Structural Reinvestigation of Ammonium Hypophosphite: Was Dihydrogen Bonding Observed Long Ago?

The past decade has seen the explosive emergence of “dihydrogen bonds”, interactions between the electrons of
M−H σ-bonds, where M is less electronegative than H (M = Al, B, Ga, Ir, Mo, Mn, Os, Re, Ru, W) and traditional
proton donors. But 70 years ago, such an interaction was proposed by Zachariasen and Mooney [J. Chem. Phys.
1934, 2, 34−37] on the basis of their single-crystal X-ray diffraction structure (heavy atoms positions only) of
ammonium hypophosphite (NH4H2PO2). We redetermined this structure at high resolution with a focus on the
hydrogen atoms, using a modern diffractometer equipped with a CCD detector. Molecular orbital calculations were
performed to investigate the charge density and the bond polarity of the P−H bonds and to assess their potential
for participation in dihydrogen bonds. Neither the theory nor the X-ray structure supports the original claim of
H···H interactions in this salt.