The triiodide ion ($I_3^-$) has structure [I-I-I]$^-$, but the resonance structure [I=I-I]$^-\leftrightarrow$ [I-I=I]$^-$ seems more favourable to me. After all, it disperses formal charge and has a higher bond order. Yet, the triiodide ion being linear shows that the first structure is the correct one. Why?
P.S.: A similar question does exist but asks why the formation of triiodide is possible, rather than why it is the way it is.
It goes the other way: triiodide ion actually has only one covalent bond, delocalized over the two linkages:
$\ce{\overset{-}{I}:}\space\space\space\space\ce{I – I <-> I – I}\space\space\space\space\ce{:\overset{-}{I}}$
The molecular orbital structure that goes along with this is given in various locations, including here.
The occupied bonding orbital has less overlap between each pair of atoms than a pairwise bond would, but the combined overlap over both linkages makes the three-center bond stronger than a simple pairwise bond would be.
