VSEPR Structures of Odd Electron Molecules

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All the usual rules of building a VSEPR structure will apply - minimize formal charge, build octect on more electronegative ligand first, etc.

Example #1: nitrogen dioxide NO2

This molecule has a total of 17 electrons to place - five from the nitrogen and 12 from the oxygens. I will go immediately to the final structure:

Notice that I show both resonance structures. Since there are three electron domains, this is a trigonal planar arrangement, but it is signified AX2e, to signal the single electron domain, also called a half-filled orbital.

The bond angles are not 120°, since the repulsive power of the single electron is less tha if there were two. So, the O-N-O bond angle moves outward to 134.3°. Adding another electron to make NO2¯ (which creates a full non-bonding electron pair, changes the O-N-O angle to 115.4° and removing an electron (to make NO2+) creates an O-N-O bond angle of 180°.

Example #2: chlorine dioxide ClO2

The substance has 19 electrons to place and is a tetrahedral family member. Its geometry can be described as AX2Ee. It will have a non-bonding pair and a non-bonding single electron.

Your assignment is to draw this structure. One hint: if you draw it singly bonding, you will arrive at a structure with a formal charge of +2 on the Cl and -1 on each oxygen. Draw a structure to remove those; the correct structure having no formal charge separation whatsoever.

The O-Cl-O bond angle in ClO2 is 118°. You may wish to ponder the effect on the bond angle in ClO2+ and ClO2¯.

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