Charge-Transfer Spectra and Stability Constants of Ion Pairs of Hexaammineruthenium(III) and Halide Ions

Stability constants and extinction coefficients for ion pairs between rutheniurn(Hi) hexaammine and chloride, bromide and iodide ions were measured spectrophotometrically. The absorption spectra of the ruthenium(III) hexaammine halide ion pairs were measured in solutions of varying sodium halide concentration, The stability constants at zero ionic strength K⁰IP and the molar extinction Δɛcoefficients at the maxima of the, absorption peaks were evaluated as follows: K⁰IP - 16 ± 1 M^-1, Δɛ255 M^-1[cm^-1 for Cl^-; K⁰IP = 11 ± 1 M^-1, Δɛ = 300 M^-1 cm^-1 for Br^-; K⁰IP = 10 ± 1 M^-1,Δɛ 251 and 252 M^-1 cm^-1 for I^-. ΔH° and ΔS° were also measured for the formation of the ion pair with the chloride ion. The spectra were interpreted as charge-transfer transitions from the halide ions to the ruthenium ion. The spectrum of the iodide ion pair consisting of two maxima was described as a superposition of two gaussians and a slightly blue-shifted Ru(NH₃)₆³⁺ spectrum. The difference between the two maxima, 6590 cm^-1, was interpreted as the ²P_3/2-²P_1/2 doublet splitting of the iodide ion. For the bromide ion, a similar procedure gave a doublet splitting of 2250 cm^-1. The transition energies of the Ru(NH₃)₆³⁺,X^- ion pairs are compared to those of the analogous cobalt(III) complexes.