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Azide is a fairly common ion, but why aren’t there similar versions such as $\ce{P3-}$ or $\ce{B3+}$? Do they exist, is there something wrong with these ions as to make them more likely to just break apart rather than for ionic compounds.

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Compare the covalent radii of $\ce{N}$ ($75\,\mathrm{pm}$) and $\ce{P}$ ($106\,\mathrm{pm}$).

Phosphorus is a bit on the chubby side. Larger atom-to-atom distance means less overlap of p-orbitals in multiple bonds: $\ce{P=P}$ and $\ce{P#P}$ bonds are much weaker than similar bonds between nitrogen atoms.

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  • $\begingroup$ Yet boron is still fairly small, barely 7 pm larger, or is this significant in atoms? $\endgroup$ – tox123 Apr 3 '15 at 20:40
  • $\begingroup$ Boron is electron deficient so while p orbital overlap is okay it would rather form clusters than linear catenated cations.You can make molecular compounds with boron-boron double and triple bonds but they are generally really unstable. $\endgroup$ – J. LS May 3 '15 at 14:11
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Nitrogen is special in that its pi bonds are stronger than its sigma bonds. This explains why organic compounds containing nitrogen connected with only single bonds or non-resonant double bonds (especially -N=N-) tend to be unstable.

Almost every other element has stronger sigma than pi bonds so they prefer single bonds. Azide analogues will most likely dissociate and/or polymerise.

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    $\begingroup$ How could one determine that this statement: 'Nitrogen is special in that its pi bonds are stronger than its sigma bonds. ' is true? Can one make a nitrogen-nitrogen pi-bond in the absence of a nitrogen sigma bond and then do a thermochemical measurement? $\endgroup$ – Lighthart Apr 3 '15 at 22:14

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