The Structure of the Hypophosphite Group as Determined from the Crystal Lattice of Ammonium Hypophosphite

The crystal structure of ammonium hypophosphite was examined in order to determine the size and shape of the hypophosphite group. The crystals are orthorhombicholohedral with the dimensions of the unit cell; a = 3.98A, b = 7.57A, c = 11.47A. There are 4 molecules NH₄(H₂PO₂) in the cell, and the space group is Acmm(V_h²¹). The complete structure is given by the following parameter values: 4 N in ±(0¼0), 4 P in ±(u₁0¼) with 2πu₁ = 195° ±5°; 8 O in ±(u₂0v₂) with 2πu₂ = 125° ±5° and 2πv₂ = 49° ±2°; 8 H in ±(U₃v₃¼) with 2πu₃ = 290° and 2πv₃ = 51°; 16 H in ±(xyz). The parameter values for phosphorus and oxygen were determined directly from the observed intensities of reflections, whereas the hydrogen values are based upon reasonable assumptions. The hypophosphite group has the shape of a distorted tetrahedron; two of the tetrahedral corners being occupied by oxygen atoms and the other two by hydrogen atoms. The phosphorus to oxygen distance is 1.51A and the angle between the phosphorus to oxygen bonds 120°. The distance from phosphorus to hydrogen is 1.5A and the angle between the P–H bonds 92°. Each ammonium radical is surrounded by four oxygen atoms at a distance of 2.81A and by four hydrogen atoms at a distance of 3.08A. There is strong indication that the ammonium radical is not rotating, and that hydrogen atoms of the NH₄‐groups are linked to the surrounding oxygen atoms as well. The hydrogen atoms of the hypophosphite group behave toward ammonium as if they were H^— ions.