# Question

Is the $5\mathrm{d}$ orbital involved in the triiodide ion, $\ce{I3-}$?

There are $5$ electron pairs around the central iodine.

# Background

(Almost) all of the hypervalent compounds involve the most electronegative elements: $\ce{F}$, $\ce{O}$, $\ce{Cl}$.

Also, the bonds involving the electronegative elements are often almost ionic:

> For sulfuric acid, computational analysis (with natural bond orbitals) confirms a clear positive charge on sulfur (theoretically $+2.45$) and a low $3\mathrm{d}$ occupancy.

So, we can say that $3\mathrm{d}$ is (almost) not involved in the central sulfur atom of sulfuric acid.

Is there any experimental data to show if $5\mathrm{d}$ is involved in the triiodide ion?