Inorganic Spectoscopy-UV-Vis

UV-visible spectroscopyUV-visible spectroscopy

Usama El-Ayaan

the Period 4 transition metals

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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

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Aqueous oxoanions of transition elements

Mn(II) Mn(VI) Mn(VII)

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

Mn(II) Mn(VI) Mn(VII)

V(V)Cr(VI)

Mn(VII)

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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+

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Linkage isomers

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Complementary colour

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The five d-orbitals in an octahedral field of ligands

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Splitting of d-orbital energies by an octahedral field of ligands

∆ is the splitting energy

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The effect of ligand on splitting energy

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Electronic Spectroscopy of Transition Metal Complexes

Chemistry 412 Experiment 1

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What is electronic spectroscopy?

Absorption

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

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

10104

UV = higher energy transitions - between ligand orbitals

visible = lower energy transitions - between d-orbitals of transition metals

- between metal and ligand orbitals

UV

400

λ / nm (wavelength)

200 700

visible

~14 000 50 00025 000

UVvisible

ν / cm-1 (frequency)−

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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, ∆oct or ∆tet and on the degree This depends on the ligand field splitting parameter, ∆oct or ∆tet 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

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Selection Rules

In electronic spectroscopy there are three selection rules which determine whether or not transitions are formally allowed:

1. Spin selection rule: ∆S = 0

allowed transitions: singlet → singlet or triplet → tripletforbidden transitions: singlet → triplet or triplet → singlet

Changes in spin multiplicity are forbiddenChanges in spin multiplicity are forbidden

Singlet state: All electrons in the molecule are spin-pairedTriplet state: One set of electron spins is unpaired Usama El-Ayaan

Selection rules

2. Laporte selection rule: there must be a change in the parity (symmetry) of the complex

Laporte-allowed transitions: g → uLaporte-forbidden transitions: g → g or u → u

Transition metal complexes get around this rule through M-L vibrations, which temporarily removes the center of symmetry

g stands for gerade – compound with a center of symmetryu stands for ungerade – compound without a center of symmetry

3. Selection rule of ∆ℓ = ± 1 (ℓ is the azimuthal or orbital quantum number, where ℓ = 0 (s orbital), 1 (p orbital), 2 (d orbital), etc.)

allowed transitions: s → p, p → d, d → f, etc.forbidden transitions: s → s, d → d, p → f, etc.

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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

Ground State

Excited State

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

During an electronic transition

the complex absorbs energy

electrons change orbital

the complex changes energy stateUsama El-Ayaan

[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

Absorption of light

A

λ / nm

This complex has a light purple colour in

solution because it absorbs green light

λmax = 510 nmUsama El-Ayaan

eg

t2g

∆o

hν

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

GS

ES

GS

ES

eg

t2g

d-d transition

[Ti(OH2)6]3+ λmax = 510 nm ∆o is ∴ 243 kJ mol-1

20 300 cm-1

An electron changes orbital; the ion changes energy state

complex in electronic Ground State (GS)

complex in electronic excited state (ES)

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Electron-electron repulsiond2 ion

eg

t2g

xy xz yz

z2 x2-y2eg

t2g

xy xz yz

z2 x2-y2

xz + z2 xy + z2xz + z2 xy + z2

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

x

z

x

z

yy

These two electron configurations do not have the same energyUsama El-Ayaan

Deriving term symbols in a free ion

- an electron is completely defined by 4 quantum numbers

n = 1,2,3… (energy and distance from the nucleus ie. what shell)

Electron configurations of the same energy are collected together into terms

l = 0, 1, 2…orbital angular momentum (what sort {shape} of orbital, s, p, d…)

ml = -l…+l component of l (what kind of orbital, eg. for d: xy, xz…)

ms = spin angular momentum (contribution from unpaired electrons, spin 1/2)

- an ion can be defined using ionic quantum numbers made up of contributions

from all the electrons in the ion…..

