ChemInform Abstract: 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 C2v symmetry, and [HgI2Br]– and [HgI2Cl]2– as closer to pyramidal, with Cs 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 C2v 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.