Download Chapter 11 Coordination Chemistry III: Electronic Spectra

Chapter 11 Coordination Chemistry III:
Electronic Spectra
11-1 Absorption of Light
11-2 Quantum Numbers of Multielectron Atoms
11-3 Electronic Spectra of Coordination Compounds
Chapter 11 Coordination Chemistry III:
Electronic Spectra
Vivid colors of coordination compound.
Dyes, gems (rubies, emeralds), blood etc.
Transitions between d orbitals of metals.
We will need to look closely at the energies of these
orbitals.
The electronic absorption spectrum provides a
convenient method for determining the magnitude of
the effect of ligands on the d orbitals of the metal.
Absorption of Light
Complementary colors: if a compound absorbs light of
one color, we see the complement of that color.
Absorption of Light
Complementary colors: if a compound absorbs light of
one color, we see the complement of that color.
600 ~ 1000 nm
Blue color
Absorption of Light
;Beer-Lambert Absorption Law
Beer-Lambert Law
log(Io/I) = A = εlc
Wavelength, wavenumber →
Energy
E = hv = hc/λ= hcν
Quantum Numbers of Multielectron Atoms
Absorption of light results in the excitation of electrons
from lower to higher energy states.
We observe absorption in band with the energy of
each band corresponding to the difference in energy
between the initial and final states.
We first need to consider electrons in atoms can
interact with each other.
Electrons tend to occupy separate orbitals ← Πc
Electrons in separate orbitals tend to have parallel
spins ← Πe
Quantum Numbers of Multielectron Atoms
Carbon atom
Energy levels for the p2 electrons → Five energy levels
Each energy levels can be described as a combination
of the ml and ms values of the 2p electrons.
2p electrons
n = 2, l = 1
ml = +1, 0, or -1
ms = +1/2 or -1/2
The orbital angular momenta and the spin angular
momenta of the 2p electrons interact in a manner
called Russell-Saunders coupling (LS coupling).
Quantum Numbers of Multielectron Atoms
Russell-Saunders coupling (LS coupling)
Orbit-orbit coupling
ML = ∑ml → L: total orbital angular momentum quantum number
Spin-spin coupling
Ms = ∑ms → S: total spin angular momentum quantum number
microstates
Spin-orbit coupling
J = L + S : total angular momentum quantum number
How many possible combinations of ml and ms values?
2p electrons
One possible set of values for the two electrons in the p2
configuration would be
n = 2, l = 1
ml = +1, 0, or -1
First electron:
ml = +1 and ms = +1/2
Notation 1+0- ms = +1/2 or -1/2
Second electron: ml = 0 and ms = -1/2
microstate
Quantum Numbers of Multielectron Atoms
Tabulate the possible microstates
1. No two electrons in the same microstate have
identical quantum numbers (the Pauli exclusion
principle)
2. Count only the unique microstates (1+0- and 0-1+)
Electronic quantum # (ml and ms) to atomic quantum #
(ML and MS)
microstates
2p electrons
n = 2, l = 1
ml = +1, 0, or -1
ms = +1/2 or -1/2
Quantum Numbers of Multielectron Atoms
Electronic quantum # (ml and ms) to atomic quantum #
(ML and MS) → describe states of multielectron atoms
Russell-Saunders coupling (LS coupling)
Orbit-orbit coupling
ML = ∑ml → L: total orbital angular momentum quantum number
Spin-spin coupling
Ms = ∑ms → S: total spin angular momentum quantum number
Spin-orbit coupling
J = L + S : total angular momentum quantum number
Quantum Numbers of Multielectron Atoms
L and S describe collections of microstates.
ML and MS describe the microstates themselves.
Atomic States
ML = 0, ±1, ± 2,… ± L
MS = S, S-1,…. –S
Individual Electrons
ml = , ±1, ± 2,… ± l
ms = +1/2, -1/2
Term symbol
Term Symbol
2S+1L
L = 0, 1, 2, 3 → S, P, D, F,
J
Quantum Numbers of Multielectron Atoms
Electronic quantum # (ml and ms) to atomic quantum #
(ML and MS) → describe states of multielectron atoms
Free-ion terms are very important in the interpretation of
the spectra of coordination compounds.
1S
(singlet S) :
S → L = 0 → ML = 0,
2S+1 =1 → S =0 → MS = 0
MS
The minimum
configuration of two
electrons
0
ML
0
0+0-
Quantum Numbers of Multielectron Atoms
2P
(doublet P) :
P → L = 1 → ML = +1,0,-1
2S+1 =2 → S =1/2 → MS = +1/2, -1/2
The minimum
configuration of one
electron
ML
MS
-1/2
+1/2
+1
1-
1+
0
0-
0+
-1
-1-
-1+
Six microstates
Quantum Numbers of Multielectron Atoms
MS
ML
-1/2
+1/2
+1
1x-
1x+
0
0x-
0x+
-1
-1
x-
-1
x+
Six microstates
The spin multiplicity is the same as the # of microstates
Quantum Numbers of Multielectron Atoms
Reduce microstate table into its commponent free-ion terms.
