In Pearson's paper, he mentions that strong acid-strong base interactions are more favourable compared to hard-soft acid-base interactions. He cites the example of $\ce{H+ + H- -> H2}$. In this example, the hydrogen cation is a strong and hard Lewis acid while the hydride ion is a strong yet soft Lewis base. By just considering hardness and softness of the acid and base, we would not expect a particularly favourable interaction between the two species. However, Pearson says it is because of a more favourable interaction which is the strong acid-strong base interaction which is at work in this case.
Some background information:
When the HOMO of the Lewis base and the LUMO of the Lewis acid are similar in energy, the bonding molecular orbitals produced are significantly lower in energy and thus, electron occupation of these bonding MOs give rise to significant stabilisation. This occurs when hard Lewis acids and hard Lewis bases react and also when soft Lewis acids and soft Lewis bases react. This is the molecular orbital basis of Pearson's Hard Soft Acid Base Principle.
Can we explain the favourable interaction of strong Lewis acids and strong Lewis bases using molecular orbitals as well?
Reference: Pearson, R. G., & Songstad, J. (1967). Application of the Principle of Hard and Soft Acids and Bases to Organic Chemistry. Journal of the American Chemical Society, 89(8). Retrieved June 5, 2017 (link to paper)
No comments:
Post a Comment