Colours and complex ions Flashcards
How does colour arise in a transition metal compound?
Ligands make d orbitals split as they approach along axis, energy corresponding to a wavelength of light is absorbed to promote e-, complementary colour is observed.
Why are transition metal complexes able to absorb light?
They are able to absorb light due to the five degenerate d orbitals splitting in terms of energy.
Summarise the splitting of d orbitals
The ligands approach the metal ion along the x,y and z axes. The electrons in d orbitals that lie along these axes will be repelled by electrons and the ligand molecules. These orbitals now have higher energy than the three d orbitals that lie between the axes and therefore the five d orbitals are no longer degerate.
How are d orbitals split? 4 mark answer
- 3d orbitals are labelled dxy, dxz, dyz, dx2y2, dz2
-dx2y2 and dz2 orbitals are raised to a higher energy level due to electrostatic repulsion from the ligands in the complex.
-energy difference between the split in the d-orbitals depends on the ligand involved, Ligands are listed in the spectrochemical series.
transition metals can absorb light because photons excite electrons in the lower d orbitals up to a higher energy d orbital - this absorbed energy dissipates as heat energy and doesnt remerge as light
What is the crystal field strength?
The difference in energy between the 2 subsets of d orbitals.
what is the electrochemical series?
A list of ligands according to how strongly they split d orbitals in a transition metal complex.
Cn- NH3. H2O. OH- F. Cl- Br- I-
strong field splitting Weak field splitting
(large E value) (Small E value)
What type of ligands cause a large difference in energy between subsets of d orbitals?
Strong field ligands, these ligands are more likely to occur in the UV region of the EM spectrum
What do weak field ligands do?
Cause a small energy difference between subsets of d orbitals, and is more likely to occur in the visible region of the EM spectrum.
When does splitting of d orbitals occur?
Occurs when the electron present in approaching ligands cause the electrons in the orbitals lying along the axes to be repelled.
Are e the d orbitals in a transition metal complex degenerate?
No
What happens if the difference in energy is equal to the energy of a colour of light?
That wavelength of light will be absorbed and an electron will, be promoted.
If one colour of light is absorbed, what will be observed?
The complementary colour ( see data booklet, colour wheel )
When is a light absorbed?
When electrons on a lower energy d orbital are promoted to a d orbital of higher energy.
What happens if absorbed energy is in the visible light section of the EM spectrum?
the complex will be coloured and a visible spectroscopy will be used.
What happens if absorbed energy is in the UV section of the EM spectrum?
The transition metal complex will be colourless and UV spectroscopy will be used.
