Phosphaethene, the unstable parent compound in the phosphaalkenes series, has been thoroughly investigated using theoretical calculations. For the first time, its electronic structure is evidenced from its photoelectron spectrum. The P E spectra of phosphaethene and 1 -phosphapropene, generated in the gas phase by the respective vacuum dehydrochlorination of chloromethylphosphine and I-chloroethylphosphine, have been recorded in situ by the use of a vacuum device directly coupled to the spectrophotometer. T h e vertical rW and np ionization energies are observed respectively a t 10.30 and 10.70 eV for phosphaethene and a t 9.75 and 10.35 eV for I-phosphapropene. The band's characterizations are discussed through correlations with the corresponding imines and with substituted phosphaalkenes and are confirmed by a b initio theoretical calculations.T h e results indicate a rather significant energetic separation between the two first rWc and np ionic states.

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Primary and secondary alpha-chlorophosphines 2a-g are prepared in ca. 70% yield by chemoselective reduction of the corresponding phosphonic and phosphinic esters with AlHCl(2) and are characterized by (31)P, (13)C, and (1)H NMR and by HMRS. They can be kept several weeks in the refrigerator after purification. They lead then to the corresponding phosphaalkenes 3a-g by HCl elimination. For the volatile alpha-chlorophosphines 2a-e HCl elimination occurs in the gas phase on solid potassium carbonate under VGSR conditions (vacuum gas-solid reactions); the corresponding phosphaalkenes 3a-e are characterized by real time HRMS analysis of the gaseous flow (VGSR/HRMS coupling) and by solid-phase IR spectroscopy after condensation of the gaseous flow on a KBr window cooled to 77 K. The decomposition of phosphaalkenes at this temperature is monitored by IR spectroscopy. The alpha-chlorophosphines 2a-g undergo a HCl elimination in the liquid phase in the presence of a Lewis base; the formation of the transient phosphaalkenes is monitored by (31)P FT-NMR. The temperature of HCl elimination is dependent both upon the P-H acidity of the phosphine precursors and the nature of the base. The (31)P NMR data of the simple phosphaalkenes 3a-g are for the first time reported. They are consistent with the proposed structure. The stereochemistry of the (Z)- and (E)-isomers is established according to the "cis-rule". Phosphaalkenes 3a-g are also characterized by chemical trapping in solution with various dienes, dipoles, or thiols. All of these experiments confirm the transient character of these species. The synthetic potential of this route is evaluated.