text: The carbon-phosphorus triple bond in phosphaalkynes represents an exception to the so-called "double bond rule", which would suggest that phosphorus tends not to form multiple bonds to carbon, and the nature of bonding within phosphaalkynes has therefore attracted much interest from synthetic and theoretical chemists. For simple phosphaalkynes such as H-C≡P and Me-C≡P, the carbon-phosphorus bond length is known by microwave spectroscopy, and for certain more complex phosphaalkynes, these bond lengths are known from single-crystal X-ray diffraction experiments. These bond lengths can be compared to the theoretical bond length for a carbon-phosphorus triple bond predicted by Pekka Pyykkö of 1.54 Å. By bond length metrics, most structurally characterized alkyl and aryl substituted phosphaalkynes contain triple bonds between carbon and phosphorus, as their bond lengths are either equal to or less than the theoretical bond distance.
  The carbon-phosphorus bond order in phosphaalkynes has also been the subject of computational inquiry, where quantum chemical calculations have been utilized to determine the nature of bonding in these molecules from first principles. In this context, natural bond orbital (NBO) theory has provided valuable insight into the bonding within these molecules. Lucas and coworkers have investigated the electronic structure of various substituted phosphaalkynes, including the cyaphide anion (C≡P), using NBO, natural resonance theory (NRT), and quantum theory of atoms in molecules (QTAIM) in an attempt to better describe the bonding in these molecules. For the simplest systems, C≡P and H-C≡P, NBO analysis suggests that the only relevant resonance structure is that in which there is a triple bond between carbon and phosphorus. For more complex molecules, such as Me-C≡P and (Me)C-C≡P, the triple bonded resonance structure is still the most relevant, but accounts for only some of the overall electron density within the molecule (81.5% and 72.1%, respectively). This is due to interactions between the two carbon-phosphorus pi-bonds and the C-H or C-C sigma-bonds of the substituents, which can be visualized by inspecting the C-P pi-bonding molecular orbitals in these molecules.