In chemistry, a phosphaalkyne (IUPAC name: alkylidynephosphane) is an organophosphorus compound containing a triple bond between phosphorus and carbon with the general chemical formula R−C≡P. Phosphaalkynes are the heavier congeners of nitriles, though, due to the similar electronegativities of phosphorus and carbon, possess reactivity patterns reminiscent of alkynes. Due to their high reactivity, phosphaalkynes are not found naturally on earth, but the simplest phosphaalkyne, phosphaethyne (H−C≡P) has been observed in the interstellar medium.

The first of preparation of a phosphaalkyne was achieved in 1961 when Thurman Gier produced phosphaethyne by passing phosphine gas at low pressure over an electric arc produced between two carbon electrodes. Condensation of the gaseous products in a −196 °C (−321 °F) trap revealed that the reaction had produced acetylene, ethylene, phosphaethyne, which was identified by infrared spectroscopy.

Following the initial synthesis of phosphaethyne, it was realized that the same compound can be prepared more expeditiously via the flash pyrolysis of methyldichlorophosphine (CH₃PCl₂), resulting in the loss of two equivalents of hydrogen chloride. This methodology has been utilized to synthesize numerous substituted phosphaalkynes, including the methyl, vinyl, chloride, and fluoride derivatives. Fluoromethylidynephosphane (F−C≡P) can also be prepared via the potassium hydroxide promoted dehydrofluorination of trifluoromethylphosphine (CF₃PH₂). It is speculated that these reactions generally proceed via an intermediate phosphaethylene with general structure RClC=PH. This hypothesis has found experimental support in the observation of F₂C=PH by ³¹P NMR spectroscopy during the synthesis of F−C≡P.

The high strength of silicon–halogen bonds can be leveraged toward the synthesis of phosphaalkynes. Heating bis-trimethylsilylated methyldichlorophosphines (((CH₃)₃Si)₂C(R)−PCl₂) under vacuum results in the expulsion of two equivalents of chlorotrimethylsilane and the ultimate formation of a new phosphaalkyne. This synthetic strategy has been applied in the synthesis of 2-phenylphosphaacetylene and 2-trimethylsilylphosphaacetylene. As in the case of synthetic routes reliant upon the elimination of a hydrogen halide, this route is suspected to involve an intermediate phosphaethylene species containing a C=P double bond, though such a species has not yet been observed.

Like the preceding method, the most popular method for synthesizing phosphaalkynes is reliant upon the expulsion of products containing strong silicon-element bonds. Specifically, it is possible to synthesize phosphaalkynes via the elimination of hexamethyldisiloxane (HMDSO) from certain silylated phosphaalkenes with the general structure RO−(Me₃Si)C=P−SiMe₃. These phosphaalkenes are formed rapidly following the synthesis of the appropriate acyl bis-trimethylsilylphosphine, which undergoes a rapid [1,3]-silyl shift to produce the relevant phosphaalkene. This synthetic strategy is particularly appealing because the precursors (an acyl chloride and tris-trimethylsilylphosphine or bis-trimethylsilylphosphide) are either readily available or simple to synthesize.

This method has been utilized to produce a variety of kinetically stable phosphaalkynes, including aryl, tertiary alkyl, secondary alkyl, and even primary alkyl phosphaalkynes in good yields.

Dihalophospaalkenes of the general form R−P=CX₂, where X is Cl, Br, or I, undergo lithium-halogen exchange with organolithium reagents to yield intermediates of the form R−P=CXLi. These species then eject the corresponding lithium halide salt, LiX, to putatively give a phospha-isocyanide, which can rearrange, much in the same way as an isocyanide, to yield the corresponding phosphaalkyne. This rearrangement has been evaluated using the tools of computational chemistry, which has shown that this isomerization process should proceed very rapidly, in line with current experimental evidence showing that phosphaisonitriles are unobservable intermediates, even at −85 °C (−121 °C).

It has been demonstrated by Cummins and coworkers that thermolysis of compounds of the general form C₁₄H₁₀PC(=PPh₃)R leads to the extrusion of C₁₄H₁₀ (anthracene), triphenylphosphine, and the corresponding substituted phosphaacetylene: R−C≡P. Unlike the previous method, which derives the phosphaalkyne substituent from an acyl chloride, this method derives the substituent from a Wittig reagent.