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