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Summary ALEVEL CHEMISTRY - Ligand Substitution Reactions Notes
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- AQA
ALEVEL CHEMISTRY - Ligand Substitution Reactions Notes
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Ligand Substitution ReactionsIn a ligand substitution reaction one ligand is substituted with a different ligand this causes a
colour change.
If the ligands substituted are smaller there is no change in coordination number.
H2O, NH3 and Cl- act as monodentate ligands. Ligands H2O and NH3 are uncharged therefore
exchange does not change the coordination number.
Cl- does change coordination number as it is slightly larger and charged so causes repulsion.
Bidentate ligands include C2O42- and H2NCH2CH2NH2.
Multidentate ligands include EDTA4-.
If a metal is dissolved in water it forms aqueous ions (H2O) not chloride ions.
Addition of Cl- causes a change in coordination number from 6 to 4.
Bidentate and multidentate ligands replace monodentate ligands to cause an increase in
entropy - chelate effect.
In many ligand substitution reactions enthalpy change is negligible as the same number and
type of bonds are broken and formed.
An increase in entropy is likely to occur to form more stable products. It can go from 4 particles
to 7 which causes an increase in entropy as there is more disorder and more ways the particles
can be rearranged in a system - chelate effect.
If the ligands are a similar size the coordination number does not change and shape stays the
same.
[Co(H2O)6]2+ + 6NH3 --> 6H2O + [Co(NH3)6]2+
If the ligands are different sizes then there is a change in coordination number and shape.
[Cu(H2O)6]2+ + 4Cl- --> 6H2O + [CuCl4]2-
Sometimes ligand substitutions are partial and only a few ligands change.
Haemoglobin and its Haem Group
Haemoglobin is a protein found in blood which helps transport oxygen around the body. It is a
multidentate. It contains Fe2+ ions which are hexa- coordinately bonded as 6 lone pairs are
donated to form 6 co-ordinate bonds.
4 of the lone pairs come from nitrogen atoms which form around the Fe2+. This part of the
molecule is called the haem group. The molecule the 4 nitrogen atoms come from is called a
porphyrin so a porphyrin ring forms. A protein called a globin and either a water or oxygen
molecule binds to the Fe2+ ion to form an octahedral structure.
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