The Truth Shall Make You Free….!!!

Tribute to Deptt. Of Chemistry

By_ Saurav K. Rawat M.Sc. (Physical Chem.)

Electronic Spectroscopy of Transition Metal Complexes

School of Chemical Science, St. John’s College, Agra

Electronic Absorption Spectroscopy

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)

Internal Energy of Molecules

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 )

® *s s 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 ® *s s transitions) shows an absorbance maximum at 125 nm. Absorption maxima due to ® *s s transitions are not seen in typical UV-VIS spectra (200 - 700 nm)

n ® *s Transitions

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

n ® *p and ® *p p Transitions

• Most absorption spectroscopy of organic compounds is based on transitions of n or p electrons to the *p 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 p electrons.

Chromophore Excitation lmax, nm Solvent

C=C p→p* 171 hexane

C=On→p*p→p*

290180

hexanehexane

N=On→p*p→p*

275200

ethanolethanol

C-X   X=Br, I

n→s*n→s*

205255

hexanehexane

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 chromophores1. 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-p* transition is typically observed at 210-220 nm, while the main p-p* transition occurs at ~190 nm.

• Aromatic side chains contribute to absorption at l> 230 nm

2. Purine and pyrimidine bases in nucleic acids andtheir 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)Cr(VI)

Mn(VII)

One of the most characteristic chemical properties of these elements is the occurrence of multiple oxidation states.

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+

Linkage isomers

An artist’s wheel

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

Mn+

• In the absence of any ligands, the five d-orbitals of a Mn+ transition metal ion are degenerate

• Repulsion between the d-electrons and ligand lone pairs raises the energy of each d-orbital

Mn+

LL

L

L

L

L

Mn+

What is electronic spectroscopy?

Absorption

Absorption of radiation leading to electronic transitions within a molecule or complex

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)

200 700

visible

Absorption

~14 000 50 00025 000

UVvisible

/ n cm-1 (frequency)-

[Ru(bpy)3]2+ [Ni(H2O)6]2+

10104

LL

L

L

L

L

Mn+Mn+

LL

L

L

L

L

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

Mn+

LL

L

L

L

L

Doct

eg

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

t2gt2g

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• The electronic transition occurs at Doct

• No other transitions are possible without changing the spin

eg

t2gt2g

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• The electronic transition occurs at Doct

• No other transitions are possible changing the spin

ground state

d6

excited stateground state

d9

excited state

Fe2+(aq) Cu2+(aq)

LL

L

L

L

L

Mn+

d2

LL

L

L

L

L

Mn+

Doct

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 >> d2eg

t2g

LL

L

L

L

L

Mn+

d1

tetragonal elongation

+½d1

d2

Doct

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 dx2

-y2 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

+½d1

Which Complexes Will Distort?

eg

t2g

+0.6 Doct

-0.4 Doct

-½d1

+⅔d2

-⅓d2

dn configuration degeneracy LFSE stabilized? distortion

t2g eg

1

Doct >>> d1 >> d2

1 3 -0.4Doct - 0.33d2 yes small

+½d1

Which Complexes Will Distort?

eg

t2g

+0.6 Doct

-0.4 Doct

-½d1

+⅔d2

-⅓d2

dn configuration degeneracy LFSE stabilized? distortion

t2g eg

1

2

3

4

5

Doct >>> d1 >> d2

1 3 -0.4Doct - 0.33d2 yes small

2 3 -0.8Doct - 0.67d2 yes small

3 1 -1.2Doct no no

3 1 2 -0.6Doct - 0.5d1 yes large

3 2 1 0 no none

Which Complexes Will Distort?

dn configuration degeneracy LFSE stabilized? distortion

t2g eg

1

2

3

4

5

6

7

8

9

Doct >>> d1 >> d2

1 3 -0.4Doct - 0.33d2 yes small

2 3 -0.8Doct - 0.67d2 yes small

3 1 -1.2Doct no no

3 1 2 -0.6Doct - 0.5d1 yes large

3 2 1 0 no none

4 3 -0.4Doct - 0.33d2 yes small

5 3 -0.8Doct - 0.67d2 yes small

6 1 -1.2Doct no no

6 2 -0.6Doct - 0.5d1 yes large

2

2

23

Which Complexes Will Distort?

dn configuration degeneracy LFSE stabilized? distortion

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

Which Complexes Will Distort?

