was formulated in 1961 by American chemist Henry Bent to explain deviations in structures predicted from the orbital the orbital hybridisation 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 molecular orbital calculations, is a useful tool in inorganic and organic chemistry. Bent based his rule on the perturbation perturbation theory, theory, and suggested that isovalent orbital 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 words, ligand orbitals tend to be rich in p character because of higher electronegativity, higher electronegativity, with with s character concentrated on the central metal. However, in cases where the metal has a lone a lone pair, the pair, the lone pair orbital is high in s character. This is because s orbitals are closer to the nucleus, the nucleus, allowing allowing for greater stabilization of the lone pair.
When there is more 2p character in the N-F
σ
bonds there is better overlap, (overlap integral
∫υ(N2p)×υ(F 2p) dτ) and this of course results in a lower energy situation. More electronegative substituents prefer site which has higher p character or in other words less s character.
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 str ucture 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 experimentally holds true for transition metal complexes.
Examples According to Bent’s rule, molecular geometry can be explained and predicted by changing the substituent group. In the molecule Me 2XCl2 (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 (CH 3)2PbF 2. This molecule is distorted following
– Pb – F bond Bent's rule, in which the bond angle of C – Pb – C (134.8°) is larger than the angle of the F (101.4°).
Exceptions
Bond angles of Me2TiCl2. The Cl – Ti – Cl bond angles suggest a deviation from Bent's rule.
According to Bent’s rule, “Atomic s character concentrates in orbitals directed towards electropositive substituents ”. However, this general statement is only true for main group elements. For the main group elements, atomic p-orbitals are directed towards more electronegative substituents. This can be rationalized by the fact that the decreased angles of bonds with more p character coincide with the decreased steric demands of more electronegative atoms. Also, electron density can be more easily withdrawn (by electronegative substituents) from higher lying p-orbitals than from s-orbitals.
Transition metal complexes are the exception to Bent’s rule. It has been experimentally determined that the group 4 transition metal compounds of Ti-Hf
do not rigorously follow Bent’s rule. With these
complexes, the more electronegative substituents have larger bond angles (indicating greater s character), which goes against Bent’s rule. This can be explained by the fact that with transition metals, the energy levels of the d-orbitals are generally below the energy of the s-orbitals, thus the more electronegative substituents will be attracted to the higher lying s-orbitals. Transition metal bonds are essentially sdx hybridized, with very little contribution from the p-orbitals.
A generalized form of Bent’s rule can be stated as follows: “The energetically lower lying valence orbital concentrates in bonds directed toward electropositive substituents .” This satisfies both main group and transition metal complexes.