The cyaphide anion $\ce{(CP)-}$ should be chemically similar to its nitrogen analogue cyanide. $\ce{(CN-})$ But unlike it's counterpart it readily polymerizes so it has only been found in outer space naturally. However, it should form compounds that should be the same like cyanides.For example sodium cyanide ($\ce{NaCN}$),would be called "sodium cyaphide" ($\ce{NaCP}$) and hydrogen cyanide ($\ce{HCN}$) would be called hydrogen cyaphide or as it's called methylidynephosphane ($\ce{HCP}$) and the anions: phosphaethynolate (cyaphate) ($\ce{PCO-}$) and thiophosphaethyonlate (thiocyaphate), ($\ce{PCS-}$) and also potentially the cyaphogen ($\ce{P2C2}$) have been observed but as far as I know the only reliable info I found on cyaphides were on Wikipedia and they were very short as well. Which source on information do I go with?

More information: https://en.wikipedia.org/wiki/Methylidynephosphane

  • $\begingroup$ Also I'm basing the nomenclature from the PCO-/PCS- anions from the cyanate and thiocyanate anions $\endgroup$ Commented May 18, 2021 at 3:18
  • 1
    $\begingroup$ en.wikipedia.org/wiki/Cyaphide $\endgroup$
    – Mithoron
    Commented May 18, 2021 at 13:59

1 Answer 1


Methylidynephosphane or phosphaethyne is known and it has been achieved in 1961 by Thurman Gier. He passed 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 phosphaethyne along with acetylene and ethylene.

$$\ce{PH3 ->[electric arc][carbon electrodes] HCP + H2C=CH2 + HC#CH}$$

The ions $\ce{PCO-}$ and $\ce{PCS-}$ are also known. From the abstract of this paper1:

A range of λ3-phosphaalkynes $\ce{P⋮C−R}$ $\ce{(R = F, Cl, O-, OLi, OH,}$ $\ce{OSiMe3, S-}$, $\ce{SLi}$, $\ce{SH, Me,}$ $\ce{SiMe3, H)}$ has been geometry-optimized at the HF, MP2, and CCSD(T) level of theory. Vibrational frequencies as well as 31P and 13C NMR chemical shifts were calculated and compared to experimental values of the large variety of λ3-phosphaalkynes prepared over several years via differing synthetic routes. With a few obvious exceptions, a good agreement of calculated and experimental parameters was found. This also opens up the possibility to make predictions on spectroscopic data of compounds not yet synthesized or fully characterized, e.g., $\ce{PCOSiMe3}$ and $\ce{PCSH}$. The bonding situation especially of the negatively charged species $\ce{P⋮C−O-}$ and $\ce{P⋮C−S-}$ was studied via NBO analyses to determine the degree of phosphaalkyne and heterocumulene contributions to the mesomeric anions. $$\ce{^{-}X-C#P <-> X=C=P- (X=O,S)}$$

Cyaphogen ($\ce{(CP)2}$) has not been synthesized yet. It is considered a pseudohalogen. Structurally, it is formulated as $\ce{P#C-C#P}$.

Currently, no binary cyaphides are known but numerous organometallic cyaphide complexes are known. Some of them are listed in the Wikipedia article.


  1. Theoretical Studies of NMR Chemical Shifts and Vibrational Frequencies in λ3-Phosphaalkynes P⋮C−R, Klaus Hübler and Peter Schwerdtfeger, Inorganic Chemistry 1999 38 (1), 157-164, DOI: 10.1021/ic9811291

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