CRYSTAL FIELD THEORY

CRYSTAL FIELD THEORY

Colour Properties

TRANSITION ELEMENTS

• Present in abundance in the periodic table

• f-block elements

• d-block elements

COORDINATION COMPLEXES𝑴

MAGNETIC & COLOUR PROPERTIES

WHY CFT???

Werner’s Theory

Valence Bond Theory

COULD NOT

EXPLAIN THE

COLOUR

PROPERTIES

Introduction To CFT• Proposed by Bethe and van Vleck.

• Orgel used the concept of crystal field theory to define nature of bonding between ligands and metals.

• It defines the bonding in ionic crystals due to which

this theory is known as crystal field theory.

• Crystal field theory (CFT) explains many important properties of transition-metal complexes, including their

ocolours,

omagnetism,

ostructures,

ostability, and

oreactivity

• d-orbitals are degenerate

• BUT, when ligands get

attached and are uniformly

distributed; their energy gets

higher

• IF, the ligands are oriented at

the axes i.e. x, y and z, the

degeneracy gets removed and

the d-orbitals split into two

groups [ 𝐞𝒈 & 𝐭2𝑔]

• the 𝐞𝐠 group consists of

𝐝𝑧2 & d𝜘2𝑦2 , these have

electrons ON the x, y and z axis

• while, the t2g has dxy, dyz dzx having electrons in BETWEEN the xy, yz and zx axes

• the energies of these two groups 𝐞𝐠 & t2g depend

on their orientations in space

APPEARANCE OF COLOURS• The striking colours that these metal complexes show are because

of the d-d transition of electrons in the d-orbitals

• Involves the jumping of electrons from lower energy orbitals and their subsequent jumping back

• Jumping occurs when electrons get excited

• In case of colour properties, excitation is caused by photons of visible light

• A photon of light can excite and can cause jumping, if its energy is equal to the crystal field splitting energy (Δo) (CFSE)

• CFSE is the difference in energy between the two sets of d-orbitals

• Energy is absorbed and then released, respectively

COLORS & HOW WE PERCEIVE IT

800

430

650 580

560

490

Artist color wheel

showing the colors which

are complementary to one

another and the wavelength

range of each color.

400

BLACK & WHITE

If a sample absorbs all wavelength of visible light, none reaches our eyes from that sample. Consequently, it appears black.

When a sample absorbs light, what we see is the sum of the remaining colors that strikes our eyes.

If the sample absorbs novisible light, it is white or colorless.

COMPLEX INFLUENCE ON COLOR

[Fe(H2O)6]3+

[Co(H2O)6]2+

[Ni(H2O)6]2+

[Cu(H2O)6]2+

[Zn(H2O)6]2+

800

430

650 580

560

490

400

Gemstone owe their color from trace transition-metal ions

Corundum mineral, Al2O3: Colorless

Cr → Al : Ruby

Mn → Al : Amethyst

Fe → Al : Topaz

TRANSITITON METAL GEMS

Ti & Co Al: Sapphire

Beryl mineral, Be3 Al 2Si6O18: Colorless

Cr → Al : Emerald

Fe → Al : Aquamarine

LIMITATIONS OF CFT1. This theory does not consider the splitting of the orbital other than d

orbital.

2. The order of ligand in the Spectro-chemical series can not be explained solely on electrostatic ground.

THANK YOU!!!