Spectrochemistry of solutions. Part 26.—Alkali-metal and alkaline-earth-metal thiocyanates in dimethylformamide and acetonitrile solutions: hot bands, stability constants and thermicity for the formation of inner- and outer-sphere ion pairs

Abstract

The IR spectra of solutions of the thiocyanates of (CH₃)₄N⁺, (C₂H₅)₄N⁺, (C₄H₉)₄N⁺, Li⁺, Na⁺, K⁺, Cs⁺, Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺ in acetonitrile and dimethylformamide have been recorded at various temperatures in the range 278–348 K. Peaks ascribed to ion pairs shift with temperature. This shift is explained in terms of the existence of hot bands. Equilibrium constant calculations have been attempted using the IR data and where possible ΔH has been estimated.

[SrNCS]⁺ forms both inner- and outer-sphere complexes with an additional thiocyanate ion in acetonitrile. The IR spectrum of the species {[SrNCS]⁺NCS^–}, in which one thiocyanate is in the inner- and one thiocyanate is in the outer-coordination sphere of the strontium cation, has been deduced with the aid of an equilibrium calculation. The separation of the absorbance maximum of this species from that of [SrNCS]⁺ is one sixth of the half-width of the bands.