..…these define the whole ionUsama El-Ayaan

Russel Saunders Coupling

L = 0, 1, 2…total orbital angular momentum (term)

ML = -L…+L component of L (ML = Σ ml)

S = total spin quantum number (S = Σ s)

Ionic quantum numbers:

Each electronic state has its own term symbol

2S+1

Lspin multiplicity L = 0 S term

L = 1 P termL = 2 D termL = 3 F term

Within each term, there can be several degenerate microstates with different ML and MS

Ms = -S….+S component of S (MS = Σ ms)

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Microstates e.g. for a free d2 ion

10 ways of arranging 2 electrons in 5 degenerate d-orbitals (Hund's rules apply)

etc

low repulsion (7 microstates) high repulsion (3 microstates)

xy + z2 xy + xz x2-y2 + z2 xz + z2

xy + yz x2-y2 + yz yz + z2

xz + yz x2-y2 + xz xy + x2-y2

2S+1

Lspin multiplicity

orbital angular momentum

xz + yz x -y + xz xy + x -y

ML = -3, -2, -1, 0, 1, 2, 3 ML = -1, 0, 1

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low repulsion (7 microstates) high repulsion (3 microstates)

ML = ∑ ml

For d-orbitals ml = -2 -1 0 1 2

x2-y2 yz z2 xz xy

For ML = -3….3 L = 3 F term

For ML = -1….1 L = 1 P term

xy + z2 xy + xz x2-y2 + z2 xz + z2

xy + yz x2-y2 + yz yz + z2

xz + yz x2-y2 + xz xy + x2-y2

ML = -3, -2, -1, 0, 1, 2, 3 ML = -1, 0, 1

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For a free d2 ion with two parallel spins there are two terms

Total spin S = no. of electrons x spin on an electron2 x 1/2 ∴ S = 1

Spin multiplicity = (2S + 1) = 3

2S+1

L F and P?

For a free d2 ion with two parallel spins there are two terms

3F 3Plow energy high energy

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3P

3F

∆ E

∆ E = 15 B

States of the same spin multiplicity

Which is the Ground State?

B is the Racah parameter and is a measure of inter-electron repulsion

within the whole ion

Relative strength of coupling interactions:

MS = Σ ms > ML = Σ ml > ML - MSUsama El-Ayaan

1. States with the highest spin multiplicity (2S+1) will be lowest in energy

(S = no. of e- x spin 1/2)

e.g. for a d5 ion 6S < 4G

Deriving the Ground State Term (Hund's rules)

2. Of two states with the same S, the one with higher L is of lower energy2. Of two states with the same S, the one with higher L is of lower energy

L = 0, S term; L = 1, P term; L = 2, D term; L = 3, F term

e.g. for a d2 ion 3F < 3P

These rules are only applicable to the ground state

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What are the ground state terms for the configurations p2 and d2?

p2: 1D 3P 1S

The ground state must be 3P because this has the largest spin multiplicity

d2: 1G 3F 1D 3P 1S

The ground state is either 3F or 3P because these have the largest spin multiplicity

Of the two, 3F must be the ground state because it has the largest value of L

(when L = 2, P term; when L = 3, F term)

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Ground States in free ions

d4 -2 -1 0 1 25D

ML = -2…2 L = 2 D term

MS = 4/2 2S+1 = 5

d3 -2 -1 0 1 24F

ML = -3…3 L = 3 F term

MS = 3/2 2S+1 = 4

d5 -2 -1 0 1 26S

ML = 0 L = 0 S term

MS = 5/2 2S+1 = 6

d6 -2 -1 0 1 25D

ML = -2….2 L = 2 D term

MS = 2 2S+1 = 5Usama El-Ayaan

Ground States in free ions

d7 -2 -1 0 1 24F

d8 -2 -1 0 1 2

ML = -3….3 L = 3 F term

MS = 3/2 2S+1 = 4

d8 -2 -1 0 1 23F

d9 -2 -1 0 1 22D

ML = -3….3 L = 3 F term

MS = 2/2 2S+1 = 3

ML = -2…2 L = 2 D term

MS = 1/2 2S+1 = 2Usama El-Ayaan

Ground State Terms for all electron configurations

Ground State

d1 d9 2D

d2 d8 3F 3P

d3 d7 4F 4P

d4 d6 5D

d5 6Sd5 6S

d-d transitions in high spin complexes

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2Eg

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

tetrahedral field free ion octahedral field

d1 ≅≅≅≅ d6

2T2g

2T2

2E

6 Dq

4 Dq

2D

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∆∆∆∆

2Eg

2D

Energy

Energy level diagram for d1 ions in an Oh field

2T2g

2D

ligand field strength, ∆oct

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

ie 5Eg and 5T2gUsama El-Ayaan

A

ν / cm-1-

30 00020 00010 000

d1 oct [Ti(OH2)6]3+

Orgel diagram for d1, d4, d6, d9

2Eg �2T2g

2Eg

2T2g

2D∆∆∆∆

E

LF strength

Orgel diagram for d , d , d , d

0 ∆∆

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

∆∆∆∆

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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 4A2gd5 (high spin) 6A1gd6 (low spin) 1A1gd8 3A2g

