F325: Module 3: Complex ions and Ligand substitution

Question 1

Explain how ethane-1,2-diamine is able to act as a bidentate ligand.

Answer 1

• Donates two lone pairs to the metal ion

- Lone pairs on N or NH2 groups

Question 2

Explain how ligand substitution reactions allow haemoglobin to transport oxygen in the blood

Answer 2

-O2 bonds to Fe2+
(Fe essential for first marking point)
-O2 substituted or O2 released (Fe not essential for second marking point)

Question 3

In the presence of carbon monoxide, less oxygen is transported in the blood. Suggest why, in terms of bond strength and stability constants.

Answer 3

• Kstab with CO grater than with complex in O2

- Coordinate bond with CO is stronger (than O2) OR CO binds more strongly

Question 4

I) What is a complex ion?

Answer 4

Transition Metal ion bonded to a number of ligands by dative covalent bonds (also known as coordinate bonding).

Question 5

I) What is a ligand?

Answer 5

An Ion or molecule that can donate a pair of electrons to a central metal ion by forming a dative covalent bond (coordinate bond) with it.

Question 6

I) What is the coordination number?

Answer 6

The total number of coordinate bonds formed between the central metal ion and any ligands.

Question 7

I) Give examples of a Monodentate, Bidentate and Multidentate ligand.

Answer 7

Monodentate - H2O, NH3, Cl-

Bidentate - Ethane-1,2-diamine (NH2CH2CH2NH2), Ethanedioate ion (C2O4 2-)

Multidentate - EDTA 4- which can for 6 coordinate bonds.

Question 8

I) What are the three most common shapes of complex ions? (Give their bond angles).

Answer 8

Octahedral - 90 degrees

Tetrahedral - 109.5 degrees

Square planar - 90 degrees

Question 9

I) What shape is normally formed from larger ligands in a complex ion? Give an example of a ligand that causes this.

Answer 9

Tetrahedral - Cl- is usually the larger ligand responsible for this.

Question 10

I) How do complex ions exhibit E/Z (Cis-Trans) isomerism?

Answer 10

The Z (Cis) isomer has two identical groups lying on one side of the metal ion.

The E (Trans) Isomer has two identical groups lying on opposite sides of the ion.

Question 11

I) What 2 types of isomerism can occur in complex ions?

Answer 11

E/Z isomerism

Optical isomerism

Question 12

I) Describe the structure of Cis-Platin.

Answer 12

Cis-Platic is composed of a central Pt2+ ion with 2 Cl- and 2 NH3 ligands in a square planar arrangement.

The 2 Cl- ligand are on the same side as each other.

Question 13

I) What is Cis-Platin used for and how does it work? Why must the drug be Cis and not trans?

Answer 13

Cis-Platin is used as an anti cancer drug.

The Cis-Platin version only works as two chloride ions are displaced and the molecule joins on to the DNA, stopping the replication of cancerous cells.

In the body, one Cl- ligand is substituted by a water molecule.

Question 14

I) What is the colour change in the reaction between:

1) Cu2+ (aq) and excess NH3
2) Cu2+ (aq) and Cl- ions (Conc HCl)
3) Co2+ (aq) and Cl- ions (Conc HCl)

Answer 14

1) Blue Solution ==> Deep blue solution
2) Blue Solution ==> Yellow solution
3) Pink Solution ==> Blue solution

Question 15

I) Describe the structure of haemoglobin.

Answer 15

In haemoglobin a central Fe2+ ion is the ion in an octahedral complex comprising of four co-ordinate bonds from the nitrogens in a porphyrin ring, and one from a nitrogen on one of the amino acids on the haemoglobin molecule.

The sixth ligand in the complex is oxygen.

Question 16

I) Why is Carbon monoxide toxic to humans?

Answer 16

It binds to the central Fe2+ ion in haemoglobin more strongly than oxygen and replaces it in the complex.

This means that the oxygen cannot be carried around the body, causing asphyxiation.

Co has a greater Kstab value than O2

Question 17

I) What is the stability constant of a complex ion?

Answer 17

The equilibrium constant for the formation of the complex ion in a solvent from its constant ions.

Question 18

I) What does a large stability constant tell you about that complex ion?

Answer 18

The larger the Kstab the more stable the complex ion is.

Question 19

I) How might you use sliver nitrate to work out the formulae of chloride containing complexes?

Answer 19

If Silver nitrate is added to a compound which has Cl- ions acting as ligands and Cl- ions outside the complex (ionically attracted), it will only form the silver chloride precipitate with the free chloride electrons outside of the complex.

If you are given details such as the amount of precipitate produced or the amount of silver nitrate reacted, you can work backwards in order to work out the original complex ion.