Transition metal complex

The Truth Shall Make You
Free….!!!
Electronic Spectroscopy
of Transition Metal
Complexes
By_ Saurav K. Rawat
M.Sc. (Physical Chem.)
School of Chemical Science,
St. John’s College, Agra
Tribute to Deptt. Of
Chemistry

Electronic Absorption Spectroscopy

Internal Energy of Molecules
Etotal=Etrans+Eelec+Evib+Erot+Enucl
Eelec- electronic transitions (UV, X-ray)
Evib- vibrational transitions (Infrared)
Erot- rotational transitions (Microwave)
Enucl- nucleus spin (NMR) or
(MRI:magnetic resonance imaging)

Electronic Spectroscopy
• Ultraviolet (UV) and visible (VIS) spectroscopy
• This is the earliest method of molecular
spectroscopy.
• A phenomenon of interaction of molecules with
ultraviolet and visible lights.
• Absorption of photon results in electronic
transition of a molecule, and electrons are
promoted from ground state to higher
electronic states.

UV and Visible Spectroscopy
• In structure determination : UV-VIS
spectroscopy is used to detect the presence of
chromophores like dienes, aromatics, polyenes,
and conjugated ketones, etc.

Electronic transitions
There are three types of electronic transition
which can be considered;
• Transitions involving p, s, and n electrons
• Transitions involving charge-transfer
electrons
• Transitions involving d and f electrons

Absorbing species containing p, s, and
n electrons
• Absorption of ultraviolet and visible
radiation in organic molecules is restricted
to certain functional groups
(chromophores) that contain valence
electrons of low excitation energy.

UV/VIS
Vacuum UV or Far UV
(λ<190 nm )

 Transitions
• An electron in a bonding s orbital is excited to
the corresponding antibonding orbital. The
energy required is large. For example, methane
(which has only C-H bonds, and can only
undergo  transitions) shows an
absorbance maximum at 125 nm. Absorption
maxima due to  transitions are not seen
in typical UV-VIS spectra (200 - 700 nm)

n  Transitions
• Saturated compounds containing atoms with
lone pairs (non-bonding electrons) are capable
of n  transitions. These transitions
usually need less energy than 
transitions. They can be initiated by light
whose wavelength is in the range 150 - 250 nm.
The number of organic functional groups with
n  peaks in the UV region is small.

n  and  Transitions
• Most absorption spectroscopy of organic
compounds is based on transitions of n or 
electrons to the  excited state.
• These transitions fall in an experimentally
convenient region of the spectrum (200 - 700
nm). These transitions need an unsaturated
group in the molecule to provide the 
electrons.

Chromophore Excitation lmax, nm Solvent
C=C →* 171 hexane
C=O
n→*
→*
290
180
hexane
hexane
N=O
n→*
→*
275
200
ethanol
ethanol
C-X
X=Br, I
n→*
n→*
205
255
hexane
hexane

Orbital Spin States
• For triplet state: Under the influence of
external field, there are three values (i.e. 3
energy states) of +1, 0, -1 times the angular
momentum. Such states are called triplet
states (T).
• According to the selection rule, S→S, T→T,
are allowed transitions, but S→T, T→S, are
forbidden transitions.

Absortpion spectroscopy
• Provide information about presence and absence of
unsaturated functional groups
• Useful adjunct to IR
• Determination of concentration, especially in
chromatography
• For structure proof, usually not critical data, but
essential for further studies
• NMR, MS not good for purity

Absorption and Emission
Emission
Absorption: A transition from a lower level to a higher level
with transfer of energy from the radiation field to an
absorber, atom, molecule, or solid.
Emission: A transition from a higher level to a lower level
with transfer of energy from the emitter to the radiation
field. If no radiation is emitted, the transition from higher to
lower energy levels is called nonradiative decay.
Absorption
http://www.chemistry.vt.edu/chem-ed/spec/spectros.html

Absorption and emission pathways
McGarvey and Gaillard, Basic Photochemistry at
http://classes.kumc.edu/grants/dpc/instruct/index2.htm

Origin of electronic spectra
Absorptions of UV-vis photons by molecule results in electronic
excitation of molecule with chromophore.
chromophore Any group of atoms that absorbs light whether or not a color is thereby
produced.
The electronic transition involves promotion of electron from a
electronic ground state to higher energy state, usually from a
molecular orbital called HOMO to LUMO.

