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Lecture 30 Electronic Spectra of Coordination Compounds 1) Jahn-Teller effect

Lecture 30 Electronic Spectra of Coordination Compounds 1) Jahn-Teller effect . Octahedral complexes can be a subject to tetragonal or trigonal distortions leading to less symmetrical but more stable structures.

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Lecture 30 Electronic Spectra of Coordination Compounds 1) Jahn-Teller effect

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  1. Lecture 30Electronic Spectra of Coordination Compounds1) Jahn-Teller effect • Octahedral complexes can be a subject to tetragonal or trigonal distortions leading to less symmetrical but more stable structures. • According to the Jahn-Teller theorem, a non-linear molecule with a not-completely filleddegenerate electronic levels can undergo a (vibrational) distortion lowering its symmetry and energy. • In the case of octahedral complexes the tetragonaldistortion reduces Oh symmetry of a complex to D4h producing either elongated or compressed tetragonal bipyramid and reduces degeneracy of eg and t2g orbitals. • The most pronounced stabilization due to the tetragonal distortion is expected for the following configurations: d1 (compression), d4, d7, d9 (various types), d2 (elongation). • No stabilization and thus no distortion is expected for d3, d5 (high spin), d6 (low spin) and d8 configurations.

  2. 2) Static and dynamic Jahn-Teller effect • The Jahn-Teller effect is more pronounced when the former eg level is not completely filled (d1 > d2), that is for configurations d4 (high spin), d7 (low spin) and d9. In these cases a static Jahn-Teller effect can be observed and the species of D4h symmetry can exist in a solid phase. • Much weaker stabilization corresponds to the case of the asymmetrically filled former t2g level such as in d1 species. • In such cases the tetragonal distortion is reflected mainly in electron absorption. We observe appearance of new bands like in the case of Ti(H2O)63+ (two new bands appears as shoulders). d1Ti(H2O)63+

  3. 3) Charge transfer bands • Similar to d-d transitions, charge-transfer (CT) transitions also involve the metal d-orbitals. CT bands are observed if the energies of empty and filled ligand- and metal-centered orbitals are similar. • The direction of the electron transfer is determined by the relative energy levels of these orbitals: i) ligand-to-metal charge transfer (LMCT) like in MnO4-, CrO42- etc. or ii) metal-to ligand charge transfer (MLCT) like in [Fe(bpy)3]2+. The simplified diagrams below are the modified versions of what we had in Lecture 26. Bold arrows show possible CT transitions.

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