ligand substitution reactions
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define ligand substitution reaction
a reaction where the ligands around a complex ion are substituted by other ligands
what happens when a ligand substitution reaction is complete with no change in coordination number
all ligands are substituted by new ones. coordination number is unchanged because number of coordinate bonds is the same
• Write an equation for the reaction between hexaaqua cobalt (II) ions with an excess of ammonia (NH3) (all water ligands are substituted) • The complex formed turns brown when left in the air for a period of time, why?
• [Co(H2O)6]2+ + 6NH3 ⎯→ [Co(NH3)6]2+ + 6H2O • Cobalt (II) oxidises to cobalt (III) due to oxygen in the air [Co(NH3)6]3+
what happens when a ligand substitution reaction is complete with change in coordination number
all ligands are substituted by new ones often new ligands are a different size so can accommodate different number of ligands
• HCl produces Cl- ions in solution that can act as ligands Write an equation for the reaction of [Cu(H2O)6]2+ with concentrated HCl • What has happened to the co-ordination number? Why?
• [Cu(H2O)6]2+ + 4Cl- ⎯→ [CuCl4]2- + 6H2O • Co-ordination number has decreased (from 6 to 4), because Cl- ligands are larger and only 4 can fit around Cu2+
what happens when a ligand substitution reaction is incomplete with no change in coordination number
only some of the ligands are substituted, but overall the number of co ordinate bonds is the same
Write an equation for the reaction of hexaaqua copper (II) ions with an excess of ammonia
[Cu(H2O)6]2+ + 4NH3 ⎯→ [Cu(H2O)2(NH3)4]2+ + 4H2O Co-ordination number unchanged Incomplete substitution
what is the chelate effect
multidentate ligands form thermodynamically stable complexes
when [Cu(H20)6] 2+ reacts with excess NH3 • what product is formed • what is the colour change
• [Cu(H2O)2(NH3)4]2+ • blue to royal blue
when [Co(H2O)6]2+ is reacted with excess NH3 • what product is formed • what is the colour change
• [Co(NH3)6]2+ • red to red brown dark brown when left in air
when [Cu(H2O)6]2+ reacts with concentrated HCl • what product is formed • what is the colour change
• [CuCl4]2- • blue to green
when [Co(H2O)6]2+ reacts with concentrated HCl • what product is formed • what is the colour change
• [CoCl4]2- • red to blue
when [Fe(H2O)6]3+ reacts with concentrated HCl • what product is formed • what is the colour change
• [FeCl4]- • brown to yellow
Write an equation for Gibbs’ Free-Energy Give the meaning of each term What is the significance of the value for ΔG?
ΔG = ΔH - T ΔS ΔG = Gibbs’ Free-energy. If less than 0, reaction feasible ΔH = enthalpy change T = temperature ΔS = entropy change
Write an equation for the reaction of [Cu(H2O)6]2+ with EDTA4-
[Cu(H2O)6]2+ + EDTA4- ⎯→ [Cu(EDTA)]2- + 6H2O
[Cu(H2O)6]2+ + EDTA4- ⎯→ [Cu(EDTA)]2- + 6H2O 1a. Explain why the entropy change for this reaction is very high 1b. Explain why the enthalpy change for this reaction is almost 0 (very small) • What does this mean about ΔG?
2 species in reactants produce 7 species in the product, therefore ΔS is very positive 6 Cu-O bonds broken, 2 Cu-N and 4 Cu-O bonds made. They have a similar bond energy so ΔH is very small ΔG is very negative. There is a strong thermodynamic stability to the product (thermodynamic driving force). This is called the Chelate effect