Explanation of steric hindrance and existence of PI5

I have come across a following line in my textbook:



> 
> $\ce{PI5}$ does not exist due to steric hindrance.
> 
> 
> 


I searched for the explanation of steric hindrance in Wikipedia but couldn't understand it. Can anyone explain steric hindrance in simpler language?


Wikipedia says: 



> 
> The existence of $\ce{PI5}$ has been claimed intermittently since the
>  early 1900s. The claim is disputed: "The pentaiodide does not exist
>  (except perhaps as $\ce{PI3\cdot I2}$, but certainly not as
>  $\ce{PI4+I-}$)".
> 
> 
> 


What does this mean?




The simplest explanation is that you can not fit five iodine atoms around a phosphorous atom.


Consider the [pentachloride](https://en.wikipedia.org/wiki/Phosphorus_pentachloride) equivalent - this is already quite crowded. Since chlorine atoms have a smaller covalent radius than iodine, you can get five of them around the phosphorous. Of course, all this depends on the bond length of P-I and P-Cl and the size of the phosphorous atom.


Interestingly, there seems to be some suggestion of an Uranium Pentaiodide compound, for example [this PubChem compound](https://pubchem.ncbi.nlm.nih.gov/compound/71308293). However, there are no references on that page, so presumably this has never been synthesised either?


In general, steric hindrance means geometric conformations of a molecule that are impossible (or difficult to achieve) due to clashes between groups and atoms within the molecule.


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**$\ce{PI5}$ versus $\ce{CI4}$**


More is involved than steric hindrance in preventing the formation of a $\ce{PI5}$ isomer with pentacoordinate phosphorus. There is a subtle antibonding interaction in this $\ce{PI5}$ isomer which does not occur in $\ce{CI4}$.


[This answer](https://chemistry.stackexchange.com/a/121199/17175) describes the hidden antibonding ligand-ligand interactions in the "nonbonding" ligand-based orbitals of "expanded octet" molecules such as $\ce{SF6}$. The ligand-ligand antibonding interaction is small enough to minimize its impact on $\ce{SF6}$. But $\ce{OF6}$, where central atom is smaller and the ligands are closer together, is less fortunate.


Similarly in $\ce{PI5}$ there is one ligand-based orbital that becomes antibonding with the ligand-ligand interactions. Occupation of that iodine-based orbital combined with its crowding around the smaller phosphorus atom thus poses a combined electronic-steric hindrance.


With $\ce{CI4}$, however, there is no such ligand-based "hidden antibonding" orbital because there is no formal octet expansion, and without this electronic factor $\ce{CI4}$ can hold together (at least if it's chilled) despite the unfavorable steric condition.\*


\*[This answer](https://chemistry.stackexchange.com/a/134588/17175) mentions the unusual deep red/purple color of this saturated compound. The relative weakness of the carbon-iodine bond, which moves normally ultraviolet electronic transitions into the visible range, explains the color.


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