Electronic spectra of anionic mixed halide complexes in solution: identification, computation of spectra and stability constants and assignment of transitions of [HgX2Y]– and [HgX2Y2]2–

Abstract

The electronic absorption spectra, in the ultraviolet region, of [HgX₂Y]^– and [HgX₂Y₂]^2–(X = Cl, Br or I, Y = Cl or Br) are here reported for the first time. Established computer techniques were used to obtain and validate the spectra of [HgX₂Y]^– free from any contributions of HgX₂ or [HgX₂Y₂]^2–. Very high mole ratios of added halide (Y) to HgX₂ were required for complete reaction to form [HgX₂Y₂]^2–(minimum 40 000). From the spectra the (stoichiometric) stability constants were computed, constant ionic strength conditions not being required, and hence by extrapolation the thermodynamic stability constants log K^′₃(T) and log K^′₄(T) were obtained as: 3.17 ± 0.01, 1.92 ± 0.05 and 0.99 ± 0.03 for log K^′₃(T) for [HgBr₂Cl]^–, [HgI₂Br]^– and [HgI₂Cl]^–, respectively; and as 0.25 ± 0.15, 0.84 ± 0.04 and –0.64 ± 0.06 for log K^′₄(T) for [HgBr₂Cl₂]^2–, [HgI₂Br₂]^2– and [HgI₂Cl₂]^2–, respectively. The log K^′₃(T) values were confirmed within experimental error by an independent graphical approach. The spectra of [HgX₂Y]^– and [HgX₂Y₂]^2– were resolved into their component Gaussian bands and the transitions identified and assigned. This permitted the identification of [HgBr₂Cl]^– as near planar, with close to C_2v symmetry, and [HgI₂Br]^– and [HgI₂Cl]^2– as closer to pyramidal, with C_s symmetry, suggesting that the solvated species are approximately trigonal-bipyramidal and tetrahedral, respectively. This allowed [HgX₂F]^– structures to now be suggested as basically pyramidal for only [HgI₂F]^–. The resolved spectra of the [HgX₂Y₂]^2– species indicated a distorted tetrahedral structure, with C_2v symmetry, but a detailed band assignment was not possible. General trends now arising from this work and our earlier studies on the spectra of HgX₂, [HgX₃]^– and [HgX₄]^2– are briefly reviewed.