Why is NaCl3 possible?

[There](http://www.gizmag.com/scientists-create-forbidden-compounds-table-salt/30520/). And [there](http://www.sci-news.com/othersciences/chemistry/science-sodium-chlorides-foundation-chemistry-01633.html). Almost a year ago, a group of scientists claimed to have reached compounds of $\ce{Na}$ and $\ce{Cl}$ with weird stochiometries ($\ce{NaCl3, Na3Cl, NaCl7, Na3Cl2}$ and $\ce{Na2Cl}$).


What one of the articles says:
------------------------------



> 
> These compounds are thermodynamically stable and, once made, remain indefinitely; nothing will make them fall apart.
> 
> 
> For example, $\ce{NaCl7, NaCl3, Na3Cl2}$, and $\ce{Na2Cl}$ are metals (that explains the apparent violation of electroneutrality since charge balance rules are inapplicable to metals), and only one semiconducting phase of $\ce{NaCl3}$ is stable in the pressure range between 250 and 480 thousand atmospheres.
> 
> 
> 


At first, I thought this whole thing was bogus. But then, I faced [another, apparently convincing, article](http://www.chemistryviews.org/details/ezine/6731321/Against_Chemical_Intuition_Na3Cl_and_NaCl3_Do_Exist.html) and even [one from RSC](http://www.rsc.org/chemistryworld/2013/12/new-sodium-chlorides-assault-chemical-rules). Then the questions popped up:


* Why does this happen? (Why is such a compound more thermodynamically stable at high pressure?)
* How can the bonding in these be explained?


![](https://i.stack.imgur.com/IUWOSm.jpg)
([Source](http://www.chemistryviews.org/SpringboardWebApp/userfiles/chem/image/2014/2014_09/hull.jpg))




Yes NaCl3 is possible. 


Think of the more common iodine analog.


$\ce{KI + I2 -> KI3}$


For a comprehensive analysis see:


[Unexpected stable stoichiometries of sodium chlorides](http://arxiv.org/ftp/arxiv/papers/1211/1211.3644.pdf)



> 
> At 25-48 GPa, NaCl3 is stable in the Pnma structure, which has 4 formula units in the
>  unit cell. Unlike all the other new phases predicted here (which are metallic), this phase is
>  a semiconductor. Its structure (Fig. 2(B)) contains almost linear asymmetric Cl3 groups.
>  Bader analysis (19) shows that the middle atom in the Cl3-group is nearly neutral, with
>  most negative charge on the side atoms (Table 1) and the total charge of this anion group
>  being ~-0.8. This result is reminiscent of well-known triiodides and azides with I3
>  - and
>  N3
>  - ions, and of recent calculations on hypothetical H3
>  - ions (20), which were found to
>  have charge configuration [H-0.81H+0.72H-0.81]-0.9. Pnma-NaCl3 still can be viewed as an
>  octet compound, with covalent bonding within [Cl3]--groups and ionic bonding between
>  Na+ and [Cl3]-. At 48 GPa, with the formation of a new phase of NaCl3, the octet rule
>  breaks down.
> 
> 
> 


This research group compressed chlorine gas between NaCl plates, while heating with lasers and observed formation of $\ce{NaCl3}$. 


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