Biological Chemistry - Coordination Complexes (9.4) Flashcards
Why are transition metals in biology important?
- Approximately half f all proteins contain metal ions, which provide an important role in biological reactions
- The transition metals are bound to the protein w/ dative covalent (coordinate) bonds
What is a coordinate (dative covalent) bond?
It is a special type of covalent bond where both electrons are donated from one atom
What is a complex ion?
It is an ion that has a central metal cation w/ a number of other molecules or ions (ligands) bonded to it by dative covalent bonds
What is a ligand?
A molecule or negative ion that surrounds that surrounds a central metal ion in a complex (in metalloproteins, these ligands are amino acids on the protein chain)
Name the common shapes of complex ions
- Octrahedral (central metal atom forms 6 bonds)
- Tetrahedral (central metal atom forms 4 bonds)
Name ligands that usually form octahedral complex ions
H2O
NH3
Name a ligand that usually forms tetrahedral complex ions
Cl^-
Name another shape of complex ion that is occasionally formed by transition metals
Square planar (central metal atom forms 4 bonds)
Square planar complexes cannot have isomers. True or false?
False - they can have geometric isomers
What is the coordination number?
The number of coordinate bonds formed between the ligands & metal ion (not always the same as no. of ligands)
What is a unidentate ligand?
They are ligands that use just one pair of electrons to form a dative covalent bond w/ the metal ions
What is a bidentate ligand?
They are ligands that use two lone pairs of electrons to form (2) covalent bonds w/ the metal ions
What is a multidentate ligand?
They are ligands that use more than 2 lone pairs of electrons to form dative covalent bonds w/ the metal ions
Give at least one example of a unidentate ligand
H2O
NH3
CN^-
Cl^-
Give at least one example of a bidentate ligand
ethandioate ion
1,2-diaminoethane
Give at least one example of a multidentate ligand
Haem
EDTA
How do you work out the charge of a complex ion?
- The charge of a complex ion is the sum of the charges on the metal ion & ligands
- The charge is spread over the whole complex so it is written outside the square brackets
Explain what happens in a ligand exchange reaction
- The ligands around a metal centre can be exchanged for others
- Exchange reactions are often equilibria, and partial substitution may occur
- The geometry may stay the same or it may change
How can ligand exchange reactions be identified?
They are often associated with a change of colour
Give an example of a ligand exchange reaction
Carbon monoxide poisoning - Fe’s affinity is stronger for CO than O2 so CO binds to Fe in haemoglobin instead
What is the chelate effect?
A specific type of ligand substitution where simple ligands are replaced by a multidentate ligand - this multi dentate ligands bends to bind to the complex ion through more than one point
Why does the chelate effect occur?
- Chelate complexes formed are more stable
- This is because there is an increase in entropy (less molecules on reactant side & more on product side) so it is more energetically favourable
Give an example of how the chelate effect can be useful
- Chelation therapy removing heavy metals from blood in heavy metal poisoning
- Reduces iron-overload in patients receiving long-term blood transfusions
