Mysteries of polarized light

1. Mysteries of polarized light • Enantiomers have identical properties except in one respect: the rotation of the plane of polarization of light • Modern symbols are (+) and (-) • Days of yore d and l (dextrose) • Racemic mixture contains equal portions of the (+) and (-)

2. Transition metal ions and spectroscopy The color of a complex corresponds to wavelengths of light that are not absorbed by the complex. The observed color is usually the complement of the color absorbed. If all wavelengths of light are absorbed, a complex appears black. If no wavelengths of light are absorbed, a complex appears white (colorless).

3. The artist’s wheel

4. Valence bond reprise • Valence bond theory is the simplest approach to an orbital picture of covalent bonds • Each covalent bond is formed by an overlap of atomic orbitals from each atom • The individual orbital identity is retained • The bond strength is proportional to the amount of orbital overlap

5. Valence bond picture in complexes • In the conventional covalent bond, each atomic orbital brings one electron with it • In the coordination complex, the ligand provides both, while the metal orbital is empty

6. Geometry and hybridization • The original atomic orbitals are mixed together and transformed into a new set of hybrid orbitals that match the directional requirements for bonding

7. Electron configurations and geometry • Electronic configuration of Co2+ is [Ar]3d7 • Empty 4s and 4p orbitals are used for bonding in tetrahedral complex • Three unpaired d electrons mean that the Co2+ is paramagnetic 3d 4s 4p Metal electrons ligand electrons

8. Square planar • Electronic configuration of Ni2+ is 3d8 • Square planar geometry is dsp2 • Use of one d orbital forces pairing of the Ni d electrons • Ni(CN)42- is diamagnetic

9. Octahedral complexes • Two options: d2sp3 or sp3d2 • Same or different? • Low spin Co(CN)63- diamagnetic • High spin CoF63- paramagnetic 3d 4s 4p 4p 4d 3d 4s

10. Let’s spin • Why are some complexes high-spin and others low spin? • Valence bond theory can describe the bonding in complexes which is consistent with observed magnetic properties; it cannot explain why the ligands dictate one over the other • Enter the crystal field theory…

11. The crystal field theory • The ligands are considered negative charges • The central ion is a positive charge • The effect of the electrostatic interactions on the energies of the d orbitals form the basis of the theory

12. Orbitals “miss” the ligands Orbitals “hit” the ligands Relative positions of ligands and d orbitals • dxy etc interact least with the ligands • dx2-y2 and dz2 interact most with the ligands in an octahedral field

13. Crystal field splitting • The orbitals that interact more strongly with the ligands are raised in energy (electrostatic repulsion) more than those that interact less strongly • The result is a splitting of the levels

14. Splitting and spectroscopy • Electrons in the incompletely filled d orbitals can be excited from lower occupied to higher unoccupied orbitals • The frequency of the absorption is proportional to the crystal field splitting: Δ = hc/λ

15. Splitting and spectroscopy • Electrons in the incompletely filled d orbitals can be excited from lower occupied to higher unoccupied orbitals • The frequency of the absorption is proportional to the crystal field splitting: Δ = hc/λ

16. Coat of many colours • Transition metal ions exhibit colours that vary strongly with the type of ligand used • Spectrochemical series orders the ligands according to the degree of crystal field splitting achieved

17. An absorption peak of 500 nm corresponds to a crystal field splitting of • On a molar basis

18. Spectrochemical series of ligands • Weak field I-<Br-<Cl-<F-<H2O<NH3<en<CN- • Strong field • When the d orbitals are empty (d0) or full (d10), the complexes are colourless – no d – d transitions • The theory successfully accounts for observed optical and magnetic properties

19. Comparison of Co(CN)63- andCoF63- • Opposition of electron-electron repulsion and lower energy of lower lying orbitals • High-spin complex: Δis lower than P (electrons unpaired, repulsion dominates) • Low-spin complex:Δis higher than P (electrons pair, lower energy of the lower orbitals)

20. Important note • Low-spin, high-spin dichotomy only occurs for d4 – d7. • d1 – d3 and d8 – d10 only have one configuration

21. Crystal field splitting in square planar and tetrahedral complexes • Tetrahedral is inverse of octahedral • Δ is lower than in octahedral because of fewer ligands – all complexes high-spin • Crystal field splitting in square planar is between the high-lying and the orbital • Square planar is favoured for d8 configuration