Phosphinous acids

Phosphinous acids are usually organophosphorus compounds with the formula R₂POH. They are pyramidal in structure. Phosphorus is in the oxidation state III. Most phosphinous acids rapidly convert to the corresponding phosphine oxide, which are tetrahedral and are assigned oxidation state V.

Synthesis

Only one example is known, bis(trifluoromethyl)phosphinous acid, (CF₃)₂POH.^[1] It is prepared in several steps from phosphorus trichloride (Et = ethyl):^[2]

PCl₃ + 2 Et₂NH → PCl₂NEt₂ + Et₂NH₂Cl

2 P(NEt₂)₃ + PCl₂NEt₂ + 2 CF₃Br → P(CF₃)₂NEt₂ + 2 BrClP(NEt₂)₃

P(CF₃)₂NEt₂ + H₂O → P(CF₃)₂OH + HNEt₂

Structure of Mo(CO)₅P(OH)₃.^[3]

Reactions

With the lone exception of the bis(trifluoromethyl) derivative, the dominant reaction of phosphinous acids is tautomerization:

PR₂OH → OPR₂H

Even the pentafluorophenyl compound P(C₆F₅)₂OH is unstable with respect to the phosphine oxide.^[2]

Although phosphinous acids are rare, their P-bonded coordination complexes are well established, e.g. Mo(CO)₅P(OH)₃.^[3]

Secondary and primary phosphine oxides

Tertiary phosphine oxides, compounds with the formula R₃PO cannot tautomerize. The situation is different for the secondary and primary phosphine oxides, with the respective formulas R₂(H)PO and R(H)₂PO.^[4]

• Secondary and primary phosphine oxides
• 
  diethylphosphite
• 
  phosphorous acid
• 
  Diphenylphosphine oxide
• 
  Hypophosphorous acid

References

1. Griffiths, James E.; Burg, Anton B. (1960). "The Phosphinous Acid (CF₃)₂POH and the Diphosphoxane (CF₃)₂POP(CF₃)₂". Journal of the American Chemical Society. 82 (6): 1507–1508. doi:10.1021/ja01491a062.
2. Hoge, Berthold; Neufeind, Stefan; Hettel, Sonja; Wiebe, Waldemar; Thösen, Christoph (2005). "Stable Phosphinous Acids". Journal of Organometallic Chemistry. 690 (10): 2382–2387. doi:10.1016/j.jorganchem.2004.09.041.
3. Xi, Chanjuan; Liu, Yuzhou; Lai, Chunbo; Zhou, Lishan (2004). "Synthesis of molybdenum complex with novel P(OH)₃ Ligand based on the One-Pot Reaction of Mo(CO)₆ with HP(O)(OEt)₂ and Water". Inorganic Chemistry Communications. 7 (11): 1202–1204. doi:10.1016/j.inoche.2004.09.012.
4. Yoshifuji, Masaaki; Shibayama, Katsuhiro; Toyota, Kozo; Inamoto, Naoki (1983). "Preparation and characterization of sterically protected primary phosphine sulfide and oxide". Tetrahedron Letters. 24 (39): 4227–4228. doi:10.1016/S0040-4039(00)88307-0.