THE SOLVATION OF CATIONS IN HYDROGEN-BONDED MOLECULAR-SOLVENTS - A NEUTRON-DIFFRACTION STUDY ON THE STRUCTURE OF NI2-GLYCOL AND IN GLYCEROL( SOLUTIONS IN ETHYLENE)

The method of isotopic substitution in time-of-flight neutron diffract ion is used to measure the coordination environment of Ni2+ in both a 0 . 956 molal solution of Ni(CF3SO3)(2) in fully deuterated ethylene g lycol (EG) and a 0 . 980 molal solution of Ni(CF3SO3)(2) in fully deut erated glycerol. The results show that EG acts as a bidentate ligand t o 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 Ni2+ ion cannot be ruled out. There is no evidence of i nner sphere complexing by the CF3SO3- anion in either solution, but th e data are consistent with a spatially well defined hydrogen bond O-D ... O of length similar to 1 . 8(1) Angstrom between the hydroxyl grou ps of the solvent molecules in the first and second coordination shell s of the cation. The number of second shell hydroxyl groups that parti cipate in this hydrogen bond process is estimated at six in both solut ions. 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 intermed iate range ordering in both solutions can be traced to the arrangement of the Ni2+ complexes which are distributed in a way that maximizes t heir mean separation. A comparison is made between the results obtaine d for the EG solution by using steady-state and pulsed neutron source instrumentation.