Structure and Bonding
The ion is linear and symmetrical. According to VSEPR theory, the central iodine atom has three equatorial lone pairs, and the terminal iodines are bonded axially in a linear fashion, due to the three lone-pairs bonding to the central iodine-atom. In the molecular orbital model, a common explanation for the hypervalent bonding on the central iodine involves a three-center four-electron bond. The I-I bond is longer than in diatomic iodine, I2.
In ionic compounds, the bond lengths and angles of triiodide vary depending on the nature of the cation. The triiodide anion is easily polarised and in many salts, one I-I bond becomes shorter than the other. Only in combination with large cations, e.g. a quaternary ammonium such as +, may the triiodide remain roughly symmetrical. The dimensions of the triiodide - bonds in a few sample compounds are shown below:
| compound | Ia–Ib (pm) | Ib–Ic (pm) | angle (°) |
|---|---|---|---|
| TlI3 | 306.3 | 282.6 | 177.9 |
| RbI3 | 305.1 | 283.3 | 178.11 |
| CsI3 | 303.8 | 284.2 | 178.00 |
| NH4I3 | 311.4 | 279.7 | 178.55 |
Read more about this topic: Triiodide
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