See Bengali video lecture bellow

Crystal field theory and coloured compound

Crystal field theory

Crystal field theory explains how transition element, d block element, and complex ion forms coloured compound.

Complex ion have two parts. One is central ion and rests are Ligand connected to central ion by coordinate bond. Ligand have lone pair electron. Lone pair electron of ligand have strong effect on d orbitals of central ion. Lone pair electron of ligand will very strongly repulse on dx²-y² and dz² than others. For that reason 5 d orbital will separated into high energy state and low energy state.  

dxy, dyz, dzx are in lowest energy state and dx²-y², dz² will be in highest energy state. There have a energy difference between them equal to ∆E, which is the main mysterious of creating color.

If the value of ∆E match with any wavelength of visible light then lowest energy state orbitals electron will absorb energy from visible light equal to ∆E and will jump to highest energy state. Again, it will return back to lowest energy state by jumping by emission of energy. This emission of wavelength energy visible as colour based on animals capabilities.