N-Trimethylsilyl-1 -methylphospholane imine has been prepared by the Staudinger reaction of 1-methylphospholane with trimethylsilyl azide. Transsilylation reactions of the phosphine imine with dichloro or dibromodimethylsilane lead to dimeric N-(halogenodimethylsilyl)-l-methylphospholane imines with an ionic type of structure, containing a dicationic four-membered Si-N ring system. Upon heating, the chloro compound undergoes a reversible isomerization to give the corresponding covalent monomer.