Mysteries of polarized Mysteries of polarized light light Enantiomers have identical properties except in one Enantiomers have identical properties except in one respect: the rotation of the plane of polarization of respect: the rotation of the plane of polarization of light light Modern symbols are (+) and (-) Modern symbols are (+) and (-) Days of yore Days of yore d and and l (dextrose) (dextrose) Racemic mixture contains equal portions of the (+) and Racemic mixture contains equal portions of the (+) and (-) (-)
Transcript
Mysteries of polarized lightMysteries of polarized light
Enantiomers have identical properties except in one respect: Enantiomers have identical properties except in one respect: the rotation of the plane of polarization of lightthe rotation of the plane of polarization of light
Modern symbols are (+) and (-)Modern symbols are (+) and (-) Days of yore Days of yore dd and and ll (dextrose) (dextrose) Racemic mixture contains equal portions of the (+) and (-)Racemic mixture contains equal portions of the (+) and (-)
Transition metal ions and Transition metal ions and spectroscopyspectroscopy
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 artist’s wheelThe artist’s wheel
Valence bond repriseValence bond reprise
Valence bond theory is the simplest approach Valence bond theory is the simplest approach to an orbital picture of covalent bondsto an orbital picture of covalent bonds
Each covalent bond is formed by an overlap of Each covalent bond is formed by an overlap of atomic orbitals from each atom atomic orbitals from each atom
The individual orbital identity is retainedThe individual orbital identity is retained The bond strength is proportional to the The bond strength is proportional to the
amount of orbital overlap amount of orbital overlap
Valence bond picture in complexesValence bond picture in complexes
In the conventional covalent bond, each atomic In the conventional covalent bond, each atomic orbital brings one electron with itorbital brings one electron with it
In the coordination complex, the ligand provides In the coordination complex, the ligand provides both, while the metal orbital is emptyboth, while the metal orbital is empty
Geometry and hybridizationGeometry and hybridization
The original atomic orbitals are mixed together and The original atomic orbitals are mixed together and transformed into a new set of hybrid orbitals that transformed into a new set of hybrid orbitals that match the directional requirements for bondingmatch the directional requirements for bonding
66 OctahedralOctahedral dd22spsp33 or or spsp33dd22 [Cr(H[Cr(H22O)O)66]]3+3+
Electron configurations and Electron configurations and geometrygeometry
Electronic configuration of CoElectronic configuration of Co2+2+ is [Ar]3d is [Ar]3d77
Empty 4s and 4p orbitals are used for bonding in Empty 4s and 4p orbitals are used for bonding in tetrahedral complextetrahedral complex
Three unpaired d electrons mean that the CoThree unpaired d electrons mean that the Co2+2+ is is paramagneticparamagnetic
3d 4p4s
Metal electrons
ligand electrons
Square planarSquare planar
Electronic configuration of NiElectronic configuration of Ni2+2+ is 3d is 3d88
Square planar geometry is dspSquare planar geometry is dsp22
Use of one d orbital forces pairing of the Ni d Use of one d orbital forces pairing of the Ni d electronselectrons
Ni(CN)Ni(CN)442-2- is diamagnetic is diamagnetic
Octahedral complexesOctahedral complexes Two options: dTwo options: d22spsp33 or sp or sp33dd22
Same or different?Same or different? Low spin Co(CN)Low spin Co(CN)66
3-3- diamagnetic diamagnetic
High spin CoFHigh spin CoF663-3- paramagnetic paramagnetic
4p4s3d
4p4s3d 4d
Let’s spinLet’s spin
Why are some complexes high-spin and others Why are some complexes high-spin and others low spin?low spin?
