Electronic Spectra of Complexes and Tanabe-Sugano Diagrams
A complex often shows more than one absorption band, which a single splitting energy cannot explain. Labelling the electronic states of the whole ion, not the orbitals, gives a diagram that accounts for every band.
What a learner can do afterwards
- Explains why several bands appear for one configuration and what each transition connects
- Reads a Tanabe-Sugano diagram to extract the splitting energy from band positions
- Distinguishes a weak d to d band from an intense charge transfer band and gives the selection-rule reason
- Explains why manganate(VII) is intensely coloured although the metal has no d electrons
1 · Read
A single gap predicts a single band, yet real complexes show two or three. The fix is to label states of the whole d shell instead of single orbitals: electron repulsion splits each configuration into several states with different energies. Every observed band is a jump between two of these many electron states.
Selection rules then thin the crowd. Spin allowed jumps appear as the main weak bands, while spin forbidden ones hide or vanish. To read a Tanabe Sugano diagram, find the ratio of the band positions, slide it along the diagram until the ratios match, then read the gap and pairing parameters off the axes.
Intensity is the second clue after position. Weak bands whisper of forbidden d to d jumps, while a towering band shouts charge transfer, which is fully allowed. Manganate(VII) glows deep purple with zero d electrons purely on ligand to metal charge transfer strength. Octahedral nickel(II) is d8, and its three weak bands are the three spin allowed jumps from the ground state.
Work every spectrum in the same order: count the bands, weigh their heights, then assign. State labels such as A and T track the symmetry of the whole ion, not of single orbitals. Position plus height together usually name the transition before any diagram is opened.
Count the bands, weigh their heights, and read whole ion state jumps off the diagram.
2 · Watch
Take it off screen
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.