Distorsion of the [Ni(OH2)6]2+ Cation in a Benzenesulfonate—Correlation with Hydrogen Bonds or Intra-Cation Interactions?

In the absence of electronic reasons for distortion and with its ligands idealized as spherical, the hexaaquanickel(II) cation may be regarded as a simple example for octahedral metal coordination. In structurally characterized derivatives, the cation must adopt a lower symmetry in which it may well be distorted. We investigate hexaaquanickel(II) 3-carboxy-4-hydroxybenzenesulfonate dihydrate, a solid in which the cation occupies a center of inversion and subtends classical O–H···O hydrogen bonds with anions and co-crystallized water. Based on high-resolution X-ray diffraction, we analyze the resulting experimental electron density in order to understand why one of the three independent Ni–O coordinative bonds is significantly shorter than the others. The electron density and its derived properties in the bond critical points do not indicate correlation between hydrogen and coordinative bonds. The electrostatic potential derived from the experimental density reflects the electron distribution in the d orbitals and suggests that intra-cation repulsions and attractions are responsible for the distorsion of the cation. The quality of our experimental results is further underlined by a very good match between the measured electron density in the bond critical points of the Ni–O coordinative bonds and a previous report in the literature.