The Triiodide Ion (\(I_3^-\)) is a chemical species composed of three iodine atoms bonded together and carrying an overall negative charge. This polyatomic ion is commonly found in solutions containing iodine and iodide salts. The bonding arrangement, which includes an extra electron, leads to electron delocalization. Since this structure cannot be perfectly captured by a single, static drawing, it suggests the involvement of chemical resonance.
The Basics of Chemical Resonance
Chemical resonance is a foundational concept used to describe bonding in molecules or ions where a single Lewis structure is insufficient. It is a theoretical tool, not a physical process where electrons rapidly flip positions. The actual structure is considered a hybrid of several contributing structures, often called canonical forms.
These canonical forms are hypothetical structures that differ only in the arrangement of electrons, such as lone pairs and double bonds, but not the arrangement of the atoms. The true structure, the resonance hybrid, is more stable than any single contributing structure. This increased stability occurs because the electrons and the associated electrical charge are spread out, or delocalized, over multiple atoms.
Determining the Structure of the Triiodide Ion
To understand the bonding in the triiodide ion, the first step is accounting for the valence electrons. Each of the three iodine atoms contributes seven valence electrons, and the single negative charge adds one, totaling 22 valence electrons. The three iodine atoms arrange themselves linearly, with one iodine atom positioned centrally between the other two.
Applying the Valence Shell Electron Pair Repulsion (VSEPR) model reveals the ion’s precise geometry. The central iodine atom is bonded to two terminal iodine atoms and holds three lone pairs of electrons. This arrangement of five electron domains results in a trigonal bipyramidal electron geometry. Because the three lone pairs occupy the equatorial positions, the resulting shape is a straight line, giving the triiodide ion a linear molecular geometry with a 180-degree bond angle.
How Resonance Stabilizes the Triiodide Ion
The triiodide ion exhibits resonance because the negative charge and the electron density between the atoms can be distributed in two equivalent ways. One canonical structure shows a single bond connecting the central iodine to the left terminal iodine, with the full negative charge on the right terminal iodine. The second canonical structure is the mirror image, where the single bond is on the right side and the negative charge is on the left terminal iodine atom.
The actual structure of \(I_3^-\) is a hybrid of these two static forms, showing the negative charge effectively shared equally between the two terminal iodine atoms. This electron delocalization averages the bond character across both iodine-iodine linkages.
Instead of one single bond and one non-bond, the hybrid structure features two bonds that are identical in character and length. This distribution of electron density across the entire ion is a highly stabilizing factor, which explains why the \(I_3^-\) ion forms readily and is stable in solution. This specific type of delocalization is often described as a 3-center, 4-electron bond, which underlies the observed bond-averaging.
Experimental Evidence Confirming Resonance
The definitive proof that resonance occurs in the triiodide ion comes from physical measurements, specifically the distance between the atomic nuclei. If the ion were represented by a single Lewis structure without resonance, one bond would be a true single bond, and the other would be much longer, resembling a non-bond. A typical single bond between two iodine atoms, as seen in the gaseous \(I_2\) molecule, has a length of approximately 266.6 picometers.
When the triiodide ion is stabilized in a perfectly symmetrical environment, experiments show that both iodine-iodine bonds are exactly the same length. This measured length is always longer than the \(I_2\) single bond, falling into an intermediate range that suggests a bond order lower than one. The observation of two identical, intermediate-length bonds confirms that the electron density is equally shared across the ion, validating the resonance hybrid model.