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CHEM2402/2912/2916 [Part 2]

CHEM2402/2912/2916 [Part 2]. Ligand-Field Theory and Reaction Kinetics. A/Prof Adam Bridgeman Room: 222 Email: A.Bridgeman@chem.usyd.edu.au www.chem.usyd.edu.au/~bridge_a/chem2 Office Hours: Monday 2-4pm, Wednesday 2-4pm Other times by appointment or by chance. Where Are We Going…?.

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CHEM2402/2912/2916 [Part 2]

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  1. CHEM2402/2912/2916 [Part 2] Ligand-Field Theory and Reaction Kinetics A/Prof Adam Bridgeman Room: 222 Email: A.Bridgeman@chem.usyd.edu.au www.chem.usyd.edu.au/~bridge_a/chem2 Office Hours: Monday 2-4pm, Wednesday 2-4pm Other times by appointment or by chance

  2. Where Are We Going…? • Shriver & Atkins, ‘Inorganic Chemistry’, OUP, 4th Edition, 2006 • Week 4: Electronic Spectroscopy • Jahn-Teller effect • Electronic spectra of multi-electron atoms • Nephelauxetic effect and the spectrochemical series • Selection rules • Charge transfer transitions • Week 5: Bonding in Complexes, Metalloproteins and Materials • π-Donor and π-Acceptor Ligands • Metal-metal bonding in clusters and solids • Role of ligand-field effects in electrochemistry • Week 6: Kinetics and Ligand-Field Effects • Differential rate laws • Ligand substiution reactions

  3. By the end of week 4… Ligand-field (‘d-d’) spectroscopy • be able to predict/explain number of bands for d1-d9 (high-spin) • be able to calculate Doct for d1, d3, d4, d6, d7, d8 and d9 • be able to explain differences in band intensity (spin forbidden, orbitally forbidden, Laporte forbidden) • be able to explain the appearance of charge transfer transitions • be able to explain and predict the occurance of the Jahn-Teller effect and its consequences (structural, spectroscopic, reaction rates) Resources • Slides for lectures 1-3 • Shriver and Atkins “Inorganic Chemistry” Chapter 19 (4th Edition)

  4. Schedule • Lecture 1: Electronic absorption spectroscopy Jahn-Teller effect and the spectra of d1, d4, d6 and d9 ions • Lecture 2: Interpreting electronic spectraInterelectron repulsion and the nephelauxetic effect • Lecture 3: Interpreting electronic spectraSelection rules and charge transfer transitions

  5. Revision – Ligand-Field Splitting • In the absence of any ligands, the five d-orbitals of a Mn+ transition metal ion are degenerate • Repulsion between the d-electrons and ligand lone pairs raises the energy of each d-orbital JKB – Lecture 3, S & A 19.1

  6. Revision – Ligand-Field Splitting • Two of the d-orbitals point along x, y and z and are more affected than the average • Three of the d-orbitals point between x, y and z and are affected less than the average • The ligand-field splitting (eg) (t2g) (Doct) eg Doct t2g JKB – Lecture 3, S & A 19.1

  7. Electronic Spectra of d1 Ions • A d1 octahedral complex can undergo 1 electronic transition • The ground state (t2g)1 comprises three degenerate arrangements • The excited state (eg)1 comprises two degenerate arrangements • The electronic transition occurs at Doct eg eg Ti3+(aq) Doct t2g t2g excited state ground state JKB – Lecture 3, S & A 19.3

  8. Electronic Spectra of High Spin d4 Ions • A high spin d4 octahedral complex can also undergo just 1 transition • The ground state (t2g)2(eg)1 comprises two degenerate arrangements • The excited state (t2g)2(eg)2 comprises three degenerate arrangements • The electronic transition occurs at Doct • No other transitions are possible without changing the spin eg eg Cr2+(aq) Doct t2g t2g ground state excited state JKB – Lecture 3, S & A 19.3

