Mercaptide

Mercaptide is the historical term for the thiolate anion, RS−, the deprotonated form of a thiol (R–SH). In broader use, it can refer to salts or complexes containing the RS− unit. Today, thiolate is the preferred terminology in most chemical contexts, but mercaptide remains encountered in older literature and in some inorganic and organometallic discussions.

Formation and examples

Mercaptides form by deprotonation of thiols with bases such as sodium or potassium hydroxide: R–SH + NaOH → RS−Na+. They also arise when thiols react with metals to give metal thiolate species, for example RS− bound to Na+, K+, or a transition metal to yield RS–M linkages. Common examples include sodium ethanethiolate (EtS−Na+) and potassium phenylthiolate (PhS−K+). Silver mercaptides, such as AgSR, have been used in various materials and catalytic contexts.

Properties and reactivity

The thiolate anion is a soft, nucleophilic species that readily attacks electrophiles, enabling the formation of thioethers by reaction with alkyl halides (R–S–R'). RS− also forms strong bonds to many metals, generating metal-thiolate complexes and clusters. Oxidation of thiolates can lead to disulfides (RSSR). The pKa of typical thiols is around 10–11, so thiols are moderately easy to deprotonate under basic conditions to give mercaptides.

Applications and context

Mercaptides (thiolates) are important as ligands in coordination and organometallic chemistry, as precursors to metal sulfide materials, and as reactive intermediates in organic synthesis. The term mercaptide is largely historical; in modern practice, thiolate is the standard term used to describe these species. Safety considerations follow those of thiols, which can be malodorous and sensitive to air or moisture in some contexts.