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Molecular Orbital Theory and Charge Transfer Excitations

Molecular Orbital Theory and Charge Transfer Excitations. Chemistry 123 Spring 2008 Dr. Woodward. Molecular Orbital Diagram H 2. Antibonding Molecular Orbital (Orbitals interfere destructively). Energy. H 1s Orbital. H 1s Orbital. Bonding Molecular Orbital

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Molecular Orbital Theory and Charge Transfer Excitations

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  1. Molecular Orbital Theory and Charge Transfer Excitations Chemistry 123 Spring 2008 Dr. Woodward

  2. Molecular Orbital Diagram H2 Antibonding Molecular Orbital (Orbitals interfere destructively) Energy H 1s Orbital H 1s Orbital Bonding Molecular Orbital (Orbitals interfere constructively)

  3. Orbital overlap – Constructive Interference Constructive Interference Between the nuclei the wavefunctions add together. Electron density maximized in the internuclear region. This type of overlap leads to formation of a covalent bond. + =

  4. Orbital overlap – Destructive Interference Destructive Interference Between the nuclei the wavefunctions cancel each other out. Electron density pushed away from internuclear region. This type of overlap works against formation of a covalent bond. + =

  5. Sigma (σ) Bondinghead on overlap Pi (π) Bondingside on overlap + = + = Atom 1 py orbital Atom 2 py orbital Bonding sigma molecular orbital Atom 1 pz orbital Atom 2 pz orbital Bonding pi molecular orbital Constructive Interference Constructive Interference + = + = Atom 1 py orbital Atom 2 py orbital Bonding sigma molecular orbital Atom 1 pz orbital Atom 2 pz orbital Antibonding pi molecular orbital Destructive Interference Destructive Interference

  6. Principles of MO Theory • 1. Conservation of Orbitals: The number of Molecular Orbitals is equal to the number of Atomic Orbitals. • 2. Conservation of Electrons:The number of electrons occupying the molecular orbitals is equal to the sum of the valence electrons on the constituent atoms. • 3.Pauli Exclusion Principle:Each MO can hold two electrons of opposite spin. • 4.Hunds Rule:When orbitals are degenerate (at the same energy) all electron spins are the same direction (up) until we have to start putting two electrons in the same orbital. • 5.Principle of Orbital Mixing:The splitting between bonding and antibonding MO’s decreases as: • The spatial overlap decreases (due to orientation of the orbitals, interatomic distance, or size of orbitals) • The orbital electronegativities become different

  7. H : N H H [Cr(NH3)6]3+Octahedron Cr3+ :NH3 5 d-orbitals on Cr (Cr3+ = d3 ion) 3 electrons in the d-orbitals 6 Ligand Orbitals Nitrogen lone pairs (all containing 2 e-) Only sigma interactions are allowed

  8. Antibonding (s*) Metal-Ligand MO’s [Cr(NH3)6]3+ eg orbitals (dz2, dx2-y2) D = Crystal Field Splitting Energy t2g orbitals (dxz, dyz, dxy) Metal (Cr) d-orbitals Nonbonding Metal d MO’s Energy Nonbonding Ligand MO’s Ligand (N) lone-pair orbitals Bonding (s) Metal-Ligand MO’s

  9. Absorption Spectra Cr3+ Solutions Antibonding (s*) Metal-Ligand MO’s Doct Nonbonding Metal d MO’s

  10. [CrO4]2- t2 orbitals (more antibonding) eorbitals (antibonding) CT Metal (Cr) d-orbitals Nonbonding Oxygen 2p MO’s Energy eorbitals (bonding) 12 Oxygen 2p orbitals (4 oxygens x 3 p orbitals) t2 orbitals (bonding)

  11. Absorption Spectra CrO42- Solutions Antibonding Cr 3d orbitals CT1 Nonbonding Oxygen 2p MO’s

  12. Absorption Spectra CrO42- Solutions Antibonding Cr 3d orbitals CT2 Nonbonding Oxygen 2p MO’s

  13. Charge Transfer Salts, ACrO4 The absorbance of SrCrO4 is similar to a concentrated solution of CrO42- ions.

  14. Charge Transfer Excitations and Periodic Trends We can expect charge transfer transitions when we have a d0 cation in a high oxidation state. How does the charge transfer change as we move around the periodic table?

  15. CrO42- vs. MnO42-

  16. Antibonding (e) Cr dx2-y2, dz2 Antibonding (e) Mn dx2-y2, dz2 CT CT Nonbonding O 2p Nonbonding O 2p [CrO4]2- [MnO4]- As the cation oxidation state increases [i.e. Cr(VI)  Mn(VII)] d-orbitals become more electronegative (lower in energy) CT Energy Gap decreases Absorption shifts to longer wavelengths

  17. SrMoO4 – SrCrO4 Series SrMoO4 SrCrO4

  18. Orbital Radii – Group 6 Cr 3d r = 0.46 Å Cr 4s r = 1.63 Å The d orbitals are always much smaller than the s and p, but the 3d orbitals are particularly small Mo 4d r = 0.73 Å Mo 5s r = 1.75 Å W 5d r = 0.78 Å W 6s r = 1.65 Å

  19. Antibonding (e) Mo dx2-y2, dz2 Antibonding (e) Cr dx2-y2, dz2 CT CT Nonbonding O 2p Nonbonding O 2p [CrO4]2- [MoO4]2- Mo 4d orbitals are larger than the Cr 3d orbitals d-orbitals interact more with O 2p orbitals – more antibonding CT Energy Gap increases Absorption shifts to shorter wavelengths

  20. [Co(H2O)6]3+ D = 2.25 eV [Rh(H2O)6]3+ D = 4.23 eV 2nd & 3rd Row Transition Metals eg (s*) • 2nd and 3rd row transition metals • d-orbitals are larger • Metal-ligand antibonding interactions are stronger • eg (s*) orbitals are more antibonding • Low spin configurations are always observed

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