Transition metal thiolate complexes are

metal complex A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...

es containing thiolate

ligand In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electr ...

s. Thiolates are ligands that can be classified as soft Lewis bases. Therefore, thiolate ligands coordinate most strongly to metals that behave as soft Lewis acids as opposed to those that behave as hard Lewis acids. Most complexes contain other ligands in addition to thiolate, but many homoleptic complexes are known with only thiolate ligands. The

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...

cysteine Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the formula . The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile. When present as a deprotonated catalytic residue, sometime ...

has a thiol functional group, consequently many cofactors in proteins and

enzymes Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different molecule ...

feature cysteinate-metal cofactors.

Synthesis

Metal thiolate complexes are commonly prepared by reactions of metal complexes with thiols (RSH), thiolates (RS⁻), and

disulfide In biochemistry, a disulfide (or disulphide in British English) refers to a functional group with the structure . The linkage is also called an SS-bond or sometimes a disulfide bridge and is usually derived by the coupling of two thiol groups. In ...

s (R₂S₂). The

salt metathesis reaction A salt metathesis reaction, sometimes called a double displacement reaction, is a chemical process involving the exchange of bonds between two reacting chemical species which results in the creation of products with similar or identical bonding a ...

route is common. In this method, an alkali metal thiolate is treated with a transition metal halide to produce an alkali metal halide and the metal thiolate complex: :LiSC₆H₅ + CuI → Cu(SC₆H₅) + LiI The thiol ligand can also effect protonolysis of anionic ligands, as illustrated by the formation of an organonickel thiolate from

nickelocene Nickelocene is the organonickel compound with the formula Ni( ''η''5-C5H5)2. Also known as bis(cyclopentadienyl)nickel or NiCp2, this bright green paramagnetic solid is of enduring academic interest, although it does not yet have any known pra ...

and

ethanethiol Ethanethiol, commonly known as ethyl mercaptan, is an organosulfur compound with the formula CH3CH2SH. is a colorless liquid with a distinct odor. Abbreviated EtSH, it consists of an ethyl group (Et), CH3CH2, attached to a thiol group, SH. Its s ...

: :2 HSC₂H₅ + 2 Ni(C₅H₅)₂ → i(SC₂H₅)(C₅H₅)sub>2 + 2 C₅H₆ Thiomersal-Skeletal-Structure-SVG

Regarding their mechanism of formation from thiols, metal thiolate complexes can arise via deprotonation of thiol complexes.

Redox routes

Many thiolate complexes are prepared by redox reactions. Organic disulfides oxidize low valence metals, as illustrated by the oxidation of

titanocene dicarbonyl Dicarbonylbis(cyclopentadienyl)titanium is the chemical compound with the formula (''η''5-C5H5)2Ti(CO)2, abbreviated Cp2Ti(CO)2. This maroon-coloured, air-sensitive species is soluble in aliphatic and aromatic solvents. It has been used for the ...

: : Some metal centers are oxidized by thiols, the coproduct being hydrogen gas: : These reactions may proceed by the

oxidative addition Oxidative addition and reductive elimination are two important and related classes of reactions in organometallic chemistry. Oxidative addition is a process that increases both the oxidation state and coordination number of a metal centre. Oxidat ...

of the thiol to Fe(0). Thiols and especially thiolate salts are

reducing agent In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ). Examples of substances that are commonly reducing agents include the Earth meta ...

s. Consequently, they induce redox reactions with certain transition metals. This phenomenon is illustrated by the synthesis of cuprous thiolates from cupric precursors: :4 HSC₆H₅ + 2 CuO → 2 Cu(SC₆H₅) + (C₆H₅S)₂ + 2 H₂O Thiolate clusters of the type e₄S₄(SR)₄sup>2− occur in

iron–sulfur protein Iron–sulfur proteins (or iron–sulphur proteins in British spelling) are proteins characterized by the presence of iron–sulfur clusters containing sulfide-linked di-, tri-, and tetrairon centers in variable oxidation states. Iron–sulfur clu ...

