The X-ray structures of solid nitroguanidine (ngoH):orthorhombic, Fdd2, a = 17.6181(14), b = 24.848(2), c = 3.5901(4) Å, V = 1571.7(3) Å3, Z = 16 and nitrosoguanidine (ngH); monoclinic, P21/n, a = 3.64510(10), b = 11.746(2), c = 8.6483(14) Å, β = 99.167(2)°, V = 365.55(9) Å3, Z = 4 have been determined utilizing single crystal X-ray diffraction methods. The results are compared with the most stable gaseous configurations derived from ab inito calculations. The lowest energy calculated configuration for the ligands and experimentally observed crystal structures are in excellent agreement. In the solid state, both the ngoH and ngH contain discrete molecules in their unit cells which are planar (within experimental error), in the diamine configurations and are structurally identical except for an oxygen atom. In solid ngH, two ligand molecules have four nitrogen atoms arranged in a plane such that they are suitable for coordination to a nickel ion (1.945, 2.064 Å), when it is at the 1/2, 1/2, 1/2 unit-cell position giving the observed complex. As far as we are aware, this is the first instance in which a ligand crystal structure is essentially the same, with minor distance, angle and torsion angle changes, as the complex it forms and suggests some potentially unique properties and applications for this material.