In chemistry, phosphaalkynes (IUPAC name: alkylidynephosphanes) are organophosphorus compounds that have a phosphorus-carbon triple bond.
Two types of phosphaalkynes are recognized. One type of phosphaalkyne is a heavier analogue of nitriles (R-C≡N), i.e. P replaces N in a nitrile. Another type of phosphaalkyne feature pentavalent, three coordinate phosphorus. Such species can also be described as ylides or phosphinocarbenes. The phosphorus version of the isonitriles R-P+≡C−, which would be called isophosphaalkynes, has not been observed.
In 1950, H. Albers reported the first indication of the existence of the parent compound of phosphaalkynes (type A), H-C≡P. This compound was identified by infrared absorption spectrometry, and its synthesis was improved by Manfred Regitz in 1987. The synthesis of the first kinetically stable phosphaalkyne, which has a tert-butyl group as a substituent R (tert-Butylphosphaacetylene), was reported in 1981 by Gerd Becker and Werner Uhl. These phosphaalkynes undergo 1,2-addition reactions and cycloadditions .
In 2000, Guy Bertrand reported the first structure of the type B phosphaalkyne. Its P-C-R bond angle is 152.6 degrees, so this type of phosphaalkyne may be best described by a phosphorus vinyl ylide structure (B2).
Cyaphide ion
The cyaphide ion P≡C− as the phosphorus cyanide cousin is not known as a salt and only observed in the gas phase. In silico measurements reveal that the -1 charge in this ion is location mainly on carbon (0.65). On the other hand, the molecule does exist as a terminal ligand in the ruthenium transition metal complex trans-[(dppe)2Ru(H)(C≡P)], which is stabilized by dppe. Recently, the synthesis and electronic structure of the first cyaphide-alkynyl complexes has been reported. The novel complexes trans-[Ru(dppe)2(C≡CR)(C≡P)] (R = CO2Me, C6H4OMe) are the first complexes to incorporate cyaphide as part of a conjugated system.