text: Secondary (2°) phosphines, with the formula RPH, are prepared analogously to the primary phosphines.  They are also obtained by alkali-metal reductive cleavage of triarylphosphines followed by hydrolysis of the resulting phosphide salt.  The latter route is employed to prepare diphenylphosphine (PhPH).  Diorganophosphinic acids, RP(O)OH, can also be reduced with diisobutylaluminium hydride.  Secondary phosphines are typically protic in character. 
  But when modified with suitable substituents, as in certain (rare) diazaphospholenes (scheme 3), the polarity of the P-H bond can be inverted (see: umpolung) and the resulting phosphine hydride can reduce a carbonyl group as in the example of benzophenone in yet another way.
  Secondary phosphines occur in cyclic forms. Three-membered rings are phosphiranes (unsaturated: phosphirenes), five-membered rings are phospholanes (unsaturated: phosphole), and six-membered rings are phosphinanes.