Biological Chemistry - Coordination Complexes (9.4)

Question 1

Why are transition metals in biology important?

Answer 1

• 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

Question 2

What is a coordinate (dative covalent) bond?

Answer 2

It is a special type of covalent bond where both electrons are donated from one atom

Question 3

What is a complex ion?

Answer 3

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

Question 4

What is a ligand?

Answer 4

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)

Question 5

Name the common shapes of complex ions

Answer 5

• Octrahedral (central metal atom forms 6 bonds)

- Tetrahedral (central metal atom forms 4 bonds)

Question 6

Name ligands that usually form octahedral complex ions

Answer 6

H2O

NH3

Question 7

Name a ligand that usually forms tetrahedral complex ions

Answer 7

Cl^-

Question 8

Name another shape of complex ion that is occasionally formed by transition metals

Answer 8

Square planar (central metal atom forms 4 bonds)

Question 9

Square planar complexes cannot have isomers. True or false?

Answer 9

False - they can have geometric isomers

Question 10

What is the coordination number?

Answer 10

The number of coordinate bonds formed between the ligands & metal ion (not always the same as no. of ligands)

Question 11

What is a unidentate ligand?

Answer 11

They are ligands that use just one pair of electrons to form a dative covalent bond w/ the metal ions

Question 12

What is a bidentate ligand?

Answer 12

They are ligands that use two lone pairs of electrons to form (2) covalent bonds w/ the metal ions

Question 13

What is a multidentate ligand?

Answer 13

They are ligands that use more than 2 lone pairs of electrons to form dative covalent bonds w/ the metal ions

Question 14

Give at least one example of a unidentate ligand

Answer 14

H2O
NH3
CN^-
Cl^-

Question 15

Give at least one example of a bidentate ligand

Answer 15

ethandioate ion

1,2-diaminoethane

Question 16

Give at least one example of a multidentate ligand

Answer 16

Haem

EDTA

Question 17

How do you work out the charge of a complex ion?

Answer 17

• 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

Question 18

Explain what happens in a ligand exchange reaction

Answer 18

• 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

Question 19

How can ligand exchange reactions be identified?

Answer 19

They are often associated with a change of colour

Question 20

Give an example of a ligand exchange reaction

Answer 20

Carbon monoxide poisoning - Fe’s affinity is stronger for CO than O2 so CO binds to Fe in haemoglobin instead

Question 21

What is the chelate effect?

Answer 21

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

Question 22

Why does the chelate effect occur?

Answer 22

• 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

Question 23

Give an example of how the chelate effect can be useful

Answer 23

• Chelation therapy removing heavy metals from blood in heavy metal poisoning
• Reduces iron-overload in patients receiving long-term blood transfusions