Formation of Coloured Ions
1. Electronic Structure of Transition Metals
Transition metals have partially filled d-orbitals. When these metals form ions, their d-electrons can
transition between energy levels, leading to the absorption of specific wavelengths of light.
2. Crystal Field Theory (CFT) and d-Orbital Splitting
In an octahedral ligand field, d-orbitals split into two energy levels: higher-energy eg orbitals and
lower-energy t2g orbitals. The energy difference (denoted as Delta) between them determines the
wavelength of light absorbed. If visible light is absorbed, the ion appears coloured.
3. Ligand Effects on Color Formation
Different ligands affect the crystal field splitting differently. Strong field ligands, such as CN- and CO,
cause a large Delta, absorbing higher-energy light (e.g., violet or blue), making the complex appear
yellow or red. Weak field ligands, like Cl- and H2O, cause a small Delta, absorbing lower-energy
light (e.g., red), making the complex appear blue or green.
4. Charge Transfer and Molecular Orbital Contributions
Some coloured ions result from charge transfer transitions, where electrons are transferred between
the metal and ligand. This produces intense colours, as seen in potassium dichromate (orange) and
permanganate (purple).
5. Examples of Colored Ions and Their Applications
1. Cu2+ (Copper(II) sulfate) - Blue
2. Fe2+ (Iron(II) sulfate) - Pale green
3. Fe3+ (Iron(III) chloride) - Yellow-brown
4. Cr3+ (Chromium(III) compounds) - Green or violet
5. MnO4- (Permanganate ion) - Purple
These coloured ions are widely used in pigments, indicators, and catalysis.
1. Electronic Structure of Transition Metals
Transition metals have partially filled d-orbitals. When these metals form ions, their d-electrons can
transition between energy levels, leading to the absorption of specific wavelengths of light.
2. Crystal Field Theory (CFT) and d-Orbital Splitting
In an octahedral ligand field, d-orbitals split into two energy levels: higher-energy eg orbitals and
lower-energy t2g orbitals. The energy difference (denoted as Delta) between them determines the
wavelength of light absorbed. If visible light is absorbed, the ion appears coloured.
3. Ligand Effects on Color Formation
Different ligands affect the crystal field splitting differently. Strong field ligands, such as CN- and CO,
cause a large Delta, absorbing higher-energy light (e.g., violet or blue), making the complex appear
yellow or red. Weak field ligands, like Cl- and H2O, cause a small Delta, absorbing lower-energy
light (e.g., red), making the complex appear blue or green.
4. Charge Transfer and Molecular Orbital Contributions
Some coloured ions result from charge transfer transitions, where electrons are transferred between
the metal and ligand. This produces intense colours, as seen in potassium dichromate (orange) and
permanganate (purple).
5. Examples of Colored Ions and Their Applications
1. Cu2+ (Copper(II) sulfate) - Blue
2. Fe2+ (Iron(II) sulfate) - Pale green
3. Fe3+ (Iron(III) chloride) - Yellow-brown
4. Cr3+ (Chromium(III) compounds) - Green or violet
5. MnO4- (Permanganate ion) - Purple
These coloured ions are widely used in pigments, indicators, and catalysis.
