Transition Metals 4 (Ligand substitution) Flashcards
What is a ligand substitution reaction?
A reaction in which one ligand is replaced by another
Why are ligands substituted?
Other ligands form stronger coordinate bonds
High concentration of other ligand, so equilibrium is changed
What happens in ligand substitution of cobalt?
[Co(H2O)6]2+
Colour: Pink
Oxidation state: 2+
Coordination number: 6
Shape: Octahedral
[Co(NH3)6]2+
Colour: Pale brown
Oxidation state: 2+
Coordination number: 6
Shape: Octahedral
What happens in the ligand substitution of [Cu(H2O)6]2+ with Cl- ions?
[Cu(H2O)6]2+
Colour: Pale blue
Oxidation state: +2
Coordination number: 6
Shape: Octahedral
[CuCl4]2-
Colour: Yellow-green
Oxidation state: +2
Coordination number: 4
Shape: Tetrahedral
What happens in the partial ligand substitution of [Cu(H2O)6]2+?
[Cu(H2O)6]2+
Colour: Pale blue
Oxidation state: +2
Coordination number: 6
Shape: Octahedral
[Cu(NH3)4(H2O)2]2+
Colour: Deep blue
Oxidation state: +2
Coordination number: 6
Shape: Elongated octahedral
What are the conditions needed for full ligand substitution?
Excess reagent/concentrated reagent
What are the conditions needed for partial ligand substitution?
Dilute reagent
Why is an elongated octahedral shape obtained?
NH3 is a better ligand since the lone pair on N is less strongly held than the lone pair in O in H2O. Hence lone pair is more readily donated to Cu2+ by N in NH3 than O in H2O. This happens because N is less electronegative than O.
Describe the chelate effect
It is the substitution of a monodentate ligand with a bidentate or multidentate ligand, hence forming a more stable complex and leading to a positive entropy change
What are the applications of EDTA?
Added to river water to remove poisonous heavy metal ions
Added to shampoos to remove calcium ions present in hard water (helps lathering)
Used in medicines
Describe ligand substitution reactions in terms of enthalpy
Metal ion and ligand bonds are broken and formed, the same number and same type of each bond are broken and made, hence a reaction would not take place due to enthalpy
Describe ligand substitution in terms on entropy
When monodentate ligands are replaced by bidentate or multidentate ligands, there is an increase in entropy. Enthalpy change is negligible, so reaction is down to entropy, deltaG is negative, so the stability of the bidentate or multidentate ligand complex is high
What is the structure of haemoglobin?
Contains Fe2+ ion with a coordination number of 6 in an octahedral shape. 4 positions from coordinate bonds, 5th for globin and 6th for H2O or O2.
Describe the ligand substitution of CO to haemoglobin
CO forms a stronger coordinate bonds with a central metal ion than O2 or H2O to form carboxyhaemoglobin and so displaces oxygen. This causes starvation of O2 in muscles and organs. Substitution cannot be undone as CO complex is more stable
What are the consequences of CO poisoning?
Headaches
Dizziness
Unconsciousness
Death
