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

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).

The artists wheel

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

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

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 The original atomic orbitals are mixed together and transformed into a new set of hybrid orbitals that match the directional requirements for bonding Coordination number Geometry Hybrid orbitals Example 2 Linearsp [Ag(NH 3 ) 2 ] + 4 Tetrahedralsp3 [CoCl 4 ] 2- 4 Square Planar dsp 2 [Ni(CN) 4 ] 2- 6 Octahedral d 2 sp 3 or sp 3 d 2 [Cr(H 2 O) 6 ] 3+

Electron configurations and geometry Electronic configuration of Co 2+ is [Ar]3d 7 Electronic configuration of Co 2+ is [Ar]3d 7 Empty 4s and 4p orbitals are used for bonding in tetrahedral complex Empty 4s and 4p orbitals are used for bonding in tetrahedral complex Three unpaired d electrons mean that the Co 2+ is paramagnetic Three unpaired d electrons mean that the Co 2+ is paramagnetic 3d4p4s Metal electrons ligand electrons

Square planar Electronic configuration of Ni 2+ is 3d 8 Electronic configuration of Ni 2+ is 3d 8 Square planar geometry is dsp 2 Square planar geometry is dsp 2 Use of one d orbital forces pairing of the Ni d electrons Use of one d orbital forces pairing of the Ni d electrons Ni(CN) 4 2- is diamagnetic Ni(CN) 4 2- is diamagnetic

Octahedral complexes Two options: d 2 sp 3 or sp 3 d 2 Two options: d 2 sp 3 or sp 3 d 2 Same or different? Same or different? Low spin Co(CN) 6 3- diamagnetic Low spin Co(CN) 6 3- diamagnetic High spin CoF 6 3- paramagnetic High spin CoF 6 3- paramagnetic 4p4s3d 4p 4s3d 4d

Lets spin Why are some complexes high-spin and others low 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 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… Enter the crystal field theory…

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

Relative positions of ligands and d orbitals d xy etc interact least with the ligands d xy etc interact least with the ligands d x2-y2 and d z2 interact most with the ligands in an octahedral field d x2-y2 and d z2 interact most with the ligands in an octahedral field Orbitals miss the ligands Orbitals hit the ligands

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 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 The result is a splitting of the levels

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

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

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

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

Spectrochemical series of ligands Weak field Weak field I - <Br - <Cl - <F - <H 2 O<NH 3 <en<CN - Strong field Strong field When the d orbitals are empty (d 0 ) or full (d 10 ), the complexes are colourless – no d – d transitions When the d orbitals are empty (d 0 ) or full (d 10 ), the complexes are colourless – no d – d transitions The theory successfully accounts for observed optical and magnetic properties The theory successfully accounts for observed optical and magnetic properties

Comparison of Co(CN) 6 3- andCoF 6 3- Opposition of electron-electron repulsion and lower energy of lower lying orbitals Opposition of electron-electron repulsion and lower energy of lower lying orbitals High-spin complex: Δ is lower than P (electrons unpaired, repulsion dominates) High-spin complex: Δ is lower than P (electrons unpaired, repulsion dominates) Low-spin complex: Δ is higher than P (electrons pair, lower energy of the lower orbitals) Low-spin complex: Δ is higher than P (electrons pair, lower energy of the lower orbitals)

Important note Low-spin, high-spin dichotomy only occurs for d 4 – d 7. Low-spin, high-spin dichotomy only occurs for d 4 – d 7. d 1 – d 3 and d 8 – d 10 only have one configuration d 1 – d 3 and d 8 – d 10 only have one configuration

Crystal field splitting in square planar and tetrahedral complexes Tetrahedral is inverse of octahedral Tetrahedral is inverse of octahedral Δ is lower than in octahedral because of fewer ligands – all complexes high-spin 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 Crystal field splitting in square planar is between the high-lying and the orbital Square planar is favoured for d 8 configuration Square planar is favoured for d 8 configuration