Hard-Soft Acid-Base Theory

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Slide 36

Other Considerations

empty 2p orbital

filled orbitals

The reactivity pattern suggests some degree of π bonding in BF3.

Slide 37

Other Considerations

Steric factors can play a role. An example is the unfavorable reaction between :N(C6H5)3 and BCl3. The large phenyl groups interact with the chlorine atoms on boron to destabilize the product.

Slide 38

Applications of Hard/Soft Theory

The Qual Scheme, a series of chemical reactions used to separate and identify the presence of dozens of metal ions, is based largely on the hard and soft properties of the metal ions.

The softer metals are precipitated out as chlorides or sulfides, with the harder ions formed as carbonates.

Slide 39

Slide 40

Evidence in Nature

In geochemistry, the elements in the earth’s crust are classified as lithophiles or chalcophiles.

The lithophile elements are typically found as silicates (bonded via the O atom): Li+, Mg2+, Ti3+, Al3+ and Cr2+,3+. These are hard Lewis acids.

Slide 41

Evidence in Nature

The chalcophile elements are typically found as sulfides or bonded to Se2- or Te2-. They include: Cd2+, Pb2+, Sb3+, and Bi3+. These are soft Lewis acids. Zinc ion, which is borderline, is typically found as a sulfide.