Biological chromophores
1. The peptide bonds and amino acids in proteins
• The p electrons of the peptide group are delocalized over the carbon,
nitrogen, and oxygen atoms. The n-* transition is typically observed at 210-
220 nm, while the main -* transition occurs at ~190 nm.
• Aromatic side chains contribute to absorption at l> 230 nm
2. Purine and pyrimidine bases in nucleic acids and
their derivatives
3. Highly conjugated double bond systems

The Period 4 transition metals

Colors of representative compounds of the Period 4 transition metals
titanium oxide
sodium chromate
potassium
ferricyanide
nickel(II) nitrate
hexahydrate
zinc sulfate
heptahydrate
scandium oxide
vanadyl sulfate
dihydrate
manganese(II)
chloride
tetrahydrate cobalt(II) chloride
hexahydrate
copper(II) sulfate
pentahydrate

Aqueous oxoanions of transition elements
Mn(II) Mn(VI) Mn(VII)
V(V)
One of the most characteristic
chemical properties of these
elements is the occurrence of
multiple oxidation states.
Cr(VI)
Mn(VII)

Effects of the metal oxidation state and of ligand identity on color
[V(H2O)6]2+ [V(H2O)6]3+
[Cr(NH3)6]3+ [Cr(NH3)5Cl ]2+

The five d-orbitals in an octahedral field of ligands

Splitting of d-orbital energies by an octahedral field of ligands
D is the splitting energy

The effect of ligand on splitting energy

Revision – Ligand-Field Splitting
• In the absence of any ligands, the five d-orbitals of a Mn+ transition metal ion are
Mn+
degenerate
• Repulsion between the d-electrons and ligand lone pairs raises the energy of
each d-orbital
Mn+

What is electronic spectroscopy?
Absorption of radiation leading to electronic transitions within a molecule or complex
Absorption
[Ru(bpy)3]2+ [Ni(H2O)6]2+
200 700
visible
UV = higher energy transitions - between ligand orbitals
visible = lower energy transitions - between d-orbitals of transition metals
- between metal and ligand orbitals
UV
400
l/nm (wavelength)
Absorption
~14 000 25 000 50 000
visible UV
n- /cm-1 (frequency)
10
104

Revision – Ligand-Field Splitting
• Two of the d-orbitals point along x, y and z and are more affected than the
average
• Three of the d-orbitals point between x, y and z and are affected less than the
average
• The ligand-field splitting
eg
Doct
t2g
(eg)
(t2g)
(Doct)

Electronic Spectra of d1 Ions
• A d1 octahedral complex can undergo 1 electronic transition
• The ground state (t2g)1 comprises three degenerate arrangements
• The excited state (eg)1 comprises two degenerate arrangements
• The electronic transition occurs at Doct
eg
t2g t2g
ground state excited state
eg
Doct
Ti3+(aq)

Electronic Spectra of High Spin d4 Ions
• A high spin d4 octahedral complex can also undergo just 1 transition
• The ground state (t2g)2(eg)1 comprises two degenerate arrangements
• The excited state (t2g)2(eg)2 comprises three degenerate arrangements
• No other transitions are possible without changing the spin
eg
t2g
t2g
ground state excited state
eg
Doct
Cr2+(aq)

Electronic Spectra of High Spin d6 and d9 Ions
• High spin d6 and d9 octahedral complexes can also undergo just 1 transition
• No other transitions are possible changing the spin
ground state
d6
excited state
ground state
d9
excited state
Fe2+(aq) Cu2+(aq)

Effect of Distortion on the d-Orbitals
• Pulling the ligands away along z splits eg and lowers the energy of dz2
• It also produces a much smaller splitting of t2g by lowering the energy of dxz and dyz
• Doct >>> d1 >> d2
d2
Doct
eg
t2g
d1
tetragonal elongation

