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In chemistry, a phosphaalkyne (IUPAC name: alkylidynephosphane) is an organophosphorus compound containing a triple bond between phosphorus and carbon with the general 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, 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 ((SiMe3)2CRPCl2) 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. tertiary alkyl, secondary 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).

The pi bonds of phosphaalkynes are weaker than most carbon phosphorus sigma bonds, rendering phosphaalkynes reactive with respect to the formation of oligomeric species containing more sigma bonds. These oligomerization reactions are triggered thermally, or can be catalyzed by transition or main group metals. thumb|311x311px|Synthesis of a cuboidal phosphaalkyne tetramer by heating a kinetically stable phosphaalkyne.

Phosphaalkynes with small substituents (H, F, Me, Ph, etc.) undergo decomposition at or below room temperature by way of polymerization/oligimerization to yield mixtures of products which are challenging to characterize. The same is largely true of kinetically stable phosphaalkynes, which undergo oligomerization reactions at elevated temperature. In spite of the challenges associated with isolating and identifying the products of these oligimerizations, however, cuboidal tetramers of tert butylphosphaalkyne and tert pentylphosphaalkyne have been isolated (albeit in low yield) and identified following heating of the respective phosphaalkyne. Computational chemistry has proved a valuable tool for studying these synthetically complex reactions, and it has been shown that while the formation of phosphaalkyne dimers is thermodynamically favorable, the formation of trimers, tetramers, and higher order oligomeric species tends to be more favorable, accounting for the generation of intractable mixtures upon inducing oligomerization of phosphaalkynes experimentally.

Unlike thermally initiated phosphaalkyne oligomerization reactions, transition metals and main group metals are capable of oligomerizing phosphaalkynes in a controlled manner, and have led to the isolation of phosphaalkyne dimers, trimers, tetramers, pentamers, and even hexamers. center|thumb|569x569px|Some of the reported phosphaalkyne oligomers generated upon treatment of a phosphaalkyne (usually tBu C≡P) with a transition metal or main group metal complex. Note that several of these species are unstable in their free forms, and instead exist stably only when bound to a transition metal. In this figure, the • symbols individually represent one C R unit, and are utilized for clarity.