Bent's Rule

Bent's Rule

Bent’s rule was formulated in 1961 by American chemist Henry Bent to explain deviations in structures predicted from the VSEPR theory. The rule states: “atomic s character tends to concentrate in orbitals that are directed toward electropositive groups and atomic p character tends to concentrate in orbitals that are directed toward electronegative groups”. This rule, which is experimentally observed and supported by molecular orbital calculations, is a useful tool in inorganic and organic chemistry. Bent based his rule on the perturbation theory, and suggested that isovalent orbital hybridization should transfer more s character to the more electropositive-bonding orbital to maximize bonding energy. Hybrid orbitals for main group elements consist of one s and three p orbitals, with the s orbital having lower energy. To have more s character means that the bonding orbital is lower in energy and shaped more like an s orbital rather than a p orbital. In other words, ligand orbitals tend to be rich in p character because of higher electronegativity, with s character concentrated on the central metal. However, in cases where the metal has a lone pair, the lone pair orbital is high in s character. This is because s orbitals are closer to the nucleus, allowing for greater stabilization of the lone pair.

Bent’s rule was derived from the comparison of experimentally determined physical properties of molecules, correlated with valence bond structures and bond hybridization. This rule has been used to qualitatively describe molecular geometries and predict the structure of substituted atoms or molecules. While Bent’s rule was originally intended to describe bonding in elements of the first row of the periodic table, it also experimentally holds true for transition metal complexes.