Structure of Ni²⁺ Solutions in Ethylene Glycol by Neutron Diffraction: An Observed Hydrogen Bond between the Solvent Ligands in the First and Second Cation Coordination Shells?

The method of isotopic substitution in neutron diffraction is applied to study the coordination environment of the Ni²⁺ ion in a 0.956 m solution of Ni(CF₃SO₃)₂ in fully deuterated ethylene glycol (EG). It is shown that EG molecules do not act as monodentate ligands but that Ni(EG)₃²⁺ tris-chelate complexes are formed. There is no evidence of inner-sphere complexing by the CF₃SO₃⁻ anion, but the data are consistent with a spatially well defined hydrogen bond between EG ligands in the first and second coordination shells of the cation. If this bond is assumed to be linear, then it is of length ~1.8 (1) Å. The number of second-shell EG molecules that participate in this hydrogen-bond process is estimated at six. Furthermore, there is evidence for weak ordering of the Ni²⁺ correlations on a length scale of 10 ≲ r (Å) ≲ 16. The stability of the Ni(EG)₃²⁺ complex is briefly discussed.