A new look at the nido-undecaborate system

The structure of the nido-undecaborate anion, [B11H14]−, has been re-examined because of what appear to be discrepancies that were observed between our determination of the structure of the anion in [(Cp2Zr)2B5H8][B11H14] (1) and previously published structures. The structure of 1 indicated the presence of two bridging H atoms and another pseudo-bridging one whereas those of a series of published structures indicate the presence of a plane of symmetry with two bridging H atoms and one endo-H atom. Thus, we undertook a series of structural determinations and also a computational study at the B3LYP/6-31++G(d,p) level. In addition to 1, the species studied included [NBnEt3][B11H14] (2), [NBnEt3][7-Br-nido-B11H13] (3) and [NBnEt3][7-(η1-dppm)-nido-B11H12] (4). Our structure of 2 indicated the presence of two bridging H atoms and an endo-hydrogen atom with some bridging character but that of 3 contained three bridging atoms. As expected the structure of 4 contains two bridging H atoms. Calculations of bond parameters fit well with the experimental data as do the 11B NMR chemical shifts. The latter were calculated for the average of the two open face configurations, one with two bridging and one endo-hydrogen and the other with three bridging hydrogen atoms. The difference in energies for these two open face configurations is calculated to be 0.36 kJ/mol, which effectively suggests that the two structures are equally favored.