What a learner can do afterwards
- Explains why the two sets of d orbitals separate in an octahedral field, from the direction each orbital points
- Draws the splitting pattern for octahedral, tetrahedral and square planar fields and compares the gap sizes
- Calculates crystal field stabilisation energy for a stated configuration
- Links the gap to the wavelength absorbed and so to the colour observed
1 · Read
Bring six ligands up along the axes and the five d orbitals stop being equal. The two orbitals aimed straight at the ligands meet their charge head on and rise in energy, while the three aimed between the axes drop. The gap between the upper pair and the lower trio is the splitting energy, and it controls the colour and magnetism of the complex.
Tetrahedral ligands sit between the axes: three d orbitals rise and two fall, reversing the octahedral order. In the ideal point-charge model, with the same metal, oxidation state, ligands and metal-ligand distance, the gap is about four ninths of the octahedral gap. Square planar has four levels with a large top step. Oxidation state sets the d count. Electrons fill the lower set singly, then pair or cross the gap at the lower cost. The small tetrahedral gap almost always favours high spin.
Stabilisation energy is bookkeeping: count minus 0.4 of the gap per lower electron and plus 0.6 per upper one. A high spin d6 ion holds four electrons below and two above, giving minus 1.6 plus 1.2, which is minus 0.4 of the gap. Its six electrons fill all five orbitals singly and then one pair forms, so 6 - 2 = 4 electrons stay unpaired.
For d to d absorption, a photon promotes an electron across the splitting gap. Your eye sees the transmitted light, so absorbing violet and blue can leave yellow. A bigger gap absorbs shorter wavelength light. Oxidation state also affects splitting. To compare ligand effects in octahedral complexes, hold the metal and oxidation state fixed. Strong field ligands give a larger gap and shorter wavelength absorption; weak field ligands give a smaller gap and longer wavelength absorption.
Ligands split the d orbitals by direction, electron counts set the stabilisation, and the gap decides which colour is absorbed.
Written for this project.
Two rewritten readings were written for this dataset; two readings written for this project. Video by Khan Academy.
2 · Watch
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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.