Novel Syntheses and Reactions of Phosphetanium Salts with a Planar C -C -C -P SkeletonCleavage of the P -Si bond of silylphosphanes R2P -SiMe3 (R = iPr, tBu, 2,4,6-Me3C6H2) with excess 1,3-dihalogenpropan%X -[CH2l3 -X (X = Br, I) affords phosphetanium salts [R,P -[CH213]+X-(3) in good yields. These strained four-membered ring systems are also obtained by alkylation of secondary phosphanes R2PH with X -[CH213 -X followed by deprotonation of