The solvation of cations in hydrogen-bonded molecular solvents: A neutron diffraction study on the structure of Ni²⁺ solutions in ethylene glycol and in glycerol

The method of isotopic substitution in time-of-flight neutron diffraction is used to measure the coordination environment of Ni^{2 +} in both a 0-956 molal solution of Ni(CF₃SO₃)₂ in fully deuterated ethylene glycol (EG) and a 0-980 molal solution of Ni(CF₃SO₃)₂ in fully deuterated glycerol. The results show that EG acts as a bidentate ligand to form [Ni(EG)₃]^{2 +} tris-chelate complexes, and are consistent with glycerol acting as a tridentate ligand to form [Ni(glycerol)₂]²⁺ bis-chelate complexes, although bidentate coordinating behaviour with respect to the Ni^{2 +} ion cannot be ruled out. There is no evidence of inner sphere complexing by the CF₃SO₃ anion in either solution, but the data are consistent with a spatially well defined hydrogen bond O—D…O of length 1-8(1) Å between the hydroxyl groups of the solvent molecules in the first and second coordination shells of the cation. The number of second shell hydroxyl groups that participate in this hydrogen bond process is estimated at six in both solutions. The structural results for the Ni²⁺ complexes are used to give a qualitative account of the mobility of these entities in the infinite dilution limit, but the hydrodynamic theory of Zwanzig does not give a sphere radius that is in agreement with the complex radius obtained microscopically from the neutron diffraction experiments. The intermediate range ordering in both solutions can be traced to the arrangement of the Ni²⁺ complexes which are distributed in a way that maximizes their mean separation. A comparison is made between the results obtained for the EG solution by using steady-state and pulsed neutron source instrumentation.