Electronic spectra of complexes and Tanabe-Sugano diagrams · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Reading the colours of metal complexes

Science · Chemistry · ages 20-22
Name ______________________   Date ____________

  1. For d to d absorption in a centrosymmetric complex, which transitions give the main weak bands, and which are much weaker or missing?

    • Spin forbidden jumps appear as the main bands, while spin allowed ones are too weak to see
    • Every possible jump appears with equal height
    • Spin allowed jumps appear as the main weak bands, while spin forbidden ones hide or vanish

  2. A complex shows three absorption bands but has one splitting energy. What does each band connect?

    • Two whole ion states of the full d shell
    • Two orbital energies of a single d electron only
    • Two different molecules in the sample

  3. Intense absorption in an octahedral complex can suggest charge transfer, but band intensity alone does not prove this assignment.

    Circle one:   True   False

  4. Why is manganate(VII) deep purple although the metal has no d electrons?

    • Its colour is an allowed ligand to metal charge transfer, which needs no d electrons
    • It still performs d to d jumps, using the empty d orbitals as both its start and its finish
    • The colour comes from spin forbidden jumps hiding in the spectrum

  5. How do you read a Tanabe Sugano diagram to get the splitting energy?

    • Take the position of the tallest band in the spectrum and use that value directly as the gap
    • Match the ratio of band energies on the diagram, then use Δ/B and B to get the gap
    • Count the bands and multiply by the first band position

  6. An octahedral nickel(II) complex shows three weak bands. How are they assigned?

    • As three intense charge transfer bands, one from each pair of trans ligands
    • As the three spin allowed jumps from the ground state of the d8 ion
    • As one real band plus two instrument artefacts

  7. Two centrosymmetric complexes each show a weak and an intense band. Use a model where weak bands are d to d and intense bands are allowed charge transfer; only these types contribute. The second intense band is at higher energy. What can you conclude?

    • Both intense bands are spin forbidden d to d transitions with no charge transfer contribution
    • Both intense bands are allowed charge transfer; the second transition needs more energy
    • The weak bands are charge transfer and the intense bands are instrument artefacts, not chemical transitions

  8. A student assigns a weak d to d band of an octahedral cobalt(II) complex using only single-orbital labels. What do the state labels A and T describe instead?

    • The direction of one d orbital toward a ligand, independently of the other electrons
    • The symmetry of the whole ion's electronic state
    • The number of ligands around the metal and the charge carried by each ligand
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Answer key

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

Reading the colours of metal complexes W1-mt_GQFMqha0es-s1

  1. Spin allowed jumps appear as the main weak bands, while spin forbidden ones hide or vanish · Spin allowed d to d transitions give the main weak bands in centrosymmetric complexes. Spin forbidden transitions are much weaker or missing.
  2. Two whole ion states of the full d shell · Electron repulsion splits each configuration into several states, and each band jumps between two of them.
  3. True · Charge transfer often gives much stronger absorption than d to d transitions. Ligand-centred transitions can also be intense, so intensity is a clue rather than proof.
  4. Its colour is an allowed ligand to metal charge transfer, which needs no d electrons · Manganate(VII) has no d electrons, so its purple cannot come from a d to d transition. It comes from intense oxygen to metal charge transfer.
  5. Match the ratio of band energies on the diagram, then use Δ/B and B to get the gap · The bands fix the parameters. Their ratio locates Δ/B, one band's E/B gives B, and Δ/B times B gives the gap.
  6. As the three spin allowed jumps from the ground state of the d8 ion · Octahedral nickel(II) is d8. Its three main weak bands are assigned to spin allowed d to d transitions from the ground state; their positions can be used with the diagram to find the gap.
  7. Both intense bands are allowed charge transfer; the second transition needs more energy · The stated model assigns both intense bands to allowed charge transfer. The second band is at higher energy, so its transition requires more energy. This conclusion uses the model, not intensity alone.
  8. The symmetry of the whole ion's electronic state · A and T describe the symmetry of the whole ion's electronic state. Single-orbital labels do not supply that state symmetry for assigning the band.
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