A two step formal insertion of 1,1,1,5,5,5-hexafluoro- (1) and l,l,l - trifluoropentane-2,4-dione (2) into the P-H bonds of phosphane gave the primary a-hydroxyphosphanes. precursors for 2-phospha-6-oxa-9- oxabicyclo[3.3.1]-nonane and 2,4,8-trioxa-6-phospha-adamantane, both formed diastereospecifically. The molecular structures of the two latter compounds were established by single-crystal X-ray structure analysis. Compound 1 reacted diastereospecifically with phosphonous acid dichlorides, RPCl2 (R = Me, Et, iPr, tBu, Me3SiCH2, PhCH2, Ph) to give in a concerted mechanism thermally stable tricyclicλ5σ5 P phosphoranes containing two five- and one six-membered ring. In one case hydrolysis gave 3,5-dihydroxy-2-oxo-1,2λ5σ4-oxaphospholane, whereas methanol added to the double bond in the six-membered ring furnishing two isomeric phosphoranes. When 2 was reacted withRPCl2 (R = Et, Me3SiCH2, PhCH2. Ph), diastereomerically pure regioisomeric phosphoranes were obtained. The solid state molecular structures of three λ5σ5P species exhibited two oxygen atoms in the axial position of a slightly distorted trigonal-bipyramidal geometry at phosphorus.