16.1 Lewis acids and bases

eqm illustrating stability of different Cr ions and respective colours

2CrO₄²⁻ + 2H⁺ ↔ Cr₂O₇²⁻ + H₂O, yellow and orange

overall reaction between Fe³⁺ and carbonate ion

2[Fe(H₂O)₆]³⁺ + 3CO₃²⁻ → 2Fe(H₂O)₃(OH)₃ + 3CO₂ + 3H₂O

Fe³⁺ is more acidic than Fe²⁺ because

Fe³⁺ is smaller and more highly charged, making it more strongly polarising so the iron ion strongly attracts electrons from the oxygen atoms of the water ligands, thus weakening the O-H bonds so the complex ion will readily release an H⁺ ion making the solution acidic

react with what to show that Al(OH)₃ is amphoteric

HCl and OH⁻

react with what to show that Cr(OH)₃ is amphoteric

H₃O⁺ and OH⁻

All Bronsted-Lowry acids are also

Lewis acids

general rule for acidity in TMs

M³⁺ > M²⁺

hydrolysis reaction

O-H bonds are broken and new species formed

why does iron (III) carbonate not exist but iron (II) carbonate does?

The CO₃²⁻ ion is able to remove protons from Fe³⁺ to form hydrated iron (III) hydroxide due to greater acidity but cannot do so from Fe²⁺ ions

overall reaction between Fe²⁺ and carbonate ion

[Fe(H₂O)₆]²⁺ + CO₃²⁻ → FeCO₃ + 6H₂O

eqn to explain Fe³⁺ acidity and another name for it and what the iron ion acts as

[Fe(H₂O)₆]³⁺ ↔ [Fe(H₂O)₅(OH)]²⁺ + H⁺, hydrolysis, Bronsted-Lowry acid

M²⁺ with OH⁻

[M(H₂O)₆]²⁺ + 2OH⁻ → M(H₂O)₄(OH)₂ + 2H₂O, M(OH)₂ is insoluble and forms a ppt, NH₃, which is basic, has the same effect as OH⁻ ions in removing protons

M³⁺ with OH⁻

[M(H₂O)₆]³⁺ + 3OH⁻ → M(H₂O)₃(OH)₃ + 3H₂O, M(OH)₃ is insoluble and forms a ppt

test for iron ions

add dilute alkali, difference in ppts

general statement concerning carbonates of TMs

carbonates of TM ions in the +2 state exist, while those of ions in the +3 state do not

→

dative bond

Lewis acid

electron pair acceptors in the formation of dative bonds

Lewis base

electron pair donors in the formation of dative bonds

in most TM compounds discussed, the metal ion exists in a cationic form but in high OS some exist

in anionic form, MnO₄⁻, CrO₄⁻, Cr₂O₇²⁻

both Fe³⁺ and Fe²⁺ ions exist in aq solution as

octahedral hexa-aqua ions

[Fe(H₂O)₆]³⁺

pale brown solution

[Fe(H₂O)₆]²⁺

pale green solution

When a ligand forms a bond to a TM ion using lone pairs,

the ligand is acting as a Lewis base and the metal ion as a Lewis acid

the smaller the value of pKa,

the stronger the acid

if we dissolve a salt of a TM, such as Fe(NO₃)₂, in water,

water molecules cluster around the Fe²⁺ ion so it actually exists as [Fe(H₂O)₆]²⁺ because 6 water molecules act as ligands bonding to the metal ion in an octahedral arrangement. A similar situation occurs with Fe³⁺, theses complexes are called aqua ions