STRUCTURE AND BONDING OF SOLVATED MERCURY(II) AND THALLIUM(III) DIHALIDE AND DICYANIDE COMPLEXES BY XAFS SPECTROSCOPIC MEASUREMENTS AND THEORETICAL CALCULATIONS

The solvation of mercury(II) complexes and ions has been studied by XA FS methods and compared to the corresponding thallium(III) species. An alyses of Hg L(III) edge extended X-ray absorption fine structure (EXA FS) spectra gave the distances 2.29(2) and 2.31(2) angstrom for HgCl2 in aqueous and dimethyl sulfoxide solutions, respectively, 2.46(2) ang strom for solid HgBr2 and 2.42(2) angstrom for HgBr2 in aqueous soluti on; for Hg(CN)2 in aqueous solution Hg-C = 2.04(2) angstrom and Hg-N = 3.18 (3) angstrom. The weakness of the EXAFS signals observed of the solvated Hg2+ ion in e.g. pyridine, acetonitrile, and aqueous solution s are interpreted as being due to dynamic distortions of the first sol vation shell by second-order Jahn-Teller effects. The pre-edge transit ions in the X-ray absorption near-edge structure (XANES) region for me rcury(II) and thallium(III) complexes have been used to distinguish be tween different coordination geometries of the solvated species. Theor etical ab initio calculations have been performed on the structures of the mercury(II) and thallium(III) dihalide and dicyanide complexes, i n order to compare the effects of differences in bonding and hydration , and also on the valence shell energy levels, to assist assignments o f pre-edge features in the XANES spectra. Relativistic effective core potentials (ECP) were constructed, both for the ground-state mercury a nd thallium atoms and for the 2p ionized state, and used in calculatio ns at the MCPF level of bond lengths and relative energy differences. The first pre-edge peak found for all complexes in their XANES spectra was assigned to a (2p) --> SIGMA(g)+ (approximately Hg 6s) excitation , with the splitting of the pre-peak for Hg(CN)2 possibly due to (2p) --> PI(C-N) at ca. 3.4 eV higher energy. Multiple scattering resonanc es have been discussed for the CN ligands. Comparisons of calculated a nd experimental bond lengths of the mercury(II) and thallium(III) dich loride and dicyanide complexes revealed unexpectedly short bond length s for the mercury(II) complexes, which have been discussed in terms of weaker solvation and stronger bonding. The bonding in the Hg(CN)2 and Tl(CN)2+ complexes were analyzed by theoretical calculations using a constrained space orbital variation (CSOV) technique, showing signific ant contributions of back-donation particularly in the Hg-CN bonds. Th e trends of the force constants from vibrational spectra are consisten t with this picture and show stronger and shorter M-C bonds but also s tronger C-N bonds in the Hg(CN)2 complex than in the Tl(CN)2+ complex.