Molecular and crystal structures of [(Ph₃P)₂N] [B₃H₇(NCS)] and [(Ph₃P)₂N] [B₃H₇(NCSe)]; the need for low-temperature x-ray crystallography

Two ion pairs of [Ph₃P)₂N] [B₃H₇(NCS)] crystallise in the centrosymmetric triclinic space group with a = 10.0206(16), b = 10.9026(13) and c = 17.482(3) Å, α = 109.063(13), β = 101.409(14) and γ = 76.480(12)° at 185 ± 1 K. The isocyanate and isoselenocyanate analogues ([Ph₃P)₂N] [B₃H₇(NCO)] and [(Ph₃P)₂N] [B₃H₇(NCSe)] are isomorphous with a = 9.839(4), b = 11.199(3), c = 17.408(5) Å, α = 110.638(23), β = 101.553(27), γ = 79.019(24)° at 291 ± 2 K), and a = 10.129(2), b = 10.887(2), c = 17.488(4) Å, α = 108.835(15), β = 101.286(19), γ = 75.764(18)° (at 185 ± 1 K) respectively. Diffraction data were collected at both ambient and low temperatures for the (NCS) and (NCSe) analogues, and for both species analysis of room and low temperature data suggests different stereochemical arrangements of the borane hydrogen atoms; one edge- and one face-bridging H is suggested from the room temperature data sets, whilst the low temperature data reveal only one (edge-) bridging hydrogen atom for each compound. The latter data afford R = 0.0471 for 5515 observed reflections (NCS) and R = 0.0493 for 4888 reflections (NCSe).