+½d1
d2
Which Complexes Will Distort?
• Relative to average: t2g go down by 0.4Doct in octahedral complex
• Relative to average: eg go up by 0.6Doct in octahedral complex
• Relative to average dz
2 is stablilized by ½d1 and dx
Doct
2
-y
2 is destablilized by ½d1
• Relative to average dxz and dyz are stablilized by ⅔d2 and dxy is destablilized by ⅓d2
eg
t2g
d1 +0.6 Doct
-0.4 Doct
-½d1
+⅔d2
-⅓d2
octahedron distorted octahedron

dn configuration degeneracy LFSE stabilized? distortion
+½d1
eg
t2g
+0.6 Doct
-0.4 Doct
-½d1
+⅔d2
-⅓d2
t2g eg
1
Doct >>> d1 >> d2
1 3 -0.4Doct - 0.33d2 yes small

+½d1
eg
t2g
+0.6 Doct
-0.4 Doct
-½d1
+⅔d2
-⅓d2
t2g eg
1
2
3
4
5
Doct >>> d1 >> d2
3 1 -1.2Doct no no
3 1 2 -0.6Doct - 0.5d1 yes large
3 2 1 0 no none

t2g eg
1
2
3
4
5
6
7
8
9
Doct >>> d1 >> d2
3 1 -1.2Doct no no
3 1 2 -0.6Doct - 0.5d1 yes large
3 2 1 0 no none
4 2
3 -0.4Doct - 0.33d2 yes small
5 2
3 -0.8Doct - 0.67d2 yes small
6 2
1 -1.2Doct no no
6 3
2 -0.6Doct - 0.5d1 yes large

t2g eg
4 4
5 5
6 6
7 6 1
Doct >>> d1 >> d2
• Low spin:
+½d1
eg
t2g
+0.6 Doct
-0.4 Doct
-½d1
+⅔d2
-⅓d2

• Large distortions (always seen crystallographically)
:
 high spin d4
 low spin d7
 d9
• Small distortions (often not seen crystallographically):
 d1
 d2
 low spin d4
 low spin d5
 high spin d6
 high spin d7
Cr2+
Co2+
Cu2+

Jahn-Teller Theorem
• This is a general result known as the Jahn-Teller theorem:
Any molecule with a degenerate ground state will distort
antibonding
bonding
+

Effect on Spectroscopy
• From Slide 6, there is one d-d transition for an octahedral d1 ion
• From Slide 15, a d1 complex will distort and will not be octahedral
• There are now 3 possible transitions
• (A) is in infrared region and is usually hidden under vibrations
• (B) and (C) are not usually resolved but act to broaden the band
eg
t2g
Ti3+(aq)
(A) (B) (C)
(B) (C)

Summary
By now you should be able to....
• Show why there is a single band in the visible spectrum for d1,
high spin d4, high spin d6 and d9 octahedral complexes
• Obtain the value of Doct from the spectrum of these ions
• Show the electronic origin of the (Jahn-Teller) distortion for high
spin d4, low spin d7 and d9 octahedral complexes
• Predict whether any molecule will be susceptible to a Jahn-Teller
distortion
• Explain how the Jahn-Teller effect leads to broadening of bands
in the UV/Visible spectrum

Absorption maxima in a visible spectrum have three important characteristics
1. Number (how many there are)
This depends on the electron configuration of the metal centre
2. Position (what wavelength/energy)
This depends on the ligand field splitting parameter, Doct or Dtet and on the degree of
inter-electron repulsion
3. Intensity
This depends on the "allowedness" of the transitions which is described by two selection
rules

Energy of transitions
Excited State
molecular rotations
lower energy
(0.01 - 1 kJ mol-1)
microwave radiation
electron transitions
higher energy
(100 - 104 kJ mol-1)
visible and UV radiation
Ground State
molecular vibrations
medium energy
(1 - 120 kJ mol-1)
IR radiation
During an electronic transition
the complex absorbs energy
electrons change orbital
the complex changes energy state

