Thiophosphate

Template:Short description Thiophosphates (or phosphorothioates, PS) are chemical compounds and anions with the general chemical formula Template:Chem/link (x = 0, 1, 2, or 3) and related derivatives where organic groups are attached to one or more O or S. Thiophosphates feature tetrahedral phosphorus(V) centers.

Organic

Organothiophosphates are a subclass of organophosphorus compounds that are structurally related to the inorganic thiophosphates. Common members have formulas of the type (RO)_3−x(RS)_xPS and related compounds where RO is replaced by RS. Many of these compounds are used as insecticides, some have medical applications, and some have been used as oil additives.

Oligonucleotide phosphorothioates (OPS) are modified oligonucleotides where one of the oxygen atoms in the phosphate moiety is replaced by sulfur. They are the basis of antisense therapy, e.g., the drugs fomivirsen (Vitravene), oblimersen, alicaforsen, and mipomersen (Kynamro).

Inorganic

File:ThiophosphateVarietyPack.png

Structures of selected thiophosphates.

The simplest thiophosphates have the formula [PS_4−xO_x]³⁻. These trianions are only observed at very high pH, instead they exist in protonated form with the formula [H_nPS_4−xO_x]⁽³⁻ⁿ⁾⁻ (x = 0, 1, 2, or 3 and (n = 1, 2, or 3).

Monothiophosphate

File:Thiophosphate-ion-3D-balls.png

Ball-and-stick model of the hypothetical monothiophosphate trianion.

Monothiophosphate is the anion [PO₃S]³⁻, which has C_3v symmetry. A common salt is sodium monothiophosphate (Na₃PO₃S). Monothiophosphate is used in research as an analogue of phosphate in biochemistry. Monothiophosphate esters are biochemical reagents used in the study of transcription, substitution interference assays. Sometimes, "monothiophosphate" refers to esters such as (CH₃O)₂POS⁻.

Dithiophosphates

Dithiophosphate has the formula [PO₂S₂]³⁻, which has C_2v symmetry. Sodium dithiophosphate, which is colorless, is the major product from the reaction of phosphorus pentasulfide with NaOH:

P₂S₅ + 6 NaOH → 2 Na₃PO₂S₂ + H₂S + 2 H₂O

Dithiophosphoric acid is obtained by treatment of barium dithiophosphate with sulfuric acid:

Ba₃(PO₂S₂)₂ + 3 H₂SO₄ → 3 BaSO₄ + 2 H₃PO₂S₂

Both Na₃PO₂S₂ and especially H₃PO₂S₂ are prone toward hydrolysis to their monothio derivatives.

Tri- and tetrathiophosphates

Trithiophosphate is the anion [POS₃]³⁻, which has C_3v symmetry. Tetrathiophosphate is the anion [PS₄]³⁻, which has T_d symmetry.

P_xS_y: binary thiophosphates and polyphosphates

File:PS3-dimerization.png

Dimerization equilibrium observed for PS₃⁻.

A number of these anions known. Some have attracted interest as components in fast ion conductors for use in solid state batteries. The binary thiophosphates do not exhibit the extensive diversity of the analogous oxyanions but contain similar structural features, for example P is 4 coordinate, P−S−P links form and there are P−P bonds. One difference is that ions may include polysulfide fragments of 2 or more S atoms whereas in the P−O anions there is only the reactive −O−O−, peroxo, unit.

• Template:Chem/link is the analogue of the nitrate ion, Template:Chem/link (there is no Template:Chem/link analogue); it was isolated as the yellow tetraphenylarsonium salt
• Template:Chem/link is the sulfur analogue of Template:Chem/link, and like Template:Chem/link is tetrahedral.
• Template:Chem/link the pyrothiophosphate ion consisting of two corner sharing PS₄ tetrahedra, analogous to the pyrophosphates.
• Template:Chem/link An ion which can be visualised either as two PS₄ tetrahedra joined by a disulfide link or a pyrothiophosphate where the bridging −S− is replaced by −S₄−.
• Template:Chem/link edge-shared bitetrahedral structure. The structure is therefore similar to the isoelectronic Al₂Cl₆ dimer. The oxygen analogue, dimetaphosphate Template:Chem/link, in contrast, is not known, the metaphosphates favour polymeric structures of chains or rings.
• Template:Chem/link and Template:Chem/link are related to Template:Chem/link but their two bridging −S− atoms are replaced by −S−S− in Template:Chem/link and by an −S−S−S− bridge in Template:Chem/link.
• P₂S These form water-stable salts. The anion has an ethane-like structure and contains a P−P bond. The formal oxidation state of phosphorus is +4. The oxygen analogue is the hypodiphosphate anion, [[hypophosphoric acid|Template:Chem/link]].
• Template:Chem/link contains a six-membered P₃S₃ ring. The ammonium salt is produced by reaction of P₄S₁₀ in liquid ammonia. Another way of visualising the structure is that it is the P₄S₁₀ adamantane (P₄O₁₀) structure with a PS³⁺ vertex removed.
• Template:Chem/link contains a square P₄ ring, Template:Chem/link contains a P₅ ring and Template:Chem/link a P₆ ring. These Template:Chem/link cyclic anions contain P with an oxidation state +3. Note they are not trigonal as arsenic(III) is in arsenites, but are tetrahedral with two bonds to other phosphorus atoms and two to sulfur. The Template:Chem/link anion is analogous to the Template:Chem/link ring anion.
• Template:Chem/link An unusual butterfly-shaped ion, SP(P₂)PS, which can be visualised as a P₄ molecule where two P−S bonds replace one P−P bond.
• Template:Chem/link is a sulfido heptaphosphane cluster anion.