about equal contributions of 8% from individual B and I atoms to the singly occupied MO (SOMO).