DM bijkm: Complexes & associated colour

Question 1

Explain in simple terms what causes colour.

Answer 1

• When light hits a substance, it may be transmitted, reflected or absorbed
• If frequency absorbed is in visible region, substance is coloured

Question 2

Explain why specific elements absorb specific frequencies of light.

Answer 2

• When light hits an atom, photons are absorbed
• Electrons promoted to higher energy levels
• Energy of a photon corresponds to gap between energy levels
• Frequency of photon, ν, related to ΔE by ΔE = hν
• Each element has unique energy levels, so absorbs unique frequencies of light

Question 3

• State what is meant by “complementary colours”.
• List them.

Answer 3

Opposite frequencies of visible light on the colour wheel which, when combined, produce white light.

Question 4

• Give the formulae of the common ions of iron
• Give their colours in aqueous solution

Answer 4

Fe²⁺, green

Fe³⁺, orange-brown

Question 5

• Give the formulae of the common ions of copper
• Give their electronic configuration with respect to Ar
• Give their colours in aqueous solution

Answer 5

Cu [Ar] 3d¹⁰4s¹

Cu⁺, [Ar] 3d¹⁰, unstable in solution

Cu²⁺, [Ar] 3d⁹​, blue

Question 6

Copper(I) ions are unstable in aqueous solution. Write an equation, showing state symbols, to suggest what they break down to.

Answer 6

2Cu⁺(aq) → Cu(s) + Cu²⁺(aq)

Question 7

What are ligands?

Answer 7

Molecules/anions which surround, and donate a lone pair of electrons to, a metal cation by coordinate (dative) bonding.

The inorganic chem equivalent of nucleophiles

Question 8

Define the following:

• Monodentate ligand
• Bidentate ligand
• Polydentate ligand

Answer 8

• Monodentate ligand: form one coordinate bond with central metal ion
• Bidentate ligand: form two coordinate bonds with central metal ion
• Polydentate ligand: form several coordinate bonds with central metal ion

Question 9

Give the corresponding prefix/suffix for naming the complex of each of the following monodentate ligands:

• H₂O:
• :OH^-
• :CN^-
• :NH₃
• Halides, :Cl^-, :Br^-, :I^-
• CH₃COO:

Answer 9

• H₂O aqua
• OH^- hydroxy
• CN^- cyano
• NH₃ amino
• Cl^- chloro, Br^- bromo, I^- iodo
• CH₃COO ethanoate

Question 10

Explain why some transition metal solutions are coloured.

Answer 10

• Ligands form dative bonds with central metal ion, which splits its d-orbitals
• Creates energy gap of right magnitude to allow absorption of visible light
• Electrons are excited to higher energy level
• Frequency absorbed is related to energy gap by ∆E = hv
• Colour seen is complementary colour to that absorbed (due to missing frequency)

​Energies of all 5 d-orbitals in transition metals are normally equal​.

Further explanation of splitting (not on spec): d-orbitals close to ligand are repelled + pushed to higher energy levels. Those further from ligand are attracted + pushed to lower energy levels

Question 11

On which factors does the colour of a transition metal complex depend?

Answer 11

• Ligand
• Ligand arrangement (affects splitting)
• Transition metal element
• Oxidation state (number of d-electrons)

Question 12

Explain why the following are not coloured:

1. Titanium(IV) oxide
2. Compounds of Sc³⁺
3. Compounds of Zn²⁺
4. Compounds of Cu⁺

Answer 12

They have the following electronic arrangements:

1. Ti⁴⁺ [Ar]
2. Sc³⁺ [Ar]
3. Zn²⁺ [Ar] 3d¹⁰
4. Cu⁺ [Ar] 3d¹⁰

No incompletely filled 3d orbitals / all are either empty or full

Dative covalent bonding with ligands cannot occur

3d electron transitions / splitting not possible

Question 13

What is a complex?

Answer 13

Central transition metal atom or ion surrounded by + bonded coordinately to several ligands.

Question 14

How would you predict the charge of a complex?

Answer 14

Overall charge of metal atom/ion + ligands.

Charge delocalised over whole ion.

Question 15

Define “coordination number”.

Answer 15

The number of dative bonds in a complex between the metal atom/ion and ligands.

D__oesn’t necessarily indicate the number of ligands.

Question 16

State the possible shapes of complexes with coordination numbers 6, 4 and 2.

Answer 16

6: octahedral
4: tetrahedral or square planar
2: linear
* May be distorted, e.g. in [Cu(H₂O)₆]²⁺ 2 bonds are shorter*

Question 17

What is the general nomenclature for complexes?

Answer 17

[no. ligands][ligand prefix] [metal] [ox state] [“ion” if applicable] Ion ends in “ate” if complex is negative​

Question 18

• Name a cobalt(II) ion complexed with 6 water molecules
• Name a copper(II) ion complexed with 4 Cl^- ions
• Name an iron(II) ion complexed with 4 Cl^- ions

Answer 18

• Hexaaqua cobalt(II) ion
• Tetrachloro cuprate(II) ion
• Tetrachloro ferrate(II) ion

Question 19

• State the coordination number of the hexaqua iron(III) ion
• State the shape of the complex ion
• Draw its structural formula and 3D shape

Answer 19

• 6
• Octahedral

Question 20

• State the coordination number of the tetrachloro nickel(II) ion
• State its shape
• Draw its structural formula and 3D shape

Answer 20

• 4
• Tetrahedral

Question 21

Draw the 3D structure of the complex which Cu²⁺ forms in aqueous solution.

