246MOLYBDENUM TRIFLUORIDE, MoFs, AND TANTALUM TRIFLUORIDE, TaF3 atoms, 2.757 and 2.759 A. in MoF a and TaF~ respectively, is not inconsistent with the accepted value of 1.36 A. for the fluorine ionic radius, but since the metal atoms will probably force the non-metal atoms apart, the effective size of the fluorine atoms in these structures cannot be deduced from their distances of closest approach.

No abstract

No abstract

No abstract

Does not sensitivity to low acetone concentration suggest effect on sensitive property of water•-e.g., surface tension-or even on nature of solution of complexes with large organic groups in water?. N. Neumann.-In looking for correlations between the properties of ions of this type and properties of the solvent mixture we have been influenced by data on the rates of dissociation of Ni(phen)32+, Fe(phen)32+ and Fe(bipy)32+ in organic-water mixtures (unpublished work of Dr. Lester Seiden). These rates show a minimum as would be expected if the behavior correlated with the viscosity. The surface tension decreases continuously on going from water to acetone, and would not lead one to expect a minimum in the rate. It seems reasonable that the rearrangement of solvent molecules would be a simpler process in the case of dissociation of a single ion than in the case of bringing two ions together, hence correlation with the dissociation data seemed more promising. It is true that the exchange re-sults considered alone show greater similarity to the surface tension than to the viscosity.Henry S. Frank.-Dr. Katzin's suggestion about surface tension may be paraphrased in more general terms by referring to the influence which non-polar groups have on water structure. For example, tetra-n-butylammonium ion affects water in such a way as to produce over 120 units of excess heat capacity beyond what can be accounted for on the basis of additivity rules. If we interpret this as an influence of the hydrophobic nature of the alkyl groups the same influence might be expected to produce a sorting tendency in mixed solvents, attracting, say, acetone, into nearest-neighbor positions, in contrast to the customary dielectric salting out effect. Something similar might happen with the tris-phenanthroline complexes, and this could account for the shape of the curve of the effect of additions of acetone.