Valence bond theory can describe the bonding Valence bond theory can describe the bonding in complexes which is consistent with in complexes which is consistent with observed magnetic properties; it cannot observed magnetic properties; it cannot explain why the ligands dictate one over the explain why the ligands dictate one over the otherother
Enter the crystal field theory…Enter the crystal field theory…
The crystal field theoryThe crystal field theory
The ligands are The ligands are considered negative considered negative chargescharges
The central ion is a The central ion is a positive chargepositive charge
The effect of the The effect of the electrostatic interactions electrostatic interactions on the energies of the d on the energies of the d orbitals form the basis orbitals form the basis of the theoryof the theory
Relative positions of ligands and d Relative positions of ligands and d orbitalsorbitals
ddxyxy etc interact least with the ligands etc interact least with the ligands
ddx2-y2x2-y2 and d and dz2z2 interact most with the ligands in an interact most with the ligands in an
octahedral fieldoctahedral fieldOrbitals “miss” the ligands
Orbitals “hit” the ligands
Crystal field splittingCrystal field splitting
The orbitals that interact more strongly with the The orbitals that interact more strongly with the ligands are raised in energy (electrostatic repulsion) ligands are raised in energy (electrostatic repulsion) more than those that interact less stronglymore than those that interact less strongly
The result is a splitting of the levelsThe result is a splitting of the levels
Splitting and spectroscopySplitting and spectroscopy
Electrons in the incompletely filled d orbitals can be Electrons in the incompletely filled d orbitals can be excited from lower occupied to higher unoccupied excited from lower occupied to higher unoccupied orbitalsorbitals
The frequency of the absorption is proportional to the The frequency of the absorption is proportional to the crystal field splitting: crystal field splitting: ΔΔ = hc/ = hc/λλ
Splitting and spectroscopySplitting and spectroscopy
Electrons in the incompletely filled d orbitals can be Electrons in the incompletely filled d orbitals can be excited from lower occupied to higher unoccupied excited from lower occupied to higher unoccupied orbitalsorbitals
The frequency of the absorption is proportional to the The frequency of the absorption is proportional to the crystal field splitting: crystal field splitting: ΔΔ = hc/ = hc/λλ
Coat of many coloursCoat of many colours
Transition metal ions exhibit colours that vary Transition metal ions exhibit colours that vary strongly with the type of ligand usedstrongly with the type of ligand used
Spectrochemical series orders the ligands according Spectrochemical series orders the ligands according to the degree of crystal field splitting achievedto the degree of crystal field splitting achieved
An absorption peak of 500 nm corresponds to An absorption peak of 500 nm corresponds to a crystal field splitting of a crystal field splitting of
On a molar basisOn a molar basis
Jxmx
smxsJx 199
834
1098.310500
)/1000.3)(.10626.6(
molkJ
molionxionJx
/240
)/1002.6)(/1098.3( 2319
Spectrochemical series of ligandsSpectrochemical series of ligands
Strong fieldStrong field When the d orbitals are empty (dWhen the d orbitals are empty (d00) or full (d) or full (d1010), ),
the complexes are colourless – no d – d the complexes are colourless – no d – d transitionstransitions
The theory successfully accounts for observed The theory successfully accounts for observed optical and magnetic propertiesoptical and magnetic properties
Comparison of Co(CN)Comparison of Co(CN)663-3- and andCoFCoF66
3-3-
Opposition of electron-electron repulsion and lower energy of Opposition of electron-electron repulsion and lower energy of lower lying orbitalslower lying orbitals
High-spin complex: High-spin complex: ΔΔ is lower than P (electrons unpaired, is lower than P (electrons unpaired, repulsion dominates)repulsion dominates)
Low-spin complex:Low-spin complex: ΔΔ is higher than P (electrons pair, lower is higher than P (electrons pair, lower energy of the lower orbitals)energy of the lower orbitals)
Important noteImportant note
Low-spin, high-spin dichotomy only occurs Low-spin, high-spin dichotomy only occurs for dfor d44 – d – d77. .
dd11 – d – d33 and d and d88 – d – d1010 only have one only have one configurationconfiguration
Crystal field splitting in square Crystal field splitting in square planar and tetrahedral complexesplanar and tetrahedral complexes
Tetrahedral is inverse of octahedralTetrahedral is inverse of octahedral ΔΔ is lower than in octahedral because of fewer is lower than in octahedral because of fewer
ligands – all complexes high-spinligands – all complexes high-spin Crystal field splitting in square planar is between Crystal field splitting in square planar is between
the high-lying and the orbitalthe high-lying and the orbital Square planar is favoured for dSquare planar is favoured for d88 configuration configuration
