6.2.4 Formation of Coloured Ions

Question 1

What does ‘degenerate’ mean in terms of electron promotion?

Answer 1

• In a transition metal ion
• The 3d orbitals are equal in energy

Question 2

What are non-generate orbitals?

Answer 2

• When ligands attach to central metal ion
• Orbitals split into 2 sets of non-degenerate orbitals

Question 3

What happens regarding ΔE when light shines on a transition metal element complex?

Answer 3

• ΔE is the difference in energy between the 2 sets of non-degenerate orbitals
• When light shines on this solution, an electron absorbs exactly that amount of energy (ΔE)
• This electron uses ΔE to excite it from a 3d orbital of lower energy to a 3d orbital of higher energy
• Electron promotion
• Other frequencies of light not absorbed are used to make the complementary colour

Question 4

What is Planck’s equation?

Answer 4

Question 5

How is the complementary colour of a complex ion formed?

Answer 5

• Transition metal ion absorbs the frequency of light corresponding to the exact energy difference ΔE between the 2 non-degenerate orbitals
• The frequencies of light that are not absorbed combine to form the complementary colour of the complex

Question 6

What factors affect ΔE?

Answer 6

• Type of ligand
• Coordination number
• Oxidation state of transition metal ion

Question 7

How do different types of ligands affect ΔE and the complementary colour of a complex?

Answer 7

• Different ligand split the d orbital by different amounts of energy
• Due to the repulsion the d orbital experience from these ligands
• So size of ΔE and frequency of light absorbed by electrons is different for each ligand
• So complementary colours observed are different for each complex, even with the same transition metal ion

Eg.
[Cu(H2O)6]2+ complex has a light blue colour
-
[Cu(NH3)4 (H2O)2]2+ has a dark blue colour
-
Evidence that ligands surrounding complex ions affect the colour of the complex

Question 8

How does the coordination number influence ΔE and the complementary colours of complexes?

Answer 8

• Strength of metal ion - ligand interactions is influenced
• Usually involves a change in ligand too, so a combination of these factors alters the strength of the interactions

Question 9

How does Oxidation state influence ΔE and the complementary colours of complexes?

Answer 9

• The higher the oxidation state
• The stronger the interaction with the ligands
• So absorbs light in the bluer (higher energy) end of the spectrum, thus appearing more orange/red in colour

Eg. [Mn(H2O)6]2+ appears pink because it absorbs in the green region of the spectrum

But [Fe(H2O)6]3+ absorbs light in the blue end of the spectrum, this appearing orange in colour

Same with Fe2+ and Fe3+ complexes

Question 10

How can the colour of complex ions be determined?

Answer 10

• Spectroscopy
• A colorimeter measures the concentration of coloured transition metal ions in solution
• Filter chosen matching complementary colour on spectrum which the complex absorbs
• So if it appears blue, it absorbs red light, so a red filter is chosen
• Some light is absorbed, the rest is passed through to the detector
• Calibration curve of absorbance against concentration is plotted (using known standard concentrations)

Question 11

What are some limitations with using visible light spectrscopy?

Answer 11

• Darkly coloured solutions may be hard for colorimeter to read accurately
• Colorimeter may not be sensitive enough to read pale solutions (ligand exchange can be used to convert complexes into a more strongly coloured one)