Electronic Spectra and Term Symbols in Inorganic Chemistry

The electronic spectra of transition-metal complexes are interpreted through free-ion term symbols and their splitting in a ligand field, summarized by Orgel and Tanabe–Sugano diagrams.

Definition

Electronic spectra and term symbols in inorganic chemistry is the interpretation of the absorption spectra of transition-metal complexes in terms of free-ion spectroscopic terms and the way a ligand field splits and orders the resulting electronic states.

Scope

This topic covers the electronic absorption spectra of d-electron complexes: the free-ion terms arising from electron repulsion, their splitting in a ligand field, the selection rules (spin and Laporte) that govern d–d band intensities, the Orgel and Tanabe–Sugano diagrams that map state energies against field strength, and charge-transfer transitions. It applies symmetry and ligand-field ideas to spectra, building on the crystal-field topic and the representation tools.

Core questions

• How do free-ion term symbols arise from a d-electron configuration?
• How does a ligand field split these terms?
• Which transitions are allowed, and why are d–d bands weak?
• How do Tanabe–Sugano diagrams assign the spectrum of a complex?

Key concepts

• Free-ion term symbols
• Splitting of terms in a ligand field
• Spin and Laporte selection rules
• d–d versus charge-transfer transitions
• Orgel diagrams
• Tanabe–Sugano diagrams

Key theories

Free-ion terms and their splitting

Electron–electron repulsion within a d configuration produces spectroscopic terms; in a ligand field these terms split according to symmetry into states whose energies depend on the field strength.

Selection rules and band intensities

The spin and Laporte (parity) selection rules make d–d transitions formally forbidden and therefore weak, with intensity gained through vibronic coupling, while charge-transfer transitions are allowed and intense.

Orgel and Tanabe–Sugano diagrams

Orgel diagrams display term splitting qualitatively, and Tanabe–Sugano diagrams plot state energies against ligand-field strength quantitatively, allowing absorption bands to be assigned and field and repulsion parameters to be extracted.

Clinical relevance

Interpreting electronic spectra lets chemists determine the geometry, oxidation state, and ligand-field strength of metal centres, including those in pigments, gemstones, catalysts, and metalloprotein active sites.

History

Building on Bethe and Van Vleck's ligand-field theory, Tanabe and Sugano published their energy-level diagrams in 1954, and Orgel developed complementary qualitative diagrams. Together with selection-rule analysis these tools turned the colours of transition-metal complexes into quantitative structural information.

Key figures

• Yukito Tanabe
• Satoru Sugano
• Leslie Orgel

Seminal works

• tanabe1954
• weller2018
• figgis2000

Frequently asked questions

Why are the d–d absorption bands of complexes usually so weak?

The Laporte rule forbids transitions between orbitals of the same parity and the spin rule forbids changes in spin, so d–d transitions are doubly hindered; they appear only weakly, gaining intensity through coupling to molecular vibrations that momentarily break the symmetry.

What does a Tanabe–Sugano diagram let you do?

It shows how the energies of a d-ion's electronic states vary with ligand-field strength, so by matching the ratios of observed absorption energies to the diagram you can assign the bands and extract the field-splitting and electron-repulsion parameters of the complex.