s. Synthetic analogues can be prepared by combined redox and salt metathesis reactions: :4 FeCl₃ + 6 NaSR + 6 NaSH → Na₂ e₄S₄(SR)₄ + 10 NaCl + 4 HCl + H₂S + R₂S₂

Structure and bonding

Divalent sulfur exhibits bond angles approaching 90°. Such acute angles are also seen in the M-S-C angles of metal thiolates. Having filled p-orbitals of suitable symmetry, thiolates are pi-donor ligands. This property plays a role in the stabilization of Fe(IV) states in the enzyme

cytochrome P450 Cytochromes P450 (CYPs) are a Protein superfamily, superfamily of enzymes containing heme as a cofactor (biochemistry), cofactor that functions as monooxygenases. In mammals, these proteins oxidize steroids, fatty acids, and xenobiotics, and are ...

Reactions

Thiolates are relatively basic ligands, being derived from conjugate acids with pK_a's of 6.5 (

thiophenol Thiophenol is an organosulfur compound with the formula C6H5SH, sometimes abbreviated as PhSH. This foul-smelling colorless liquid is the simplest aromatic thiol. The chemical structures of thiophenol and its derivatives are analogous to phenols ...

) to 10.5 (

butanethiol 1-Butanethiol, also known as butyl mercaptan, is a volatile, clear to yellowish liquid with a fetid (extremely foul-smelling) odor, commonly described as "skunk" odor. In fact, 1-butanethiol is structurally similar to several major constituents o ...

). Consequently, thiolate ligand often

bridge A bridge is a structure built to span a physical obstacle (such as a body of water, valley, road, or rail) without blocking the way underneath. It is constructed for the purpose of providing passage over the obstacle, which is usually somethi ...

pairs of metals. One example is Fe₂(SCH₃)₂(CO)₆. Thiolate ligands, especially when nonbridging, are susceptible to attack by electrophiles including acids,

alkylating agent Alkylation is the transfer of an alkyl group from one molecule to another. The alkyl group may be transferred as an alkyl carbocation, a free radical, a carbanion, or a carbene (or their equivalents). Alkylating agents are reagents for effecting al ...

s, and oxidants. Fe2 SMe 2 CO 6

Occurrence and applications

Metal thiolate functionality is pervasive in

metalloenzyme Metalloprotein is a generic term for a protein that contains a metal ion cofactor. A large proportion of all proteins are part of this category. For instance, at least 1000 human proteins (out of ~20,000) contain zinc-binding protein domains al ...

Iron-sulfur proteins Iron–sulfur proteins (or iron–sulphur proteins in British spelling) are proteins characterized by the presence of iron–sulfur clusters containing sulfide-linked di-, tri-, and tetrairon centers in variable oxidation states. Iron–sulfur clu ...

blue copper protein Copper proteins are proteins that contain one or more copper ions as prosthetic groups. Copper proteins are found in all forms of air-breathing life. These proteins are usually associated with electron-transfer with or without the involvement o ...

s, and the zinc-containing enzyme

liver alcohol dehydrogenase Alcohol dehydrogenases (ADH) () are a group of dehydrogenase enzymes that occur in many organisms and facilitate the interconversion between alcohols and aldehydes or ketones with the reduction of nicotinamide adenine dinucleotide (NAD+) to N ...

feature thiolate ligands. Commonly thiolate is ligand is provided from the

residue. All molybdoproteins feature thiolates in the form of cysteinyl and/or

molybdopterin Molybdopterins are a class of cofactors found in most molybdenum-containing and all tungsten-containing enzymes. Synonyms for molybdopterin are: MPT and pyranopterin-dithiolate. The nomenclature for this biomolecule can be confusing: Molybdopter ...

.S. J. Lippard, J. M. Berg “Principles of Bioinorganic Chemistry” University Science Books: Mill Valley, CA; 1994. . The copper site in plastocyanin features two imidazole,_a_thioether,_and_a_thiolate_ligand..html" ;"title="thioether.html" ;"title="imidazole, a thioether">imidazole, a thioether, and a thiolate ligand.">thioether.html" ;"title="imidazole, a thioether">imidazole, a thioether, and a thiolate ligand.

References

Biochemistry Inorganic chemistry Coordination complexes {{Coordination complexes