Lecture Twenty One - Crystal field theory

Question 1

What are strong and weak field ligands?

Answer 1

Δoct is a measure of the electrostatic repulsion between the metal d electrons ad the ‘point charge’ ligands.

Ligands that cause large Δoct are called strong field ligands.** **

• Bind to metal ion strongly = larger splitting energy = greater ligand-electron repulsion = low spin.

E.g. CN-.

Ligands that casue small Δoct are called weak field ligands.

• Bind to metal ion weakly = smaller splitting energy = less ligand-electron repulsion = high spin.

E.g. Cl-.

Reminder: Δoct is the distance between the eg orbitals and the t2g orbitals.

Question 2

What is high and low spin?

Answer 2

High spin occurs when Hunds rule is obeyed and each orbital is filled with one electron before pairing of electrons occurs.

This can occur only when the Δoct is small enough that the energy required to put an electron in a higher energy orbital (eg orbitals) is less than the energy required to put two electrons together in one of the lower energy orbitals (t2g).

Low spin occurs when Hunds rule is disobeyed and not all orbitals are filled before electrons begin to be paired.

The t2g orbitals in this case are filled with electron pairs before any electrons are then placed in the eg orbitals.

This is because it requires less energy to pair electrons in the lower energy t2g orbitals than to place single electrons in the higher energy eg orbitals.

Question 3

Why are transition metals coloured?

Answer 3

An object is coloured because it reflects or transmits that colour (e.g. absorbs all except green).

Absorbs complementary colour (e.g. absorbs red and looks green).

The colour in transition metals is because depending on the Δoct, they will reflect different wavelengths of light.

ΔE electron = ΔE photon = hv = hc/λ

Where

h = Planck’s constant.

v = Frequency of light.

c = Speed of light.

λ = wavelength.

E.g.

Consider [Ti(H2O)6]3+, λmax = 510 nm.

H2O is a weak field ligand, so Δoct will be quite small.

If H2O is exchanged of Cl- (an even weaker ligand), Δoct will be even smaller and λmax will shift to longer wavelengths.

The greater the Δoct, the shorter the wavelength.