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Jahn Teller effect LO_0.ppt
- 1. The Jahn-Teller Effect Theory and examples Created by Adam R. Johnson, Harvey Mudd College (adam_johnson@hmc.edu) and posted on VIPEr on August 7, 2020. Copyright Adam R. Johnson, 2020. This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike License. To view a copy of this license visit http://creativecommons.org/licenses/by-nc-sa/4.0/
- 3. Bond Lengths (Å) Co1 N1 1.962(2) Co1 N2 1.964(2) Co1 N3 1.961(2) Co1 N4 1.961(2) Co1 N5 1.961(2) Co1 N6 1.971(2) [Co(en)3]3+ Data collected at Harvey Mudd College for the tartrate chloride pentahydrate salt (Spring 2019)
- 4. Data from: Simmons, C. J.; Stratemeier, H.; Hitchman, M. A.; Riley, M. J. Inorg. Chem. 2013, 52(18), 10481-10499. (10.1021/ic401385f) Bond Lengths (Å) Cu1 O9 1.9471(13) Cu1 O7 2.0233(18) Cu1 O8 2.3061(12)
- 7. Jahn-Teller effect • A complex with electrons in degenerate energy levels will distort to lower symmetry and decrease the degeneracy if that distortion lowers the energy. • Mn(III) d4 vs Cr(III) d3
- 8. • A complex with electrons in degenerate energy levels will distort to lower symmetry and decrease the degeneracy if that distortion lowers the energy. • Cr(III) d3 vs Co(III) d6 (low spin) Jahn-Teller effect
- 9. Co(acac)3 (ls d6) Data collected at Harvey Mudd College (Spring 2020)
- 10. Mn(acac)3 (hs d4) Data collected at Harvey Mudd College (Spring 2020)
- 11. Fe(acac)3 (hs d5) Data collected at Harvey Mudd College (Spring 2020)
Editor's Notes
- The Janh-Teller effect is a theorem that states that molecules with a degenerate electronic ground state will distort in order to remove the degeneracy if that distortion lowers the overall energy of the molecule.
- Here is the MO diagram for an octahedral complex. For [Co(en)3]3+. This molecule does not have a distortion.
- ORTEP diagram from crystals prepared and diffracted at Harvey Mudd College. Bond lengths are all 1.96 Å, bond angles are all very close to 90°
- The classic J-T distortion is observed in Cu(II) complexes. For example [Cu(H2O)6]2+ would be expected to be octahedral, but as it is a d9 system, it exhibits marked distortions (usually elongation along the z axis) to give the tetragonally distorted square-bipyramidal geometry. The MO diagram for [Cu(H2O)6]2+ is shown. Lets examine what distortions might occur and how it would result in a lower energy for the complex Only 3 bond lengths because there is an inversion center
- Starting with an octahedral complex, two easy to visualize distortions are tetragonal out along z (this slide) or tetragonal in along z (next slide) The distortions are exaggerated in this animation The z-out distortion results in the same energy splitting that is observed upon taking an octahedral complex to a square planar complex (in terms of the crystal field effects on the resulting orbitals) but since the ligands are not being removed from the complex, the magnitude of the shifting is lower.
- Starting with an octahedral complex, two easy to visualize distortions are tetragonal out along z (previous slide) or tetragonal in along z (this slide) The distortions are exaggerated in this animation Note: JT theory predicts that a distortion will occur, but it does not predict WHAT that distortion will be. It could be bond angles changing, twists, etc. However, most can be modeled as pairs of ligands elongating along their bond axis
- Mn(III) is a d4 configuration and if it undergoes a z-out distortion, the resulting d orbital splitting is as shown: dxz/dyz lowest with dxy rising up a little, dz2 drops below dx—y2. this is the same pattern that would be seen moving from Oh to D4h in crystal field theory. The small energy gaps are called ∆(JT) for this example, and are on the order of 10% of the ∆o splitting in the octahedral complex The resulting tetragonally distorted complex would have lower energy because the single electron in the upper “eg” manifold is lower than it was before. The 3 electrons in the t2g manifold are the same net energy because two of the orbitals and 2 of the electrons have gone down by 1/3 ∆(JT) while one orbital and one electron have gone up by 2/3 ∆(JT) Cr(III) is a d3 system and would not be expected to undergo a JT distortion, because no net energy lowering would occur Cu(II) d9 would be expected to, however, as the ”t2g” orbitals are filled and the “eg” orbitals are unequally filled
- Cr(III) is a d3 system and would not be expected to undergo a JT distortion, because no net energy lowering would occur Co(III) d6 low spin similarly would not be expected to undergo a JT distortion.
- Co(acac)3 is a ls d6 configuration, no JT distortion observed Crystal grown and diffracted at HMC in spring 2020
- Mn(III) is a d4 configuration and exhibits a distortion. O1 and O4 are compressed relative to O2, O3, O5 and O6. it is difficult to argue that O2, O3, O5 and O6 have different bond lengths, but ignoring the error bars, O1 and O4 are “short”, O2 and O6 are “medium” and O3 and O5 are “long” bonds. This is just one possible distortion. For a good description of a variety of possible JT distortions observed at different temperatures in Mn(acac)3, see J. Chem. Educ. 2005, 82, 3, 460.
- Fe(III) is a hs d5 configuration and does not exhibit a distortion. Ls d5 would be expected to have a distortion.