[Ti(OH2)6]3+ = d1 ion, octahedral complex
white light
400-800 nm
blue: 400-490 nm
yellow-green: 490-580 nm
red: 580-700 nm
3+
Ti
A
l / nm
This complex is has a light purple colour in
solution because it absorbs green light
lmax = 510 nm
Absorption of light

The energy of the absorption by [Ti(OH2)6]3+ is the ligand-field splitting, Do
eg
t2g
Do
hn
d-d transition
ES
complex in electronic
Ground State (GS)
ES
complex in electronic
excited state (ES)
[Ti(OH2)6]3+ lmax = 510 nm Do is  243 kJ mol-1
20 300 cm-1
An electron changes orbital; the ion changes energy state
GS
GS
eg
t2g

d2 ion Electron-electron repulsion
eg
eg
z2 x2-y2 t2g
xy xz yz
t2g
z2 x2-y2
xy xz yz
xz + z2 xy + z2
x
z
y
x
z
y
lobes overlap, large electron repulsion lobes far apart, small electron repulsion
These two electron configurations do not have the same energy

3P
3F
D E
D E = 15 B
Which is the Ground State?
B is the Racah parameter and is a measure of inter-electron repulsion within
the whole ion
States of the same
spin multiplicity
Relative strength of coupling interactions:
MS = S ms > ML = S ml > ML - MS

Effect of a crystal field on the free ion term of a d1 complex
d1  d6
tetrahedral field free ion octahedral field
2Eg
2T2g
2T2
2E
6 Dq
4 Dq
2D

Energy level diagram for d1 ions in an Oh field
D
2Eg
2T2g
Energy
2D
ligand field strength, Doct
For d6 ions in an Oh field, the splitting is the same, but the multiplicity of the states is 5, ie 5Eg
and 5T2g

d1 oct [Ti(OH2)6]3+
A
10 000 20 000 30 000
n / cm-1 -
Orgel diagram for d1, d4, d6, d9
E
D
D 0 D
LF strength
d4, d9 tetrahedral
T2g or T2
T2g or T2
d4, d9 octahedral
Eg or E
d1, d6 tetrahedral
Eg or E
d1, d6 octahedral
2Eg
 2T2g
2Eg
2T2g
2D
D
D

The Jahn-Teller Distortion: Any non-linear molecule in a degenerate electronic state will
undergo distortion to lower it's symmetry and lift the degeneracy
A
n / cm-1 -
[Ti(H2O)6]3+, d1
10 000 20 000 30 000
2Eg
2T2g
2B1g
2A1g
d3 4A2g
d5 (high spin) 6A1g
d6 (low spin) 1A1g
d8 3A2g
Degenerate electronic ground state: T or E
Non-degenerate ground state: A

Racah Parameters
Free ion [Co2+]: B = 971 cm-1
[Co(H2O)6]2+ [CoCl4]2-
d7 tetrahedral complex
15 B' = 10 900 cm-1
B' = 727 cm-1
d7 octahedral complex
15 B' = 13 800 cm-1
B' = 920 cm-1
B' = 0.95
B
B' = 0.75
B
Nephelauxetic ratio, b
b is a measure of the decrease in electron-electron repulsion on complexation

The Nephelauxetic Effect cloud expanding
- some covalency in M-L bonds – M and L share electrons
-effective size of metal orbitals increases
-electron-electron repulsion decreases
Nephelauxetic series of ligands
F- < H2O < NH3 < en < [oxalate]2- < [NCS]- < Cl- < Br- < I-Nephelauxetic
series of metal ions
Mn(II) < Ni(II) Co(II) < Mo(II) > Re (IV) < Fe(III) < Ir(III) < Co(III) < Mn(IV)