• 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

bonding

antibonding

+

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

molecular rotationslower energy (0.01 - 1 kJ mol-1)microwave radiation

electron transitionshigher energy (100 - 104 kJ mol-1)visible and UV radiation

molecular vibrationsmedium energy (1 - 120 kJ mol-1)IR radiation

Ground State

Excited State

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 light400-800 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

eg

t2g

Do

hn

d-d transition

[Ti(OH2)6]3+ lmax = 510 nm Do is 243 kJ mol-1

20 300 cm-1

The energy of the absorption by [Ti(OH2)6]3+ is the ligand-field splitting, Do

An electron changes orbital; the ion changes energy state

complex in electronic Ground State (GS)

complex in electronic excited state (ES)

GS

ES

GS

ES

eg

t2g

Electron-electron repulsiond2 ion

eg

t2g

xy xz yz

z2 x2-y2eg

t2g

xy xz yz

z2 x2-y2

xz + z2 xy + z2

lobes overlap, large electron repulsion lobes far apart, small electron repulsion

x

z

x

z

y y

These two electron configurations do not have the same energy

3P

3F

D E

D E = 15 B

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

Which is the Ground State?

2Eg

2T2g

Effect of a crystal field on the free ion term of a d1 complex

2T2

2E

6 Dq

4 Dq

2D

tetrahedral field free ion octahedral field

d1 d6

D

2Eg

2T2g

2D

Energy

ligand field strength, Doct

Energy level diagram for d1 ions in an Oh field

For d6 ions in an Oh field, the splitting is the same, but the multiplicity of the states is 5, ie 5Eg

and 5T2g

A

n / cm-1-

30 00020 00010 000

d1 oct [Ti(OH2)6]3+

E

LF strength

Orgel diagram for d1, d4, d6, d9

0 DD

D

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

A

n / cm-1-30 00020 00010 000

[Ti(H2O)6]3+, d1

2T2g

2Eg

2B1g

2A1g

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

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

d7 tetrahedral complex

15 B' = 10 900 cm-1

B' = 727 cm-1

[CoCl4]2-[Co(H2O)6]2+

d7 octahedral complex

15 B' = 13 800 cm-1

B' = 920 cm-1

Free ion [Co2+]: B = 971 cm-1

B' = 0.95B

B' = 0.75B

Nephelauxetic ratio, b

b is a measure of the decrease in electron-electron repulsion on complexation

- 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)

cloud expandingThe Nephelauxetic Effect

Selection Rules

Transition e complexes

Spin forbidden 10-3 – 1 Many d5 Oh cxsLaporte forbidden [Mn(OH2)6]2+

Spin allowedLaporte 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 ligandsLaporte 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

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

D D

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 d3 and d8:3 bands v1 = Doct B from calculation

Octahedral d7:3 bands Doct = v2 – v1 B from calculation

Features of an Electronic Spectrum

Ni2+, d8:

13800 cm-1 25300 cm-18500 cm-1

• 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

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 photons are absorbed:low area or weak bands weak colour

If there is no change, there is no interaction and no photons are absorbed

Selection Rules

Selection rules tell us which transitions give no change in dipole moment and hence which will have zero intensity

• 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 photons are absorbed:low area or weak bands weak colour

If there is no change, there is no interaction and no photons are absorbed

Selection Rules - IR

• During the transition, there must be a change in the dipole moment of the molecule

• Octahedral ML6 complexes undergo 3 types of M-L stretching vibration:

dipole momentchange?

no yes no

• There is one band in the M-L stretching region of the IR spectrum

[Co(CN)6]3-

Selection Rules – Spin Selection Rule

The spin cannot change during an electronic transition

eg

t2gt2g

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

ground state

d5

AJB lecture 1

NO spin allowed transitions for high spin d5

Selection Rules – Orbital Selection Rule

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)

• A photon has 1 unit of angular momentum• When a photon is absorbed or emitted, this momentum must be conserved

…so why do we see ‘d-d’ bands?

