Colour of complex compounds

Colour of Complex Compounds

Many coordination compounds are brightly coloured, and their colours are due to electronic transitions within the d-orbitals of the central metal ion. The observed colour depends on factors such as the type of metal ion, its oxidation number, and the ligands coordinated to it.

Origin of Colour

The colour of a complex compound mainly arises due to d-d transitions. When white light falls on a complex, certain wavelengths are absorbed as electrons jump between split d-orbitals. The portion of light that is not absorbed and instead reflected or transmitted is what gives the complex its visible colour.

Crystal Field Splitting

When ligands surround the central metal ion in a coordination complex, the originally equal-energy d-orbitals split into two distinct energy levels. This is known as crystal field splitting.

For example, in an octahedral field, the d-orbitals split into two sets: lower energy t_2g and higher energy e_g. When electrons absorb energy equal to the gap between these levels (Δ), they get excited from t_2g to e_g.

Factors Affecting Colour

• Nature of Ligands: Strong field ligands (like CN⁻) cause greater splitting and often change the colour compared to weak field ligands (like H₂O or Cl⁻).
• Oxidation State of Metal: Higher oxidation states usually result in larger splitting, affecting colour.
• Type of Geometry: Octahedral and tetrahedral complexes split d-orbitals differently, affecting the transitions and hence the observed colour.
• Electronic Configuration: Colour is only observed when d-d transitions are allowed. For example, d⁰ and d¹⁰ configurations are usually colourless due to the absence of such transitions.

Examples

• [Ti(H₂O)₆]³⁺ appears violet due to d-d transition of a single d-electron.
• [Cu(H₂O)₆]²⁺ appears blue.
• [Fe(CN)₆]³⁻ appears yellow due to strong field ligands causing greater splitting.

Why Some Complexes are Colourless

Complexes with completely filled (d¹⁰) or empty (d⁰) d-orbitals do not show d-d transitions, so they appear colourless. Example: [Zn(H₂O)₆]²⁺ is colourless as Zn²⁺ has d¹⁰ configuration.

Relevance to JEE Main

Understanding the cause of colour in complex compounds is important in JEE Main. Questions may involve identifying coloured ions, predicting colour changes based on ligand strength, or applying crystal field theory concepts.