Selection Rules
Transition e complexes
Spin forbidden 10-3 – 1 Many d5 Oh cxs
Laporte forbidden [Mn(OH2)6]2+
Spin allowed
Laporte forbidden 1 – 10 Many Oh cxs
[Ni(OH2)6]2+
10 – 100 Some square planar cxs
[PdCl4]2-
100 – 1000 6-coordinate complexes of low symmetry,
many square planar cxs particularly with
organic ligands
Spin allowed 102 – 103 Some MLCT bands in cxs with unsaturated ligands
Laporte allowed
102 – 104 Acentric complexes with ligands such as acac, or
with P donor atoms
103 – 106 Many CT bands, transitions in organic species

eg
D D
t 2g
eg
t 2g
weak field ligands
e.g. H2O
high spin complexes
strong field ligands
e.g. CN-low
spin complexes
I- < Br- < S2- < SCN- < Cl-< NO3
- < F- < OH- < ox2-
< H2O < NCS- < CH3CN < NH3 < en < bpy
< phen < NO2
- < phosph < CN- < CO
The Spectrochemical Series
The Spin Transition

Energies of d-d Transitions
Octahedral d1, d4, d6 and d9:
1 band energy = Doct
Octahedral d2:
3 bands Doct and B from calculation
Octahedral d7:
3 bands Doct = v2 – v1 B from calculation
Octahedral d3 and d8:
3 bands v1 = Doct B from calculation

Features of an Electronic Spectrum
• The frequency, wavelength or energy of a transition relates to the energy
required to excite an electron:
 depends on Doct and B for ligand-field spectra
 decides colour of molecule
• The width of a band relates to the vibrational excitation that accompanies the
electronic transition:
 narrow bands: excited state has similar geometry to the ground state
 broad bands: excited state has different geometry to the ground state
• The height or area of a band relates to the number of photons absorbed
 depends on concentration and path length
 transition probability
 decides intensity or depth of colour
Ni2+, d8:
8500 cm-1 13800 cm-1 25300 cm-1

Transition Probability
• When light is shined on a sample, some of the light may be absorbed and some
may pass straight through
 the proportion that is absorbed depends on the ‘transition probability’
• To be absorbed, the light must interact with the molecule:
 the oscillating electric field in the light must interact with an oscillating
electric field in the molecule
• During the transition, there must be a change in the dipole moment of the
molecule:
 if there is a large change, the light / molecule interaction is strong and many
photons are absorbed:
large area or intense bands  intense colour
 if there is a small change, the light / molecule interaction is weak and few
low area or weak bands  weak colour
 If there is no change, there is no interaction and no photons are absorbed

Selection Rules
molecule:
 if there is a large change, the light / molecule interaction is strong and many
large area or intense bands  intense colour
 if there is a small change, the light / molecule interaction is weak and few
low area or weak bands  weak colour
 If there is no change, there is no interaction and no photons are absorbed
Selection rules tell us which transitions give no change in dipole
moment and hence which will have zero intensity

Selection Rules - IR
molecule
• Octahedral ML6 complexes undergo 3 types of M-L stretching vibration:
[Co(CN)6]3-
dipole moment
change?
no yes no
• There is one band in the M-L stretching region of the IR spectrum

Selection Rules – Spin Selection Rule
The spin cannot change during an electronic transition
eg
t2g
t2g
ground state 1st excited state
eg
d4
t2g
2nd excited state
eg
AJB lecture 1
Only one spin
allowed
transition

Selection Rules – Spin Selection Rule
The spin cannot change during an electronic transition
eg
t2g
d5
ground state
AJB lecture 1
NO spin allowed transitions for
high spin d5

Selection Rules – Orbital Selection Rule
• A photon has 1 unit of angular momentum
• When a photon is absorbed or emitted, this momentum must be conserved
Dl = ±1 or:
‘s ↔ p’, ‘p ↔ d’, ‘d ↔ f’ etc allowed (Dl = ±1)
‘s ↔ d’, ‘p ↔ f’ etc forbidden (Dl = ±2)
‘s ↔ s’, ‘p ↔ p’ , ‘d ↔ d’, ‘f ↔ f’ etc forbidden (Dl = 0)
…so why do we see ‘d-d’ bands?