M

L

L

L

LM

L

L

L

L

‘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 not atoms

labelling the orbitals as ‘d’ (atomic orbitals) is incorrect if there is any covalency

M

L

L

L

LM

L

L

L

L

A metal p-orbital overlaps with ligand orbitals

M

L

L

L

LM

L

L

L

L

A metal d-orbital overlaps with the same ligand orbitals

M

L

L

L

LM

L

L

L

L

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

L

L

LL

LL

L

L

LL

LL

L

L

LL

LL

L

L

LL

LL

Laporte Selection Rule

• This way of ‘relaxing’ the orbital selection rule is not available in octahedral complexes

A metal p-orbital overlaps with ligand orbitals

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)

L

L

LL

LL

L

L

LL

LL

L

L

LL

LL

L

L

LL

LLin phase

out of phase

no overlap

‘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

spin-orbit coupling gets stronger as elements get heavier and so spin forbidden transitions get more important

• Mn2+ d5: all transitions are spin forbidden

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 forbiddenorbitally forbidden,Laporte forbidden

octahedral d5 complexes

(e.g. [Mn(H2O)6]2+)

1 – 10spin forbidden

orbitally forbidden,tetrahedral d5

complexes (e.g. [MnCl4]2-+)

10 – 102

spin allowed,orbitally forbiddenLaporte forbidden

octahedral and square planar complexes

10 – 103 spin allowed,orbitally forbidden tetrahedral complexes

> 103 LMCT, MLCT, IVT

verypale colours

intensecolours

Tanabe-Sugano diagrams

E/B

D/B

2T2g

4A1g, 4E

4T2g

4T1g

4T2g

4T1g

2A1g

4T2g

2T2g

6A1g

2Eg

4A2g, 2T1g

2T1g

2A1g

4EgAll 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

E/B

D/B

[V(H2O)6]3+: Three spin allowed transitions

n1 = 17 800 cm-1 visible

n2 = 25 700 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

D/B = 32

n1 = 17 800 cm-1

n2 = 25 700 cm-1

n1

n2E/B = 43 cm-1

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

E/B

D/B

[Cr(H2O)6]3+: Three spin allowed transitionsn1 = 17 400 cm-1 visible

n2 = 24 500 cm-1 visible

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

TiF4 d0 ion

TiCl4 d0 ion

TiBr4 d0 ion

TiI4 d0 ion

d0 and d10 ion have no d-d transitions

[MnO4]- Mn(VII) d0 ion

[Cr2O7]- Cr(VI) d0 ion

[Cu(MeCN)4]+ Cu(I) d10 ion

[Cu(phen)2]+ Cu(I) d10 ion

Zn2+ d10 ion

extremely purplebright orange

d0 and d10 ions

white

white

orange

dark brown

colourless

dark orange

white

Charge Transfer Transitions

Charge Transfer Transitions

Ligand-to-metal charge transfer

LMCT transitions

Metal-to-ligand charge transfer

MLCT transitions

MdLp

Ls

Lp*

t2g*

eg*

d-d transitions

Charge Transfer 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

M O

O

OO

Ligand to Metal Charge Transfer• These involve excitation of an electron from a ligand-based orbital into a d-

orbital

• 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

M O

O

OO

visible lightM O

O

OO

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+

VO43-

V5+

CrO42-

Cr6+

MnO4-

Mn7+

more easily reduced

in far UV ~39500 cm-1 ~22200 cm-1 ~19000 cm-1

white white yellow purple

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

NN

N

N

M = Fe2+, Ru2+, Os2+

N

MN

NN

N

N

• Sunlight excites electron from M2+ (t2g)6 into empty ligand p* 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

• Colour arises from excitation of an electron from Fe2+ to Fe3+

“Prussian blue”contains Fe2+

and 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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