I previously explored stabilized “carbenes” with the formal structures (R_{2}N)_{2}C:, concluding that perhaps the alternative ionic representation R_{2}N^{+}=C^{–}NR_{2} might reflect their structures better. Here I take a broader look at the “carbene” landscape before asking the question “what about nitrenes?”

The top row shows the compounds for which no crystal structure could be found.^{‡} This includes the traditional carbon-substituted unstabilized carbenes, as well as those substituted with either group 4A or 6A elements (Si, S, etc). Isolated hits **were** however found as follows for other combinations (all interesting, but I do not discuss them here).

- R
_{2}N-C-CR_{3 }[cite]10.5517/CC12GP8H[/cite],[cite]10.1016/j.jorganchem.2014.05.036[/cite],[cite]10.5517/CC7YK1R[/cite],[cite]10.1021/ja047503f[/cite],[cite]10.5517/CCPVPSZ[/cite],[cite]10.1073/pnas.0705809104[/cite] - R
_{2}P-C-Si [cite]10.5517/CCP00F3[/cite],[cite]10.1016/j.ica.2007.02.050[/cite] - R
_{2}P-C-PR_{2}[cite]10.5517/CC8GST9[/cite],[cite]10.1002/anie.200462239[/cite] - R
_{2}N-C-OR [cite]10.1021/ja9819312[/cite] - R
_{2}N-C-SR [cite]10.1002/jlac.199719970213[/cite]

At this point I turned to nitrenes. As with unstabilised carbenes, the nitrogen is described as having one covalent bond, one unshared spin-paired lone pair of electrons and two further unpaired electrons to give a total valence shell count of six (and in fact a triplet spin state). Are there any examples? Just one, formally corresponding to R_{2}P-N (DEGSEP[cite]10.5517/CCYPH14[/cite]). To explore what the nature of the single P-N bond is, I did a search for the P-N bond lengths of all P-N compounds in the CSD (of any bond type). The distribution shows ~1.48Å as the shortest and ~1.8Å as the longest.There is no sign of a multimodal distribution indicating partitioning into *e.g.* single, double or triple bonds.

So what about our nitrene? The P-N bond is 1.456Å, which is very much at the short end of the spectrum above, and so pretty far from the formal simple definition of a nitrene given above. So now for a ωB97XD/Def2-TZVPP calculation of the singlet wavefunction[cite]10.14469/hpc/1625[/cite] (the triplet state is 22 kcal/mol higher in energy[cite]10.14469/hpc/1626[/cite]) for a model compound with calculated CN distance 1.493Å and from which an ELF-based localisation and integration of the electron basins can be derived. The total basin integration for the N can be taken as 7.77e (close to the octet) if basins 15 and 16 are assumed to be shared (covalent) and this gives the P-N bond double bond character. If basins 15 and 16 are not included (and taken as localised just on the P), the N has 5.81e and an associated single bond.

A search of the CSD specifying P≡N as the search query returns lengths in the range 1.47-1.56Å, which our example certainly conforms to. So perhaps we can tentatively conclude that the only example thus far reported of a crystalline nitrene in fact sustains a very short bond to the nitrogen. It could be considered as the N having a filled octet in its valence shell, and certainly a bond order higher than one, if not actually triple.

^{‡}For the search query, see [cite]10.14469/hpc/1624[/cite]

Although I drew a blank for R

_{3}C-C-CR_{3}, there are two examples of R_{3}C-Si-CR_{3}; 10.5517/CC10Q5SP and 10.5517/CC4L999 which could fairly be said to be true silyenes.