Examples
According to Bent’s rule, molecular geometry can be explained and predicted by changing the substituent group. In the molecule Me2XCl2 (where X=main group elements C, Si, Ge, Sn, Pb), the bond angle Cl–X–Cl is smaller than the C–X–C bond angle. With the highly electronegative halogen substituent, Cl, more p character is concentrated on central atom in X–Cl than X–C bonds. Subsequently, bonds with greater p character have smaller bond angles than those with greater s character. For example, when X=C, the Cl–C–Cl has 108.3° bond angle that is smaller than C–C–C bond angle, 113.0°. In addition, this can be applied to heavier main group elements. When X=Si, Cl–Si–Cl has a bond angle (107.2°) that is smaller than that of C–Si–C (114.7°). In another example, Cl can be substituted to form the molecule (CH3)2PbF2. This molecule is distorted following Bent's rule, in which the bond angle of C–Pb–C (134.8°) is larger than the angle of the F–Pb–F bond (101.4°).
The molecule ClF5 has a square pyramidal structure, with two identical F–Cl–F three-center four-electron bonds on equatorial position and one Cl–F sp hybridized bond on axial position. This molecule follows Bent’s rule based on the difference in electronegativity between two atoms. Increased s character of the lone pair on the central atom, Cl, leads to reduced s character (greater p character) on the axial Cl–F bond. Although the overlap of the axial orbital decreases due to the reduced s character of central atom, the central atom, Cl, becomes less electronegative toward F on axial position. Therefore, the bond of Cl–F on axial position becomes shorter and stronger than other equatorial bonds according to the Pauling’s ionic resonance energy and Schomaker–Stevenson equation.
The molecule XSF4 (X = LP, O and CH2) also follows Bent’s rule. In SF4, the bond angle of the axial F–S–F bond is 173° (ideally 180°) and equatorial F–S–F bond angle is 101° (ideally 120°). The axial bond angle has changed slightly due to VSEPR effects. Because of the increased s character of lone pair, more p character is concentrated on the equatorial fluorine atoms, leading to a decrease in the F–S–F bond angles. In OSF4, the axial F–S–F has a bond angle of 164° (ideally 180°), whereas the equatorial F-S-F has a bond angle of 115° (ideally 120°). These smaller than ideal bond angles are due to VSEPR effects arising from the sterics of the lone pair.
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