Sulfide
Systematic name Sulfanediide[1] (substitutive) Sulfide(2-)[1] (additive)
Identifiers
CAS number	18496-25-8 Y
PubChem	29109
ChemSpider	27079 Y
ChEBI	CHEBI:15138
Jmol-3D images	Image 1
SMILES [S--]
InChI InChI=1S/S/q-2 Y Key: UCKMPCXJQFINFW-UHFFFAOYSA-N Y
Properties
Molecular formula	S2−
Molar mass	32.065 g mol-1
Related compounds
Other anions	Telluride
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.

Properties[link]

The dianion S²⁻ exists only in strongly alkaline aqueous solutions. Such solutions can form by dissolution of H₂S or alkali metal salts such as Li₂S, Na₂S, and K₂S in the presence of extra hydroxide. The ion S²⁻ is exceptionally basic with a pK_a > 14. It does not exist in appreciable concentrations even in highly alkaline water, being undetectable at pH < ~15 (8 M NaOH).^[2]

Hydrogen sulfide ion

Hydrogen sulfide

Instead, sulfide combines with protons to form HS⁻, which is variously called hydrogen sulfide ion, hydrosulfide ion, sulfhydryl ion, or bisulfide ion. At still lower pH (<7), HS⁻ converts to H₂S, hydrogen sulfide.^[3]

Sulfides are moderately strong reducing agents. They react with oxygen in the air in elevated temperatures to form higher-valence sulfur salts, such as sulfates and sulfur dioxide.

Metal derivatives[link]

Aqueous solutions of transition metals cations react with sulfide sources (H₂S, NaHS, Na₂S) to precipitate solid sulfides. Such inorganic sulfides typically have very low solubility in water, and many are related to minerals with the same composition (see below). One famous example is the bright yellow species CdS or "cadmium yellow". The black tarnish formed on sterling silver is Ag₂S. Such species are sometimes referred to as salts. In fact, the bonding in transition metal sulfides is highly covalent, which gives rise to their semiconductor properties, which in turn is related to the deep colors. Several have practical applications as pigments, in solar cells, and as catalysts.

Geology[link]

Many important metal ores are sulfides.^[4] Significant examples include: argentite (silver sulfide), cinnabar (mercury), galena (lead sulfide), molybdenite (molybdenum sulfide), pentlandite (nickel sulfide]), realgar (arsenic sulfide), and stibnite (antimony), sphalerite (zinc sulfide), and pyrite (iron disulfide), and chalcopyrite (iron-copper sulfide).

Corrosion induced by sulfide[link]

Dissolved free sulfides (H₂S, HS⁻ and S²⁻) are very aggressive species for the corrosion of many metals such as steel, stainless steel, and copper. Sulfides present in aqueous solution are responsible for stress corrosion cracking (SCC) of steel, and is also known as sulfide stress cracking. Corrosion is a major concern in many industrial installations processing sulfides: sulfide ore mills, deep oil wells, pipeline transporting soured oil, Kraft paper factories. Microbially-induced corrosion (MIC) or biogenic sulfide corrosion are also caused by sulfate reducing bacteria producing sulfide.

Oxidation of sulfide can also form thiosulfate (S₂O₃²⁻) an intermediate species responsible for severe problems of pitting corrosion of steel and stainless steel while the medium is also acidified by the production of sulfuric acid when oxidation is more advanced.

Organic chemistry[link]

In organic chemistry, "sulfide" usually refers to the linkage C-S-C, although the term thioether is less ambiguous. For example, the thioether dimethyl sulfide is CH₃-S-CH₃. Polyphenylene sulfide (see below) has the empirical formula C₆H₄S. Occasionally, the term sulfide refers to molecules containing the -SH functional group. For example, methyl sulfide can mean CH₃-SH. The preferred descriptor for such SH-containing compounds is thiol or mercaptan, i.e. methanethiol, or methyl mercaptan.

Disulfides[link]

Confusion arises from the different meanings of the term "disulfide". Molybdenum disulfide (MoS₂) consists of separated sulfide centers, in association with molybdenum in the formal 4+ oxidation state (Mo⁴⁺). Iron disulfide (pyrite, FeS₂) on the other hand consists of S₂²⁻, or ⁻S–S⁻ dianion, in association with divalent iron in the formal 2+ oxidation state (ferrous ion: Fe²⁺). Dimethyldisulfide has the chemical binding CH₃-S–S-CH₃, whereas carbon disulfide has no S–S bond, being S=C=S (linear molecule analog to CO₂). Most often in sulfur chemistry and in biochemistry, the disulfide term is commonly ascribed to the sulfur analogue of the peroxide −O–O− bond. The disulfide bond (−S–S−) plays a major role in the conformation of proteins and in the catalytic activity of enzymes.

Examples[link]

Preparation[link]

Sulfide compounds can be prepared in several different ways:^[5]

1. Direct combination of elements:

Example: Fe_(s) + S_(s) --> FeS_(s)

2. Reduction of a sulfate:

Example: MgSO_4(s) + 4C_(s) --> MgS_(s) + 4CO_(g)

3. Precipitation of an insoluble sulfide:

Example: M²⁺ + H₂S_(g) --> MS_(s) + 2H⁺_(aq)

Safety[link]

Many metal sulfides are so insoluble in water that they are probably not very toxic. Some metal sulfides, when exposed to a strong mineral acid, including gastric acids, will release toxic hydrogen sulfide.

Organic sulfides are highly flammable. When a sulfide burns it produces sulfur dioxide (SO₂) gas.

Hydrogen sulfide, some of its salts, and almost all organic sulfides have a strong and putrid stench; rotting biomass releases these.

See also[link]

• Dithionite
• Sulfate
• Sulfide stress cracking
• Sulfite
• Tetrathionate
• Thioether
• Thiosulfate

References[link]