colours of transition metals

why are the colours of different transition metal complexes different?

• when a ligand bonds to a transition metal, the d orbitals split in E

• when they absorb visible light, the e^- move up to higher E orbitals i.e. from the ground state to an excited state - this is called excitation

• the difference in E is called ΔE

• remaining wavelengths of light are transmitted/reflected by the complex and combine to form the colour, which is complementary to the wavelengths absorbed 

give the eqn for calculating the difference in E and state what each symbol corresponds to:

ΔE = h ν = hc / λ, where:

• ΔE: energy absorbed / J

• h: Planck’s constant = 6.63 × 10^-34 Js

• v (Greek letter nu): freq of light absorbed / Hz

• c: speed of light = 2.998 × 10⁸ ms^-1

• λ: wavelength of light absorbed / m

give 3 factors which might affect the colour of a transition metal complex:

• change in oxidation state

• change in coordination number

• change in ligand

how can we use colorimetry to determine the concentration of transition metal ions in a solution of unknown concentration?

1. add an appropriate ligand to intensity colour

2. make up solutions of known concentrations and measure absorption/transmission

3. plot calibration curve

4. measure absorption of unknown and use calibration curve to deduce concentration