"More electronegative constituents 'prefer' hybrid orbitals having less s character and more electropositive substituents 'prefer' hybrid orbitals having more s character."
An example of Bent rule is provided by the fluoromethanes. In CH₂F₂ the F-C-F bond angle is less than 109.5°, indicating less than 25% s character, but the H-C-H bond angle is larger and C-H bond has more s character. The bond angle in the other fluoromethanes yield similar results.
The tendency of more electronegative substituents to seek out the. low electronegative pxdx² apical orbital in TBP structures is often termed "apicophilicity". It is well illustrated in a series of oxysulfuranes of the type :
prepared by Martin and Co-workers. These, as well as related phosphoranes provide interesting insight into certain molecular rearrangements.
Bent's rule is also consistent with and may provide alternative rationalization for Gillespie's VSEPR model. Thus the Bent's rule prediction that highly electronegative constituents will 'attract' p character and reduce bond angles is compatible with the reduction in regular volume of the bonding pair when held tightly by an electronegative substituents. Strong, s-rich covalent bonds require a larger volume in space to bond. Thus double bonded oxygen, despite the high electro negativity of oxygen, seeks s-rich orbitals because of the shortness and better overlap of the double bond. Again, the explanation, whether in purely s-character terms (bent's rule) or in larger angular volume for a double bond (VSEPR), predicts the correct structure.
The mechanism operating behind Bent's rule is not completely clear. One factor favouring increased p character in electronegative substituents is the decreased bond angles of p orbitals and the decreased steric requirements of electronegative substituents. There may also be an optimum strategy of bonding for a molecule in which the character is concentrated in those bond in which the electronegativity difference is small and covalent bonding is important. The p character, if any, is then directed towards bonds to electronegative groups. The latter will result in greater ionic bonding in a situation in which covalent bonding would be low anyway because of electronegativity difference.
Some light may be thrown on the workings of Bent's rule by observations of apparent exceptions to it. The rate exceptions to broadly useful rules are unfortunate with respect to the universal applications of those rules. They also have the annoying tendency to be confusing to someone who is encountering the rule for the first time. On the other hand, any such exception or apparent exception is a boon to the research. since it almost always provides insight into the mechanism operating behind the rule.
Consider the cyclic bromophosphate ester :
The phosphorus atom is in an approximately tetrahedral environment using four o bonds of approximately sp³ character. We should expect the more electronegative oxygen atoms to bond to s-poor orbitals on the phosphorus and the two oxygen atoms in the ring do attract hybridizations of about 20% s. The most electropositive constituent on the phosphorus is the bromine atom and Bent's rule would predict an s-rich orbital, but instead it draws another s-poor orbital on the phosphorus atom is that involved in o bond to the exocyclic oxygen. This orbital has nearly 40% s-character. The oxygen atom ought to be about as electronegative as the other two, so why the difference? The answer probably lies in the overlap aspect.
1. The large bromine atom has diffuse orbitals that overlap poorly with the relatively small phosphorus atom. Thus, even though the bromine is less electronegative than the oxygen, it probably does not form as strong a covalent bond.
2. The presence of a π bond shortens the exocyclic double bond and increases the overlap of the o orbitals. If molecules respond to increase in overlap by rehybridization in order to profit from it, the increased s-character then becomes reasonable. From this point of view, Bent's rule might be rewarded. The p character tends to concentrate in orbitals with weak covalently (from either electro negativity or overlap considerations), and s-character tends to concentrate in orbitals with strong covalently matched electro negativities and good overlap.
Some quantitative support for the above qualitative arguments comes from average bond energies of phosphours, bromine and oxygen.
| P – Br | 264 KJ MOL-1 |
| P – O | 335 KJ MOL-1 |
| P = O | 544 KJ MOL-1 |
Bent's rule is a useful tool in inorganic and organic chemistry. For example, it has been used to supplement the VSPER interpretation of the structures of various non-metal fluorides, and should be applicable to a wide range of question on molecular structure.
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