  9. Electronic Spectra of High Spin d6 and d9 Ions • High spin d6 and d9 octahedral complexes can also undergo just 1 transition • The electronic transition occurs at Doct • No other transitions are possible changing the spin d6 d9 excited state ground state excited state ground state Cu2+(aq) Fe2+(aq)

  10. Effect of Distortion on the d-Orbitals • Pulling the ligands away along z splits eg and lowers the energy of dz2 • It also produces a much smaller splitting of t2g by lowering the energy of dxz and dyz • Doct >>> d1 >> d2 eg d1 Doct t2g d2 tetragonal elongation JKB – Lecture 6, S & A p467

  11. Which Complexes Will Distort? • Relative to average: t2g go down by 0.4Doct in octahedral complex • Relative to average: eg go up by 0.6Doct in octahedral complex • Relative to averagedz2 is stablilized by ½d1 and dx2-y2 is destablilized by ½d1 • Relative to averagedxz and dyz are stablilized by ⅔d2 and dxy is destablilized by ⅓d2 +½d1 d1 eg +0.6 Doct -½d1 Doct +⅔d2 d2 t2g -0.4 Doct -⅓d2 distorted octahedron octahedron

  12. Which Complexes Will Distort? Doct >>> d1 >> d2 3 1 -0.4Doct - 0.33d2 yes small +½d1 eg +0.6 Doct -½d1 +⅔d2 t2g -0.4 Doct -⅓d2

  13. Which Complexes Will Distort? Doct >>> d1 >> d2 3 1 -0.4Doct - 0.33d2 yes small 2 3 -0.8Doct - 0.67d2 yes small 1 -1.2Doct 3 no no 2 -0.6Doct - 0.5d1 3 yes large 1 1 3 0 no none 2 +½d1 eg +0.6 Doct -½d1 +⅔d2 t2g -0.4 Doct -⅓d2

  14. Which Complexes Will Distort? Doct >>> d1 >> d2 3 1 -0.4Doct - 0.33d2 yes small 2 3 -0.8Doct - 0.67d2 yes small 1 -1.2Doct 3 no no 2 -0.6Doct - 0.5d1 3 yes large 1 1 3 0 no none 2 3 4 2 -0.4Doct - 0.33d2 yes small 5 3 -0.8Doct - 0.67d2 2 yes small 1 -1.2Doct 6 no no 2 3 2 -0.6Doct - 0.5d1 6 yes large

  15. Which Complexes Will Distort? Doct >>> d1 >> d2 • Low spin: +½d1 eg +0.6 Doct -½d1 +⅔d2 t2g -0.4 Doct -⅓d2

  16. Which Complexes Will Distort? • Large distortions (always seen crystallographically) • : • high spin d4 • low spin d7 • d9 Cr2+ Co2+ Cu2+ • Small distortions (often not seen crystallographically): • d1 • d2 • low spin d4 • low spin d5 • high spin d6 • high spin d7

  17. Jahn-Teller Theorem • This is a general result known as the Jahn-Teller theorem: Any molecule with a degenerate ground state will distort antibonding bonding +

  18. Effect on Spectroscopy • From Slide 6, there is one d-d transition for an octahedral d1ion • From Slide 15, a d1complex will distort and will not be octahedral • There are now 3 possible transitions • (A) is in infrared region and is usually hidden under vibrations • (B) and (C) are not usually resolved but act to broaden the band Ti3+(aq) eg (B) (C) t2g (C) (A) (B)

  19. Summary By now you should be able to.... • Show why there is a single band in the visible spectrum for d1, high spin d4, high spin d6and d9 octahedral complexes • Obtain the value of Doct from the spectrum of these ions • Show the electronic origin of the (Jahn-Teller) distortion for high spin d4, low spin d7 and d9octahedral complexes • Predict whether any molecule will be susceptible to a Jahn-Teller distortion • Explain how the Jahn-Teller effect leads to broadening of bands in the UV/Visible spectrum • Next lecture • Effects of interelectron repulsion

  20. Practice

  21. Practice

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