In studies of the reactivity and thermal stability of various alkylidenephos-phines prepared from benzophenone and organylbis(trimethylsilyl)phosphines via a NaOH-catalyzed elimination of hexamethyldisiloxane (eq. la), detailed analyses of nmr-spectra and x-ray structure determinations prove the methyl derivative la to dimerize to an 1,3-diphosphetane (2a), whereas from the iso-propyl compound 1b the 1,2-diphosphetane 2b is obtained [1]. This observation confirms our hypothesis that the formation of 1,2- or 1,3-diphosphetanes is determined by the steric requirements of the substituent at phosphorus
