The central iodine atom has the ground state electronic configuration as 5s2sp5 . It involves sp3d hybridisation involving one 5s, three 5p and one vacant 5d orbital. The half filled sp3d hybrid orbital forms a covalent bond with one iodine atom and an empty sp3d hybrid orbital accepts an electron pair from I ion to form a coordinate bond.
Thus, the geometry of the molecule is trigonal bipyramidal in which three positions are occupied by lone pairs.
Because of the equivalent positions of lone pairs, the resulting I3− ion is linear.
