Lewis acids and bases

• Lewis acid - electron pair acceptor

• Lewis base - electron pair donor

• H+ + NH3 --> NH4+

• We can also talk about reactions like

• NH3 + BF3 --> NH3BF3

• Note - usually refer to as “Lewis acids” to avoid confusion with BL acids and bases

Lewis acids

• 1) vacant valence orbitals (BF3)

• 2) Simple cations - Fe+3

– Fe+3 + 6 CN- --> Fe(CN)6-3

• 3) Molecule with vacant p bonds involving oxygen

• Consider H2O + CO2 --> H2CO3

• base? Acid?

Hydrolysis of water by metals

• What happens when a cation interacts with water?

• Which would be more acidic a water near an aluminum ion or near a sodium ion?

• Fe(H2O)6 +3 = Fe(H2O)5 (OH)+2 + H+

– Ka = 2 *10-3

Hard and soft Lewis acids and bases

• Copper, lead and mercury are mostly found as sulfides in the earth

• Na and K are founds as chlorides

• Mg and Ca as carbonates

• Al, Ti and Fe as oxides

• Why?

• The strength with which an atom holds its electrons, determines the Lewis base strength

Polarizability

• How does polarizability change across the periodic table (L to R)? (Z_eff)(size)

• How does polarizability change down the periodic table (L to R)? (size)

• If polarizability is low, “hard”

• If polarizability is high, “soft”

• What is a hard Lewis base? Soft?

• What is a hard Lewis acid? Soft?

Hard-Soft acid-base (HSAB) principle (empirical)

• Hard Lewis acids tend to combine with hard Lewis bases

• Soft Lewis acids tend to combine with soft Lewis bases

• Should Fe react with oxygen?

• Should Au react with oxygen?

Ranking hardness and softness

• Rank the Lewis acids from hardest to softest

– H₂S, H₂O, H₂Se

– H₂O, NH₃, PH₃

– BCl₃, GaCl₃, AlCl₃

^– Fe⁰, Fe⁺³, Fe⁺²

Metathesis reactions

• BI₃ + GaF₃ --> BF₃ + GaI₃

• What is the driving force?