The spectrochemical series organizes ligands according to how strongly they split a metal ion’s d orbitals, and also describes trends among metal ions. Understanding these patterns helps explain the electronic, magnetic, and optical properties of coordination complexes.
When a ligand binds to a metal ion, it changes the energies of the metal’s d orbitals. The energy difference between the resulting orbital groups is called the ligand-field splitting parameter in ligand field theory, or the crystal-field splitting parameter in crystal field theory. It is commonly represented by Δ.
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What is the spectrochemical series?
It is an ordering of ligands by the size of the d-orbital splitting, Δ, they generally produce around a metal ion. A related ordering can also compare metal ions for their general effect on Δ.
What is the difference between weak-field and strong-field ligands?
Weak-field ligands generally produce a smaller Δ and appear toward the left of the ligand series. Strong-field ligands generally produce a larger Δ and appear toward the right.
How do π-donor ligands tend to affect Δ?
π-donor ligands, such as halides and OH⁻, donate electron density through π interactions as well as σ bonding. This tends to reduce the effective splitting Δ.
Why are CN⁻ and CO strong-field ligands?
They can act as π acceptors: suitable vacant orbitals allow metal-to-ligand π backbonding. This additional interaction tends to increase Δ.
What properties can be influenced by Δ?
Δ can influence a complex’s electron arrangement and spin state, its magnetic behavior, and the energies of electronic transitions that contribute to its color and absorption spectrum.
Quiz questions
What does the ligand spectrochemical series rank?Answer: Ligands by the d-orbital splitting they generally produce.
Which ligand generally produces a larger Δ than NH₃ in the listed series?Answer: en
For many octahedral 3d complexes, what can a weak field favor when pairing costs more energy than the splitting?Answer: A high-spin arrangement with more unpaired electrons.
How can Δ affect the color of a coordination complex?Answer: It affects electronic-transition energies and therefore the wavelengths absorbed.
What does the observed ligand ordering suggest about a purely ionic bonding model?Answer: Covalent interactions and π bonding can also influence metal–ligand bonding.