Why do sulfur-containing compounds bind readily to metals such as mercury and silver?

Sulfur-containing compounds, such as thiols and sulfides, bind readily to heavy metal ions like silver and mercury primarily due to the high polarizability of the sulfur atom.1

Because sulfur is a third-period element, its valence electrons are held in larger orbitals and are further from the nucleus than those of lighter elements like oxygen or nitrogen. This makes the electron cloud more easily deformed, or polarizable, allowing sulfur to engage in bonding interactions with electrophilic partners more readily.1

This behavior is also explained by the hard and soft acids and bases (HSAB) theory. In this framework, heavy metal ions like silver and mercury are classified as "soft" acids, while divalent sulfur is a "soft" base. According to the principle that soft acids react preferentially with soft bases, these compounds form strong, stable interactions. Additionally, in protic solvents, sulfur is more weakly solvated than lighter heteroatoms, which enhances its nucleophilicity and facilitates its ability to bind to these metals.231