Structural Chemistry of <i>arachno</i>-Nonaboranes

Single-crystal conventional-tube and synchrotron X-ray diffraction studies of the anions in [NMe4][arachno-B9H12-4,8-Br2] 1 and K[arachno-B9H14] 2, and also of the series of adducts [arachno-B9H13-4-L], where L is P(CCH)3 (3), NHEt2 (4), NC5H5 (5), or NH2CH2Ph (6), are reported. Structural studies of 1−6, determined at low temperatures, located all atoms, including bridging and endo-terminal hydrogen atoms. The basic boron-hydride clusters of these, and of all the other known species with the arachno nine-vertex i-nonanborane geometry reported in the literature, are isostructural and feature three bridging and two endo-terminal hydrogen atoms on the open face. This arrangement is different from that previously reported for Cs[arachno-B9H14] 7 and for [arachno-B9H13-4-(NCMe)] 9. However, a new X-ray diffraction data set and refinement experimentally confirm the {3 × μ-H, 2×endo} arrangement for 9 also. The experimental results for 1−6 support recently reported calculations for [B9H14]-, which predict both the structures and the 11B NMR chemical shifts. These conclusions are also supported by calculations for 3, 4, and 9 and also for the [arachno-B9H13-4-(NCS)]- anion in [NMe4][B9H13(NCS)] 8.