Thiophosphate

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-xR_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

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 formul [H_nPS_4−xO_x]^(3-n)-.

Monothiophosphate

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.

PxSy: binary thiophosphates and polyphosphates

There are a number of these anions known. There has been interest in compounds containing these anions due to their potential application as fast ion conductors for use in solid state batteries. The binary thiophosphates do not exhibit the extensive diversity of the analogous P-O anions 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 thations 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.

PS−
3 is the analogue of the nitrate ion, NO−
3 (there is no PO−
3 analogue)- it was isolated as the tetraphenylarsonium salt

PS₄ ³⁻ is the sulfur analogue of PO3−
4, and like PO3−
4 is tetrahedral.

P₂S4−
7 the pyrothiophosphate ion consisting of two corner sharing PS₄ tetrahedra, analogous to the pyrophosphates.

P₂S4−
10 An ion which can be visualised either as two PS₄ tetrahedra joined by a disulfide link or a pyro-thiophosphate where the bridging -S- is replaced by -S₄-.

P₂S2−
6 with a structure consisting of two edge sharing tetrahedra. The structure is therefore similar to the isoelectronic Al₂Cl₆ dimer. The oxygen analogue, dimetaphosphate P₂O₆ ²⁻, in contrast, is not known, the metaphosphates favour polymeric structures of chains or rings.

P₂S2−
8 and P₂S2−
10 are related to P₂S2−
6 but their two bridging -S- atoms are replaced by -S-S- in P₂S2−
8 and by an -S-S-S- bridge in P₂S2−
10.

P₂S4−
6 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, P₂O4−
6.

P₃S3−
9 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.

P₄S4−
8 contains a square P₄ ring, P₅S5−
10 contains a P₅ ring and P₆S6−
12 a P₆ ring. These (PS−
2)_n 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 P₆S6−
12 anion is analogous to the P₆O6−
12 ring anion.

P₄S−
2 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.

P₇(S)3−
3 is a sulfido heptaphosphane cluster anion.