Answer 21

Question 22

Answer 22

D

Although 3 is true for 3d elements, it doesn’t include all transition metal elements, i.e. 4d, 5d, etc

Question 23

• The ethanedioate ion is an example of which type of ligand?
• Draw the complex it forms with a metal ion, “M”
• Name the ring formed

Answer 23

• Bidentate ligand (several lone pairs but only forms 2 bonds)
• Chelate ring (five-membered)

Question 24

Which statement(s) about ethanedioate ions is/are correct?

1. Its formula is C₂O₄^2–
2. It is a bidentate ligand
3. Up to three ethanedioate ions can complex with a metal ion

A 1, 2 and 3

B Only 1 and 2

C Only 2 and 3

D Only 1

Answer 24

A

Question 25

Answer 25

B

Question 26

• Write the equation for the reaction between hydrochloric acid and aqueous Cu(II) ions
• What type of reaction is this?

Answer 26

• [Cu(H₂O)₆]²⁺(aq) + 4Cl^-(aq) → [CuCl₄]^2-(aq) + 6H₂O(l)
• Ligand exchange / substitution

Question 27

The addition of OH^- (from aqueous NaOH or ammonia) to aqueous solutions of Cu²⁺, Fe²⁺ and Fe³⁺ results in the formation of coloured precipitates.

For each, write a simplified ionic equation to represent the interaction and state the changes which would be observed.

Answer 27

Cu²⁺(aq) + 2OH^-(aq) → Cu(OH)₂(s)

Blue solution → pale blue precipitate

Fe²⁺(aq) + 2OH^-(aq) → Fe(OH)₂(s)

Green solution → dark green precipitate

Fe³⁺(aq) + 3OH^-(aq) → Fe(OH)₃(s)

Orange-brown solution → red-brown precipitate

Question 28

The addition of OH^- to aqueous solutions of Cu²⁺, Fe²⁺ and Fe³⁺ results in the formation of coloured precipitates.

Write full equations for each.

Answer 28

[Cu(H₂O)₆]²⁺(aq) + 2OH^-(aq) → Cu(OH)₂(H₂O)₄ + 2H₂​O

[Fe(H₂O)₆]²⁺(aq) + 2OH^-(aq) → Fe(OH)₂(H₂O)₄ + 2H₂​O

[Fe(H₂O)₆]³⁺(aq) + 3OH^-(aq) → Fe(OH)₃(H₂O)₃ + 3H₂O

Question 29

The addition of OH^- to aqueous solutions of Cu²⁺, Fe²⁺ and Fe³⁺ results in the formation of coloured precipitates.

Name the type of reaction which occurs.

Answer 29

Acid-base / deprotonation

Not ligand exchange; OH doesn’t replace the water, it deprotonates it

Question 30

The addition of ammonia solution to aqueous solutions of Cu²⁺, Fe²⁺ and Fe³⁺ results in the formation of coloured precipitates.

For which will a change be seen if excess ammonia solution is added? Describe and explain the change observed.

Answer 30

• Cu(OH)₂ re-dissolves in ammonia
• Tetraaminodiaqua copper(II) ion is formed: [Cu(NH₃)₄(H₂O)₂]²⁺
• Pale blue precipitate becomes deep blue-violet solution

Question 31

• Write an ionic equation representing the effect of adding ammonia solution, not in excess, to a solution of Cu²⁺
• Write an equation showing what happens when ammonia is in excess
• Write an overall equation
• Describe the changes observed throughout

Answer 31

Ammonia = weak base: NH₃ + H₂O → NH₄⁺ + OH^-

Ammonia not in excess:

Cu(H₂O)₆]²⁺(aq) + 2OH^- → Cu(H₂O)₄(OH)₂ + 2H₂O(l)

Ammonia in excess:

Cu(H₂O)₄(OH)₂ + 4NH₃(aq) → [Cu(NH₃)₄(H₂O)₂]²⁺(aq) + 2H₂O(l) + 2OH^-(aq)

Overall:

[Cu(H₂O)₆]²⁺(aq) + 4NH₃(aq) → [Cu(NH₃)₄(H₂O)₂]²⁺(aq) + 4H₂O(l)

Blue solution → pale blue precipitate → dark blue-violet solution

Question 32

Adding excess ammonia solution to an aqueous solution of Cu²⁺ results in a colour change of blue to deep blue-purple. Which ligand, of water and ammonia, therefore has the greater splitting effect?

Answer 32

• Blue-purple is higher frequency than blue
• Larger gap between energy levels since ΔE = hv
• Ammonia has greater splitting effect

Question 33

Which of the complex ions of copper(II) with water or ammonia is more stable and why?

Answer 33

• Ammonia
• Undergoes ligand substitution

Question 34

Explain the term complex ion, and why different complexes of d-block elements have different
colours. (6)

Answer 34

Definition of a complex ion:

• Ligands form dative (coordinate) bonds to a central metal ion

Origin of colour:

• Ligands split d-orbitals
• Creates energy gap of right magnitude to allow absorption of visible light
• Electrons are excited to higher energy level
• Frequency absorbed is related to energy gap by ∆E = hv
• Colour seen is complementary colour (due to
  missing frequency)

Cause of different colours:

• Different gaps in split d-orbitals, depending on:
  
  • Ligand
  • Ligand arrangement (affects splitting)
  • D-block element
  • Oxidation state (number of d-electrons)
• So different frequencies are absorbed
• Gives different complementary colours