What a learner can do afterwards
- Compares pairing energy with the splitting energy to assign a high or low spin configuration
- Places common ligands in order of field strength and explains why the order does not follow charge
- Predicts the number of unpaired electrons and converts it to a spin-only magnetic moment
- Explains why tetrahedral complexes are almost always high spin
1 · Read
High spin or low spin is a contest of two prices. Promoting an electron across the gap costs the splitting energy, while doubling up inside one orbital costs the pairing energy. If the gap costs more, electrons spread out and stay high spin; if pairing costs less, they double up and go low spin. Whichever bill is smaller wins.
The spectrochemical series ranks ligands by the split they produce, and it is an experimental fact to be used, not derived. Strong field ligands such as cyanide and carbon monoxide sit at the top and force low spin; weak field ligands such as fluoride and water sit at the bottom and allow high spin. Charge does not rule: neutral carbon monoxide outranks negatively charged fluoride, so memorise the order instead of deriving it from charge.
A d5 ion shows the stakes. High spin it carries five unpaired electrons, low spin just one, and magnetism tells you which case you hold. Four unpaired electrons give about 4.90 Bohr magnetons, so a measured moment near that value pins the configuration down. Run the logic backwards: count unpaired electrons from the moment, then ask which arrangement gives that count.
Tetrahedral fields are a special case: the gap is so tiny that the pairing price is never worth paying, so tetrahedral complexes are almost always high spin. Octahedral fields allow both outcomes, which is where the series earns its keep.
Compare the gap with the pairing cost, read the ligand's rank in the series, and let the measured moment confirm the spin state.
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.