5. ligand substitution

ligand substitution

ligands can be swapped for another ligand causing a colour change

similar sized ligands

H₂O and NH₃

similar sized ligands do not change coordination number of complexes

or the complex shape

Cl ligand is larger than H₂O and NH₃

substitution H₂O/NH₃ for Cl would change the coordination number and shape

partial substitution of ligands

sometimes not all 6 of H₂O ligands are substituted

haemoglobin

a protein found in blood that helps transport oxygen around the body

haemoglobin contains Fe²⁺ ions which are hexa-co-ordinated

6 lone pairs are donated to them to form 6 coordinate bonds

4 of the lone pairs come from nitrogen atoms which form a circle around the Fe²⁺

this part of the molecule is called haem

porphyrin

the molecule that the four nitrogen atoms are a part of is called a mulidentate ligand

a protein called globin and either an oxygen or water molecule also binds to Fe²⁺ ion to form an octahedral structure

both water and oxygen will bind to Fe²⁺ ions as ligands

so the complex can transport oxygen to where its needed then swap it for a water molecule

in the lungs where oxygen concentration is high

water ligands are substituted for oxygen molecules to form oxyhaemoglobin and then transported around the body

when oxyhaemoglobin gets to places where oxygen is needed

the oxygen molecules are exchanged for water molecules

haemoglobin then returns to the lungs

and the whole process repeats

inhaled carbon monoxide can substitute water molecules on haemoglobin

forming carboxyhaemoglobin

carbon monoxide forms a very strong bond with the Fe²⁺ ion and doesnt readily exchange with oxygen or water ligands

meaning the haemoglobin cant transport oxygen anymore

carbon monoxide poisoning starves the organs of oxygen causing

• headaches

• dizziness

• unconsciousness

• death if not treated

ligan exchange reactions can be easily reversed

unless the new complex ion is much more stable than the old one

if the new ligands form stronger bonds with central metal ions than the old ligands

the change is less easy to reverse

multidentate ligands like EDTA⁺ form more stable complexes than monodentate ligands

ligand exchange reactions involving bidentate ligands and multidentate ligands are hard to reverse

when ligand exchange reactions occur

bonds are broken and formed

the strength of the bonds being broken is often similar to the strength of the new bonds formed

so the enthalpy change for a ligand exchange reaction is usually very small

when monodentate ligands are substituted with bidentate or multidentate ligands

the number of particles increases, more particles increase the enthropy

reactiosn that result in an increase in entropy are more likely to occur

this is why multidentate ligands always form much more stable complexes than monodentate ligands AKA CHELATE EFFECT

when the hexadentate ligand EDTA⁺ replaces monodentate or bidentate ligands

the complex formed is loads more stable

its difficult to reverse these reactions

because reversing them would decrease the systems enthropy