Why complexes have colour

Science · Chemistry · ages 19-20

Name ______________________ Date ____________

In the ideal point-charge model, keep the metal, oxidation state, ligands and metal-ligand distance fixed. How do tetrahedral d orbital ordering and gap size compare with the octahedral pattern?
- Much bigger, more than double the octahedral gap
- Inverted and less than half the size
- Exactly equal in size, but with the order reversed
Why do d orbitals split into two sets in an octahedral field?
- Two orbitals point at the ligands and rise, three point between and sink
- Three orbitals point at the ligands and rise, two point between and sink
- All five orbitals rise by the same amount, so no gap opens between them
In the simple splitting-gap model, a solution absorbs light through d to d excitation across its gap. Its transmitted colour comes from the light left unabsorbed.
Circle one: True False
A high spin octahedral d7 ion puts one electron in each of the five orbitals before any pair. Type its number of unpaired electrons.
Answer: ______________
A high spin octahedral d7 ion has five electrons in the lower set and two in the upper set. Counting -0.4 of the gap for each lower electron and +0.6 for each upper one, and ignoring pairing energy, type its crystal field stabilisation energy in units of the gap.
Answer: ______________
A stronger field ligand widens the gap. Which light gets absorbed now?
- Longer wavelength light, towards the red end
- Shorter wavelength light, towards the blue end
- The same wavelength, since the metal is unchanged
Tetrahedral complexes are usually low spin because their gap is tiny.
Circle one: True False
A student compares splitting-gap absorption in two octahedral complexes using ligand field strength. Which comparison supports predicting that the weak field complex absorbs at longer wavelength?
- Different metals and oxidation states, with the weak field complex assumed to have the larger gap
- The same metal in different oxidation states, with ligand strength assumed to be the only factor
- The same metal and oxidation state, with weak versus strong field ligands

Answer key

For grown-ups. Fold this page away before handing over the rest.

Why complexes have colour W1-mt_YfIBGXj8su-s1

Inverted and less than half the size · Under the stated model conditions, the ordering reverses and the tetrahedral gap is about four ninths of the octahedral gap. Four ninths is less than half.
Two orbitals point at the ligands and rise, three point between and sink · Direction decides energy: head-on orbitals cost more.
True · In this model, light absorbed in exciting an electron is removed from the transmitted light. The light left unabsorbed gives the transmitted colour.
3 · Five electrons go in singly, then the sixth and seventh each pair with one of them. Two pairs out of seven electrons leave three unpaired.
-0.8 · Five lower electrons give 5 x -0.4 = -2.0 and two upper electrons give 2 x +0.6 = +1.2, so the total is -0.8 of the gap.
Shorter wavelength light, towards the blue end · Bigger gaps match higher energy, shorter wavelength photons.
False · With a tiny gap, pairing is almost never worth the energy cost, so tetrahedral complexes stay high spin.
The same metal and oxidation state, with weak versus strong field ligands · Hold the metal and oxidation state fixed to compare ligand effects. Weak field ligands then give the smaller gap, requiring lower photon energy and longer wavelength.