Theoretical Studies of Xanthates in Heavy Metal Complexation: Understanding the Structural, Thermochemical, and Electronic Aspects

Heavy metals have been widely discussed as a significant cause of environmental disasters in recent decades. One of the main focuses has been exploring agents and mechanisms capable of removing these metals from nature. Under this perspective, xanthates have been studied as sequestering agents of these metals in aqueous environments. In this theoretical study, complexes of cadmium(II) and mercury(II), with different coordination modes and geometric isomers of the xanthate ligands (n-propyl, n-butyl, and n-pentyl xanthates), were evaluated using density functional theory. To verify the versatility of coordination modes of the xanthates, monodentate and bidentate structures were optimized. For these compounds, structural, thermochemical, and electronic parameters were studied. The thermochemical results showed that all complexes are formed through exothermic and spontaneous processes. Electronic analysis showed that the mercury(II) complexes have a higher covalent character of the Hg-S bond, in contrast with the cadmium(II) complexes, which exhibited a higher ionic character of the Cd-S bond. Through the analysis of the natural bond orbitals, the Lewis acid-base nature of these complexes was confirmed and corroborated by the softness calculations, with the participation of the p orbitals of the sulfur and s orbitals of the metal centers.

Keywords:
xanthates; Cd²⁺; Hg²⁺; EDA; NBO; thermochemical parameters