The spin multiplicity is the same as the # of microstates.
Each terms has different energies; they represent three states with
different degrees of electron-electron interactions.
Which term has the lowest energy. This can be done by using two of
Hund’s rules.
1. The ground term (term of lowest energy) has the highest spin
multiplicity. (Hund’s rule of maximum multiplicity)
2. If two or more terms share the maximum spin multiplicity, the
ground term is one having the highest value of L.
Quantum Numbers of Multielectron Atoms
Quantum Numbers of Multielectron Atoms
d2
l=2
ml = +2, +1 0, -1, -2
ms = +1/2 or -1/2
1S
1D
1G
3P
3F
ML
MS
+1
+4
+3
+2
+1
0
-1
-2
-3
-4
x
x
xx
xx
xx
x
x
0
x
xxx
xxx
xxxx
xxxxx
xxxx
xxx
xxx
x
-1
x
x
xx
xx
xx
x
x
Quantum Numbers of Multielectron Atoms
d6
l=2
ml = +2, +1 0, -1, -2
ms = +1/2 or -1/2
High spin
5D
Low spin
1I
Spin-Orbit Coupling
The spin and orbital angular momenta couple each other
→ spin-orbit coupling
J = L + S : total angular moment quantum number
J may have the following values
J = L+S, L+S-1, L+S-2,…. |L-S|
Term Symbol
2S+1L
J
Spin-orbit coupling can have significant effects on the
electronic spectra of coordination compounds, especially
involving heavy metals.
Spin-orbit coupling acts to split free-ion terms into states of
different energies.
Spin-Orbit Coupling
J may have the following values
J = L+S, L+S-1, L+S-2,…. |L-S|
Term Symbol
2S+1L
J
Spin-orbit coupling acts to split free-ion terms into states of
different energies.
1S
1S
0
1S, 1D
1D
1D
2
p2
3P
2
3P
3P
3P
1
3P
0
Spin-Orbit Coupling
1S
1S
0
1D
1D
2
1S, 1D
p2
3P
2
3P
3P
3P
1
3P
0
Total energy level diagram for the carbon atom. (five energy
states)
The state of lowest energy can be predicted from Hund’s
third rule.
3. For subshells that are less than half-filled, the state
having the lowest J value has the lowest energy.
For subshells that are more than half-filled, the state
having the highest J value has the lowest energy.
Half-filled subshells have only one possible J value.
Electronic Spectra
of Coordination Compounds
Microstates and free-ion terms for electron configurations
Identify the lowest-energy term
Electronic Spectra
of Coordination Compounds
Identify the lowest-energy term
1. Sketch the energy levels, showing
the d electrons.
2. Spin multiplicity of lowest-energy
state = number of unpaired
electrons + 1.
3. Determine the maximum possible
value of ML for the configuration
as shown. This determines the
type of free-ion term.
4. Combine results of steps 2 and 3
to get ground term.
Spin multiplicity = 3+1=4
Max. of ML: 2+1+0 =3
4F
Electronic Spectra of Coordination
Compounds: Selection Rules
On the basis of the symmetry and spin multiplicity of
ground and excited electronic states
1. Transitions between states of the
same parity are forbidden
(symmerty with respect to a center
of inversion.: Laporte selection
rule
2. Transitions between states of
different spin multiplicities are
forbidden: spin selection rule
Between d orbitals are forbidden
g → g transition
Between d and p orbitals are
allowed; g → u transition
4A
2
and 4T1: spin-allowed
4A
2
and 2T2: spin-forbidden
Electronic Spectra of Coordination
Compounds: Selection Rules
Some rules for relaxation of selection rules
1. Vibrations may temporarily change the symmetry(the
center of symmetry is temporarily lost: vibronic coupling
relax the first selection rule:d-d transition
2. Tetrahedral complexes often absorb more strongly than
Oh complexes. Metal-ligand sigma bonds can be
described as involving a combination of sp3 and sd3
hybridization of the metal orbitals: relax the first selection
rule
3. spin-orbit coupling provides a mechanism of relaxing
the second selection rule
Electronic Spectra of Coordination
Compounds: correlation diagrams
To relate the electronic spectra of transition metal complexes
to the ligand field splitting: correlation diagrams and TanabeSugano diagrams
1. Free ions (no ligand field): d2; 3F, 3P, 1G, 1D, 1S.
2. Strong ligand field.
t2g2
t2geg
eg2
Electronic Spectra of Coordination
Compounds: correlation diagrams
Electronic Spectra of Coordination
Compounds: correlation diagrams
The free-ion terms will be split into states corresponding to the
irreducible representation.
