What is the hybridization of the central iodine atom in i3−?

Answer is: the central iodine atom in triiodide has sp3d hybridization.
In triiodide anion the central iodine atom has three equatorial lone pairs of electrons and the terminal iodines are bonded axially in a linear shape. Electrons in sp3d hybridization are arranged in trigonal bipyramidal symmetry.

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