‘Relaxing’ The Orbital Selection Rule
• The selection rules are exact and cannot be circumnavigated
• It is our model which is too simple:
 the ligand-field transitions described in Lectures 2 and 3 are in molecules
 labelling the orbitals as ‘d’ (atomic orbitals) is incorrect if there is any
M
L
L
L
not atoms
covalency
L
L
L
M
L
L
A metal p-orbital overlaps with
ligand orbitals
M
L
L
L
L
A metal d-orbital overlaps with
the same ligand orbitals
Through covalent overlap with the ligands, the
metal ‘d’ and ‘p’ orbitals are mixed

‘Relaxing’ the Orbital Selection Rule
Through covalent overlap with the ligands, the
metal ‘d’ and ‘p’ orbitals are mixed
• As the molecular orbitals are actually mixtures of d and p-orbitals, they are actually
allowed as Dl =±1
• But, if covalency is small, mixing is small and transitions have low intensity
In tetrahedral complexes, the ‘d-d’ transitions become allowed through
covalency but the ‘d-d’ bands are still weak as covalency is small

Laporte Selection Rule
• This way of ‘relaxing’ the orbital selection rule is not available in octahedral
L
L L
L L
L
complexes
A metal p-orbital overlaps with
ligand orbitals
L
L L
L L
L
in phase
no overlap
out of phase
A metal d-orbital cannot overlap
with the same ligand orbitals
In general, no mixing of the ‘d’ and ‘p’ orbitals is possible if the
molecule has a centre of inversion (Laporte rule)

‘Relaxing’ the Laporte Selection Rule
• Again our model is deficient:
 molecules are not rigid but are always vibrating
During this vibration, centre of
inversion is temporarily lost:
d-p mixing can occur
• Vibrational amplitude is small so deviation and mixing is small:
 octahedral complexes have lower intensity bands than tetrahedral
complexes
 the intensity of the bands increases with temperature as amplitude
increases

‘Relaxing’ the Spin Selection Rule
• Again our model from lectures 1 and 2 is deficient:
 electrons can have magnetism due to the spin and orbital motions
 this coupling allows the spin forbidden transitions to occur
spin-orbit coupling: the interaction between spin and orbital magnetism
• Mn2+ d5: all transitions are spin forbidden
spin-orbit coupling gets stronger as elements get heavier and so spin forbidden
transitions get more important

Selection Rules and Band Intensity
• The height of the band in the spectrum is called the ‘molar extinction
cofficient’ – symbol e:
e (mol-1 cm-1) type of transition type of complex
10-3 - 1
spin forbidden
orbitally forbidden,
Laporte forbidden
octahedral d5 complexes
(e.g. [Mn(H2O)6]2+)
1 – 10
spin forbidden
orbitally forbidden,
tetrahedral d5 complexes
(e.g. [MnCl4]2-+)
10 – 102
spin allowed,
orbitally forbidden
Laporte forbidden
octahedral and square
planar complexes
10 – 103 spin allowed,
orbitally forbidden
tetrahedral complexes
> 103 LMCT, MLCT, IVT
very
pale colours
intense
colours

Tanabe-Sugano diagrams
E/B
D/B
4T2g
2A1g
4T1g
4Eg
4T2g
4A1g,
2A1g
2T1g
2T2g
4E
2Eg
4A2g,
2T1g
4T2g
6A1g
4T1g
2T2g
All terms included
Ground state assigned to E = 0
Higher levels drawn relative to GS
Energy in terms of B
High-spin and low-spin configurations
Critical value of D
d5
WEAK FIELD STRONG FIELD

Tanabe-Sugano diagram for d2 ions
[V(H2O)6]3+: Three spin allowed transitions
E/B
D/B
n1 = 17 800 cm-1 visible
n3 = obscured by CT transition in UV
10 000
e
30 000
n- /cm-1
10
20 000
5
25 700 = 1.44
17 800
D/B = 32
n3 = 2.1n1 = 2.1 x 17 800
 n3 = 37 000 cm-1
= 32

E/B
n E/B = 43 cm-1 2
D/B = 32
n1 = 17 800 cm-1
n2 = 25 700 cm-1
n1
E/B = 30 cm-1
E/B = 43 cm-1 E = 25 700 cm-1
B = 600 cm-1
Do / B = 32
Do = 19 200 cm-1

