A Short, Slightly Asymmetrical, Intramolecular Hydrogen Bond: A Neutron Diffraction Study of Bis(2‐amino‐2‐methyl‐3‐butanone Oximato)Nickel(II) Chloride Monohydrate {Ni(C₅H₁₁N₂O)₂H}⁺Cl⁻·H₂O

A neutron diffraction study of bis(2‐amino‐2‐methyl‐3‐butanone oximato)nickel(II) chloride monohydrate {Ni(C₅H₁₁N₂O)₂H}⁺Cl⁻·H₂O has provided a wealth of precise information concerning the short intramolecular hydrogen bond, the dynamics of rotating methyl groups, and the effects of intermolecular environment on potential functions. The intensities of 3500 single‐crystal reflections were measured at the Brookhaven High Flux Beam Reactor. These data were used in conjunction with the earlier x‐ray determination to locate all atoms including the 25 hydrogen atoms. After least squares refinement with anisotropic temperature factors, the agreement factor $R = ΣΔ(F2) / ΣF02$ was 0.055. The short intramolecular hydrogen bond O···O [2.420(3) Å] is unique in that no bond symmetry is imposed by the space group, and it is slightly asymmetrical. The O–H bond lengths for the hydrogen bond are 1.242(5) and 1.187(5) Å, and the O–H–O angle is 169.9(3)°. The potential apparently has a broad, flat, single minimum, shifted toward one oxygen, possibly because of intermolecular packing forces. The N–O bonds adjacent to the hydrogen bond are of lengths 1.335(2) and 1.346(2) Å with the shorter N–O distance associated with the longer O–H distance. The water molecule and chloride ion are extensively hydrogen bonded to each other and to the amine hydrogens of the cation. The methyl groups attached to $sp2$ carbon atoms have large rotational amplitudes (19–30°) resulting in short apparent C–H bond lengths (1.006 Å average) from the harmonic treatment; the bond length shortening is well correlated with the rms rotational amplitude. Other average uncorrected bond distances involving hydrogens are O–H in H₂O (0.950 Å); N–H (1.024 Å); and C–H (1.081 Å) for methyl groups attached to the $sp3$ carbons.