Electronic Spectra of Coordination
Compounds: correlation diagrams
Electronic Spectra of Coordination
Compounds: correlation diagrams
Irreducible representations may be obtained for the strong-field
limit configurations.
Each free-ion irreducible representation is matched with a
strong-field irreducible representation.
The spin multiplicity of the ground state.
Electronic Spectra of Coordination
Compounds: correlation diagrams
Electronic Spectra of Coordination
Compounds: Tanabe-Sugano diagrams
B = Racah parameter, a measure of
the repulsion between terms of the
same multiplicity; the energy
difference between 3F and 3P is 15B.
E is the energy above the ground
state.
Electronic Spectra of Coordination
Compounds: Tanabe-Sugano diagrams
Electronic Spectra of Coordination
Compounds: Tanabe-Sugano diagrams
High spin vs low spin
High spin
Low spin
Ground state and spin multiplicity changed
Electronic Spectra of Coordination
Compounds: Tanabe-Sugano diagrams
Jahn-Teller Distortions and Spectra
d1 d9 complexes: might expect each to exhibit one
absorption band: excitation from the t2g to the eg levels.
eg
t2g
eg
t2g
Two closely overlapping absorption bands.
Jahn-Teller Distortions and Spectra
To lower the symmetry of the molecule and to reduce
the degeneracy.
Distortion from Oh to D4h: results in stabilization of the
molecule.
The most common distortion observed is elongation
along z axis.
Jahn-Teller Distortions and Spectra
: Symmetry labels for configurations
Electron configurations have symmetry labels that
match their degeneracies.
T
E
A or B
Triply degenerate
asymmetrically
occupied state
Doubly degenerate
asymmetrically
occupied state
Nondegenerate
state
Jahn-Teller Distortions and Spectra
: Symmetry labels for configurations
2D
term for d9
Lower energy
Symmetry label
2E
g
Higher energy
2T
2g
the opposite of the order of
energies of the orbitals
Too weak
Distortions can be splitting of bands.
Tanabe-Sugano Diagrams: Determining ∆o
from Spectra;d1, d4(high spin), d6(high spin), d9
Tanabe-Sugano Diagrams:
Determining ∆o from Spectra
Tanabe-Sugano Diagrams:
Determining ∆o from Spectra;d3, d8
Ground state F term
To find ∆o, we simply find
the energy of the lowestenergy transition
Tanabe-Sugano Diagrams:
Determining ∆o from Spectra;d2, d7 (high spin)
Ground state F term
eg2
t2geg
t2g2
3T
1g state arising from the
3P free-ion terms, causing
a slight curvature of the
both in the Tanabe-Sugano
diagram.
An Alternative way.
Tanabe-Sugano Diagrams:
Determining ∆o from Spectra;d2, d7 (high spin)
t2g2
t2geg
eg2
Tanabe-Sugano Diagrams:
Determining ∆o ;d5 (high spin), d4 to d7 (low spin)
d5
Low spin from d4 to d7
Electronic Spectra of Coordination
Compounds: Tanabe-Sugano diagrams
Tetrahedral Complexes
The lack of a center of symmetry: makes transitions
between d orbitals more allowed; much more intense
absorption bands.
Hole formalism: d1 Oh configuration is analogous to
the d9 Td configuration: the hole in d9 results in the
same symmetry as the single electron in d1.
We can use the correlation diagram for d10-n
configuration in Oh geometry
hole
eg
t2g
octahedral
t2
e
tetrahedral
Charge-Transfer Spectra
Charge-transfer absorptions is much more intense
than d-d transitions.
Involve the transfer of electrons from molecular
orbitals that are primarily ligand in character to orbitals
that are primarily metal in character (or vice versa)
Formal
reduction of the
metal: Co(III) to
Co(II)
LMCT
Charge-Transfer Spectra
IrBr62- (d5): two band
IrBr63- (d6): one band
Why?
Formal
reduction of the
metal: Co(III) to
Co(II)
LMCT
Charge-Transfer Spectra
MLCT
π-acceptor ligand (π* orbitals): CO, CN-, SCN-,
bipyridine..
Oxidation of the metal
d-d transitions may be completely overwhelmed and
essentially impossible to observe.
Formal
oxidation of the
metal: Fe(III) to
Fe(IV)
MLCT