a brief note on chirality - the dionne group
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A brief note on chirality
Stereogenic Chirality
Axial Chirality(heptahelicene)
Chiral Planes Chiral Planes (trans-cyclooctene)
Lecture 17Lecture 17
Crystal & Ligand Field Theory
Suggested reading: Shriver & Atkins, Chapter 20
Recall from last class: 2 Models of Complexes
Crystal Field TheoryCrystal Field Theory
• emerged from studying d-metal ions in solids •based on an electrostatic model of bonding, where ligands are
modeled as point charges around the metal•The negative charge from the ligand repels the electrons in the
d orbitals of the metald b ta t e eta• rationalizes optical spectra, thermodynamic stability, and
magnetic properties
Ligand Field Theory
• applies Molecular Orbital theory • applies Molecular Orbital theory •More accurate than crystal field theory
•Accounts for the overlap of ligand and metal atom orbitals•Explains a wider range of properties (such as the
spectrochemical series)
Crystal Field Theory: Octahedral complexes
A i l i Slight repulsive -
-
Attractive electrostatic interaction
Slight repulsive interactions
-
+--
-
Quiz Answer
Magnetic Measurements
•Ground-state configurations are experimentally determined by magnetic measurements
•In a free atom or ion, both orbital and spin angular momentum contribute to paramagnetism
•When the atom/ion is part of a complex, orbital angular momentum is normally quenchedy q
Spin Magnetic Moment Spin Magnetic Moment
BSS 2/112 B me2
em2
S=1/2 x # unpaired electrons
Example: Co(II)
The measured magnetic moment of a Co(II) complex is 4.0 μB. What is the ground state configuration?
Co (II) is d7. Possible configurations:
t2g5eg
2 (high spin, s=3/2)t2g
6eg1 (low spin, s=1/2)
Magnetic moments:
t2g5eg
2 μ=2(3/2(3/2+1))1/2 μB =3.87 μB
t 6 1 2(1/ (1/ +1))1/2 1 73t2g6eg
1 μ=2(1/2(1/2+1))1/2 μB =1.73 μB
Significance of LFSE: Hydration Enthalpies
Li t d di t d b i i Linear trend predicted by ionic radii, going left to right across a
period (∆H~ 1/(r++r-))
Actual hydration enthaphy:Actual hydration enthaphy:
CaO: ∆H=3460TiO ∆H 3878
LFSE=0LFSE 0 8 ∆TiO: ∆H=3878
VO: ∆H=3913MnO: ∆H=3810 LFSE=0
LFSE=0.8 ∆O
LFSE=1.2 ∆O
All have an octahedral coordination of the metal ions in a rock-salt configuration
Tetrahedral ComplexesSecond in abundance to octahedral complexesp
Tetrahedral Crystal Field
OT
Tetragonally Distorted ComplexesSix-coordinate complexes sometimes depart considerably from an p p yoctahedral geometry and show pronounced tetragonal distortions
Usually occurs when an odd number of electrons occupy the eUsually occurs when an odd number of electrons occupy the eg
orbitals. i.e., d9 complexes of Cu(II), high-spin d4 complexes (Mn3+), low-spin d7 (Ni3+)
238 pm
195
Hexaaquacopper complex
195 pm
Jahn-Teller EffectIf the ground-state electronic configuration of a complex is
orbitally -degenerate and asymetrically-filled, the complex will distort to remove the degeneracy and lower it’s energy
Jahn-Teller Effect
w: weak Jahn–Teller effect (t2g orbitals unevenly occupied) s: strong Jahn–Teller effect expected (eg orbitals unevenly occupied)
bl k h T ll ff dblank: no Jahn–Teller effect expected.
Ligand Field Theory
•Metal atom orbitals and symmetry adapted linear combinations of •Metal atom orbitals and symmetry-adapted linear combinations of ligand orbitals
•Consider an octahedral complex in which each ligand has a single valence orbital directed toward the central metal atom.
•First, assume the ligand orbitals have σ symmetry with respect to the M-L axis (i.e., F- or NH3)
Ligand Field Theory: σ Bonding
Ligand Field Theory: σ BondingNon-bonding d
l bi lmetal orbitals
Ligand Field Theory: σ Bonding
•Most of the bonding bi l li d i orbitals are ligand in
characterThe electrons that we
regard as provided by the ligands are largely
confined to the ligandsconfined to the ligands
•Remaining n electrons id d b h l provided by the metal
enter the non-bonding t2g
and antibonding egg g
orbitals
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