VSEPR Theory - Odd-electron Molecules

Odd-electron Molecules

The VSEPR theory can be extended to molecules with an odd number of electrons by treating the unpaired electron as a "half electron pair". In effect, the odd electron has an influence on the geometry which is similar to a full electron pair, but less pronounced so that the geometry may be intermediate between the molecule with a full electron pair and the molecule with one less electron pair on the central atom.

For example, nitrogen dioxide (NO₂) is an AX₂E_0.5 molecule, with an unpaired electron on the central nitrogen. VSEPR predicts a geometry similar to the NO−
2 ion (AX₂E₁, bent, bond angle approx. 120°) but intermediate between NO−
2 and NO+
2 (AX₂E₀, linear, 180°). In fact NO₂ is bent with an angle of 134° which is closer to 120° than to 180°, in qualitative agreement with the theory.

Similarly chlorine dioxide (ClO₂, AX₂E_1.5) has a geometry similar to ClO−
2 but intermediate between ClO−
2 and ClO+
2.

Finally the methyl radical (CH₃) is predicted to be trigonal pyramidal like the methyl anion (CH−
3) but with a larger bond angle as in the trigonal planar methyl cation (CH+
3). However in this case the VSEPR prediction is not quite true as CH₃ is actually planar, although its distortion to a pyramidal geometry requires very little energy.