MATRIX SPECTRA, FORCE CONSTANTS, AND STRUCTURES FOR M+CLO4-AND HYDRATED ALKALI-METAL PERCHLORATE ION-PAIRS

The effect of hydration on the ion pairs Li+C104-and K+C104'isolated in argon matrices has been followed by observing the progressive reduction in the magnitude of the splitting between components of the V3(f) antisymmetric stretching mode. Though this splitting is strongly reduced (by a factor of 3) for the hydrated ion pairs, MC104-H20, relative to the simple ion pairs, M+C104-, the splitting pattern remains that calculated for a bidentate coordination of the metal with the anion. This frequency splitting pattern has been predicted by decreasing the two coordinated Cl-0 bond force constants while increasing the FCio values for the noncoordinated bonds. This procedure generates a frequency vs. ∆FC10plot for each C104'mode of a M+C104'bidentate complex. Besides reflecting the fact that the observed frequency pattern indicates a bidentate structure for all cases, this procedure has also permitted stretching force constants to be estimated for the various hydrates, M+C104-°H20, where n varies from 0 to 4. The new data are consistent with the idea that the C104'ion is only very weakly distorted through interactions with either a water or an ammonia solvent medium and, together with similar results for M+N03-ion pairs, establish the general nature of the marked reduction in the strength of contact cation-anion interactions that accompanies Lewis base solvation of the cation. © 1979, American Chemical Society. All rights reserved.