Hard-Soft Acid-Base Theory

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Slide 10

Lewis Acids & Bases

A Lewis base has an electron pair in its highest occupied molecular orbital (HOMO) of suitable symmetry to interact with the LUMO of the Lewis acid. The closer the two orbitals are in energy, the stronger the bond in the adduct.

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Hard and Soft Acids and Bases

The polarizability of an acid or base plays a role in its reactivity. Hard acids and bases are small, compact, and non-polarizable.

Soft acids and bases are larger, with a more diffuse distribution of electrons.

Slide 12

Hard and Soft Acids and Bases

In addition to their intrinsic strength,

Hard acids react preferentially with hard bases, and soft acids react preferentially with soft bases.

Slide 13

Examples: Aqueous Solubility Silver Halides

Compound solubility product

AgF 205

AgCl 1.8 x 10-10

AgBr 5.2 x 10-13

AgI 8.3 x 10-17

AgX(s) + H2O(l) ↔ Ag+(aq) + X-(aq)

Slide 14

Solubility of Lithium Halides

LiBr> LiCl> LiI> LiF

LiF should have a higher ∆solv than the other salts, yet it is the least soluble in water. This is due to the strong hard acid (Li+)/hard base (F-) interaction.

Slide 15

Example: Thiocyanate Bonding

SCN- displays linkage isomerism as the ligand coordinates to metals via the sulfur or the nitrogen. Mercury (II) ion bonds to the sulfur (a soft-soft interaction) whereas zinc ion bonds to the nitrogen atom.

Slide 16

Example: K for ligand exchange reactions

Compare:

[MeHg(H2O)]+ + HCl MeHgCl + H3O+

K= 1.8 x 1012

[MeHg(H2O)]+ + HF MeHgF + H3O+

K= 4.5 x 10-2

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Slide 18

Hard and Soft Acids & Bases

There have been many attempts to categorize various metal ions and anions to predict reactivity, solubility, etc.

R.G. Pearson (1963) categorized acids and bases as either hard or soft (using Kf values).

Hard acids bond in the order: F->Cl->Br->I-

Soft acids bond in the order: I- >Br- >Cl- > F-

Slide 19

Hard and Soft Acids & Bases