Weak-field ligands leave electrons unpaired while strong-field ligands cause pairing in lower orbitals.
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For octahedral complexes of metals with four to seven d electrons, two spin states are possible depending on the size of the crystal field splitting. When the splitting is small, as with weak field ligands, electrons occupy the eg orbitals before pairing in t2g, giving a high spin complex with many unpaired electrons. When the splitting is large, as with strong field ligands, electrons pair in the lower t2g orbitals before occupying eg, giving a low spin complex. The energy cost of pairing must be compared with delta_o to decide which configuration is adopted. The spin state determines magnetism and often colour.
Key ideas: Rule: low spin if delta_o is greater than the pairing energy P. Applies for d4 to d7 in octahedral complexes. Count unpaired electrons: d6 high spin has 4, low spin has 0; d5 high spin has 5, low spin has 1. Fe(III) with CN- is low spin, with F- is high spin. Second and third row metals are almost always low spin.
Common mistakes and connections: Typical errors are applying the idea to d1 to d3 or d8 to d10, where only one arrangement is possible, and assuming that tetrahedral complexes can be low spin (they virtually are not). Also not recalling that low spin complexes have higher CFSE and are often inert. Links to spectrochemical series, magnetism and reaction rates.
When Delta o is below the pairing energy, as with water, Fe2+ is high spin; cyanide gives a larger Delta o and low spin. The interactive lesson shows this as a live animation or chart.
[Fe(CN)6]4- is low spin and diamagnetic, whereas [Fe(H2O)6]2+ is high spin and paramagnetic.
Weak-field ligands leave electrons unpaired while strong-field ligands cause pairing in lower orbitals.
High Spin and Low Spin Complexes is a advanced topic in Bonding & Molecular Structure. It builds on beginner and intermediate topics, so review those first if needed.
Study these earlier topics in Bonding & Molecular Structure first: Chelation, Crystal Field Theory, Spectrochemical Series.
The next topics in Bonding & Molecular Structure are Color of Transition Metal Complexes and Isomerism in Complexes.
Yes. The interactive lesson includes a self-check question on High Spin and Low Spin Complexes, and the Bonding & Molecular Structure quiz draws on every topic in this area.
High Spin and Low Spin Complexes is topic 94 of 104 in Bonding & Molecular Structure.