About 18 months ago, there was much discussion on this blog about a system reported by Bob Pascal and co-workers containing a short H…H contact of ~1.5Å[cite]10.1021/ja407398w[/cite]. In this system, the hydrogens were both attached to Si as Si-H…H-Si and compressed together by rings.
The crystal structures of ammonium tetraphenylborate, NH4 +·BPh4 −, are determined by neutron diffraction at 20 and 293 K. At both temperatures, all four N—H vectors of the ammonium ion are time-average-oriented at the midpoints of the phenyl rings of neighboring anions. The N—H...Ph distances, H...M 2.067 and N...M 3.023 Å, are exceptionally short (M = aromatic midpoint). Even at 20 K the ammonium ion performs large amplitude motions which allow the N—H vectors to sample the entire face of the aromatic system.