ELECTRONIC-SPECTRA, FORMATION-CONSTANTS, AND GEOMETRIES OF HGX3- IN METHANOL

The electronic absorption spectra, in the ultraviolet region, of HgX3-(X = Cl, Br, or I) in methanol have for the first time been obtained, by computer techniques, free from any contributions of HgX2 or HgX4(2-). Four different and independent methods are described and evaluated. Where the desired spectrum is completely hidden below other bands, here for HgBr3- and HgCl3-, a combination of two methods is needed. Additional tests validated the computed spectra. From these spectra, the stepwise formation constant K3 was computed and found independent of ionic strength at low concentrations. The infinite dilution values are log K3(HgCl3-) = 1.28 +/- 0.01, log K3(HgBr3-) = 3.08 +/- 0.08, and log K3(HgI3-) = 5.26 +/- 0.02. The spectra of HgX3- were resolved into their component Gaussian bands and the transitions identified and assigned. This permitted the identification of HgCl3- as planar, with D3h symmetry, and HgI3- as pyramidal, with C3v symmetry, which suggests that the solvated species are trigonalbipyramidal and tetrahedral, respectively. The spectrum of HgBr3- appears to show features of both symmetries but is here concluded to be closer to pyramidal geometry. Comparisons are made with the structures of HgX3- species obtained in dipolar aprotic solvents by other techniques.