Thiophosphate

Thiophosphates (or phosphorothioates, PS) are chemical compounds and anions with the general chemical formula PS
_4−xO³⁻
_x (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.^[1]

Organic

Organothiophosphates are a subclass of

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).^[3]

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 formula [H_nPS_4−xO_x]⁽³⁻ⁿ⁾⁻ (x = 0, 1, 2, or 3 and (n = 1, 2, or 3).

Monothiophosphate

Monothiophosphate is the anion [PO₃S]³⁻, which has C_3v

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

A number of these anions known. Some have attracted interest as components in

• PS⁻
  ₃ is the analogue of the nitrate ion, NO⁻
  ₃ (there is no PO⁻
  ₃ analogue); it was isolated as the yellow tetraphenylarsonium salt^[8]
• PS³⁻
  ₄ is the sulfur analogue of PO³⁻
  ₄, and like PO³⁻
  ₄ is tetrahedral.
• P
  ₂S⁴⁻
  ₇ the pyrothiophosphate ion consisting of two corner sharing PS₄ tetrahedra, analogous to the pyrophosphates.^[9]
• P
  ₂S⁴⁻
  ₁₀ 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−.^[10]
• P
  ₂S²⁻
  ₆ edge-shared bitetrahedral structure. The structure is therefore similar to the isoelectronic Al₂Cl₆ dimer. The oxygen analogue, dimetaphosphate P
  ₂O²⁻
  ₆, in contrast, is not known, the metaphosphates favour polymeric structures of chains or rings.^[9]
• P
  ₂S²⁻
  ₈ and P
  ₂S²⁻
  ₁₀ are related to P
  ₂S²⁻
  ₆ but their two bridging −S− atoms are replaced by −S−S− in P
  ₂S²⁻
  ₈ and by an −S−S−S− bridge in P
  ₂S²⁻
  ₁₀.^[11]
• P₂S⁴⁻
  ₆ These form water-stable salts.^[12] 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, P
  ₂O⁴⁻
  ₆.
• P
  ₃S³⁻
  ₉ contains a six-membered P₃S₃ ring. The ammonium salt is produced by reaction of P₄S₁₀ in liquid ammonia.^[13] Another way of visualising the structure is that it is the P₄S₁₀ adamantane (P₄O₁₀) structure with a PS³⁺ vertex removed.
• P
  ₄S⁴⁻
  ₈ contains a square P₄ ring,^[8] P
  ₅S⁵⁻
  ₁₀ contains a P₅ ring and P
  ₆S⁶⁻
  ₁₂ a P₆ ring.^[11] 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 P
  ₆S⁶⁻
  ₁₂ anion is analogous to the P
  ₆O⁶⁻
  ₁₂ ring anion.^[14]
• P
  ₄S²⁻
  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.^[15]
• P
  ₇S³⁻
  ₃ is a sulfido heptaphosphane cluster anion.^[8]

References