Tanabe-Sugano diagram for d3 ions
[Cr(H2O)6]3+: Three spin allowed transitions
E/B
D/B
n3 = obscured by CT transition
24 500 = 1.41
17 400
D/B = 24
n3 = 2.1n1 = 2.1 x 17 400
 n3 = 36 500 cm-1
= 24

Calculating n3
E/B
D/B
n1 = 17 400 cm-1
n2 = 24 500 cm-1
= 24
E/B = 34 cm-1
E/B = 24 cm-1
When n1 = E =17 400 cm-1
E/B = 24
so B = 725 cm-1
When n2 = E =24 500 cm-1
E/B = 34
so B = 725 cm-1
If D/B = 24
D = 24 x 725 = 17 400 cm-1

d0 and d10 ions
Zn2+ d10 ion
TiF4 d0 ion
TiCl4 d0 ion
TiBr4 d0 ion
TiI4 d0 ion
d0 and d10 ion have no d-d transitions
white
white
orange
dark brown
[MnO4]- Mn(VII) d0 ion
[Cr2O7]- Cr(VI) d0 ion
extremely purple
bright orange
[Cu(MeCN)4]+ Cu(I) d10 ion
[Cu(phen)2]+ Cu(I) d10 ion
colourless
dark orange
white
Charge Transfer Transitions

Ligand-to-metal charge transfer
LMCT transitions
Metal-to-ligand charge transfer
MLCT transitions
Md
L
L
L
*
*
eg
t2g
d-d transitions

• As well as ‘d-d’ transitions, the electronic spectra of transition metal
complexes may 3 others types of electronic transition:
 Ligand to metal charge transfer (LMCT)
 Metal to ligand charge transfer (MLCT)
 Intervalence transitions (IVT)
• All complexes show LMCT transitions, some show MLCT, a few show IVT

Ligand to Metal Charge Transfer
• These involve excitation of an electron from a ligand-based orbital into a d-orbital
M O
O
O
O
visible light
M O
O
O
O
• This is always possible but LMCT transitions are usually in the ultraviolet
• They occur in the visible or near-ultraviolet if
 metal is easily reduced (for example metal in high oxidation state)
 ligand is easily oxidized
If they occur in the visible or near-ultraviolet, they are much more intense
than ‘d-d’ bands and the latter will not be seen

Ligand to Metal Charge Transfer
•They occur in the visible or near-ultraviolet if
 metal is easily reduced (for example metal in high oxidation state)
TiO2
Ti4+
3-
V5+
VO4
2-
CrO4
Cr6+
-
MnO4
Mn7+
in far UV ~39500 cm-1 ~22200 cm-1 ~19000 cm-1
white white yellow purple
more easily reduced
d0

Metal to Ligand Charge Transfer
• They occur in the visible or near-ultraviolet if
 metal is easily oxidized and ligand has low lying empty orbitals
N N
N
N
M = Fe2+, Ru2+, Os2+
N
N N
M
N
N
N
• Sunlight excites electron from M2+ (t2g)6 into empty ligand * orbital
 method of capturing and storing solar energy

Intervalence Transitions
• Complexes containing metals in two oxidation states can be coloured due to
excitation of an electron from one metal to another
“Prussian blue”
contains Fe2+ and Fe3+
• Colour arises from excitation of an electron from Fe2+ to Fe3+

Summary
By now, you should be able to ....
• Explain that the spin cannot change during an electronic
transition
• Explain that pure ‘d-d’ transitions cannot occur
• Explain that d-p mixing in complexes without centre of
inversion (e.g. tetrahedron) ‘relaxes’ this rule
• Explain that ‘d-p’ mixing for complexes with a centre of
inversion (e.g. octahedron or square planar) can only occur
due to molecular vibrations
• Explain that origin of LMCT, MLCT and IVT transitions
• Predict the relative intensities of spin, Laporte and orbitally
forbidden transitions

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Transition metal complex

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Transition metal complex