From dots to orbitals in metal complexes

Science · Chemistry · ages 20-21

Name ______________________ Date ____________

In ligand field theory, each ligand group orbital mixes only with a metal orbital of the same symmetry label.
Circle one: True False
For an octahedral complex, what does crystal field theory get right, and what bonding effect does it omit?
- It gets the two up three down pattern right but cannot explain pi effects
- It explains pi bonding well but gets the two up, three down pattern wrong
- It explains every complex, including why fluoride and cyanide differ
How does a pi donor ligand such as fluoride move the splitting gap?
- It pushes the lower set up and narrows the gap
- It pulls the lower set down and widens the gap
- It leaves every level exactly where it was
Cyanide gives a much larger splitting than fluoride. Which pi bonding difference helps explain this?
- Cyanide carries a larger negative charge than fluoride, so it repels the d orbitals more
- Both ligands are pure sigma donors with no pi role
- Cyanide accepts pi density while fluoride donates it, so their gaps move opposite ways
Why does carbon monoxide sit at the strong end of the series although it is neutral?
- Its large size crushes the metal orbitals together
- It accepts metal density back into empty pi orbitals, which widens the gap
- It donates filled pi density to the metal, which raises the lower set further
How do you build the octahedral orbital diagram in ligand field theory?
- Mix ligand group orbitals with metal orbitals of the same label, then read the gap
- Place six point charges around the metal and read the gap from their total charge
- Add each ligand's field strength from the series and read the gap from the sum of the six values
Replacing water ligands with cyanide widens the gap of a complex. In the pi bonding picture, which level moves and how?
- The upper set drops while the lower set stays fixed
- Both sets rise together by the same amount
- The lower set is pulled down by back donation, so the space above it grows
A student explains the series order using ligand charges alone. What breaks in that argument?
- Nothing breaks, since charge predicts the whole series
- Neutral ligands such as CO would rank low, yet CO tops the series through back donation
- Charged ligands would all rank identically, which they do

Answer key

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From dots to orbitals in metal complexes W1-mt_Gq3JG7F7Nl-s1

True · Symmetry matching is how the theory rebuilds the gap from overlap.
It gets the two up three down pattern right but cannot explain pi effects · Electrostatic repulsion explains the octahedral two-up, three-down pattern. The point-charge model omits covalent pi donation and acceptance.
It pushes the lower set up and narrows the gap · Filled ligand pi orbitals push the metal lower set up.
Cyanide accepts pi density while fluoride donates it, so their gaps move opposite ways · Pi acceptance widens the gap while pi donation narrows it, explaining the order of the two ligands.
It accepts metal density back into empty pi orbitals, which widens the gap · Back donation stabilises the lower set and widens the gap.
Mix ligand group orbitals with metal orbitals of the same label, then read the gap · Mixing by symmetry splits the levels, and pi effects then shift the levels.
The lower set is pulled down by back donation, so the space above it grows · Swapping a donor for an acceptor flips the pi shift from pushing up to pulling down.
Neutral ligands such as CO would rank low, yet CO tops the series through back donation · Charge language cannot see pi acceptance, so it misplaces the neutral acceptor CO.