Degenerate electronic ground state: T or E

Non-degenerate ground state: A

A

ν / cm-1-30 00020 00010 000

[Ti(H2O)6]3+, d1

2T2g

2Eg

2B1g

2A1g

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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, ββββ

ββββ is a measure of the decrease in electron-electron repulsion on complexationUsama El-Ayaan

- some covalency in M-L bonds – M and L share electrons

-effective size of metal orbitals increases

-electron-electron repulsion decreases

cloud expandingThe Nephelauxetic Effect

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)

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Selection Rules

Transition εεεε 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[PdCl ]2-[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 speciesUsama El-Ayaan

eg

t 2g

eg

t 2g

I- < Br- < S2- < SCN- < Cl-< NO3- < F- < OH- < ox2-

< H2O < NCS- < CH3CN < NH3 < en < bpy

< phen < NO2- < phosph < CN- < CO

The Spectrochemical Series

∆ ∆

weak field ligands

e.g. H2O

high spin complexes

strong field ligands

e.g. CN-

low spin complexes

The Spin Transition

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Tanabe-Sugano diagrams

E/B

2T2g

4A1g, 4E

4T2g

4T1g

2A1g

4T2g

2Eg

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

d5

∆/B

4T2g

4T1g

2T2g

6A1g

Eg

4A2g, 2T1g Critical value of ∆

WEAK FIELD STRONG FIELDUsama El-Ayaan

Tanabe-Sugano diagram for d2 ions

E/B

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

νννν1 = 17 800 cm-1 visible


νννν = obscured by CT transition in UV

10 000

ε

30 000ν / cm-1−

10

20 000

5

∆/B

νννν3 = obscured by CT transition in UV

25 700 = 1.44

17 800

∆/B = 32

νννν3 = 2.1νννν1 = 2.1 x 17 800

∴ νννν3 = 37 000 cm-1

= 32 Usama El-Ayaan

E/B

νννν1 = 17 800 cm-1

νννν2 = 25 700 cm-1

νννν1

νννν2E/B = 43 cm-1

E/B = 30 cm-1

∆/B = 32

νννν1E/B = 30 cm-1

E/B = 43 cm-1 E = 25 700 cm-1

B = 600 cm-1

∆o / B = 32

∆o = 19 200 cm-1

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Tanabe-Sugano diagram for d3 ions

E/B

[Cr(H2O)6]3+: Three spin allowed transitionsνννν1 = 17 400 cm-1 visible


νννν3 = obscured by CT transition

24 500 = 1.41

17 400

∆/B = 24

∆/B

νννν3 = 2.1νννν1 = 2.1 x 17 400

∴ νννν3 = 36 500 cm-1

= 24 Usama El-Ayaan

Calculating ν3

E/B

νννν1 = 17 400 cm-1

νννν2 = 24 500 cm-1

When νννν1 = E =17 400 cm-1

E/B = 24

so B = 725 cm-1

When νννν2 = E =24 500 cm-1

E/B = 34

∆/B = 24

E/B = 34 cm-1

E/B = 24 cm-1

E/B = 34

so B = 725 cm-1

If ∆/B = 24

∆ = 24 x 725 = 17 400 cm-1

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TiF4 d0 ion

TiCl4 d0 ion

TiBr4 d0 ion

TiI4 d0 ion

d0 and d10 ion have no d-d transitions

Zn2+ d10 ion

d0 and d10 ions

white

white

orange

dark brown

white

[MnO4]- Mn(VII) d0 ion

[Cr2O7]- Cr(VI) d0 ion

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

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

extremely purple

bright orange

colourless

dark orange

Charge Transfer TransitionsUsama El-Ayaan

Charge Transfer Transitions

Ligand-to-metal charge transfer

LMCT transitions

Metal-to-ligand charge transfer

MLCT transitions

Lπ∗

t *

eg*

d-d transitions

MdLπ

Lσ

t2g*

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Transition Metals, Compounds and Complexesor

Electronic Spectroscopy of Transition Metal Complexes

Dr. E.R. Schofield

Lecture 3: Interpretation of weak field spectraLecture 3: Interpretation of weak field spectra

Electronic spectra from Orgel diagrams

Orgel diagram for d2, d3, d7, d8 ions

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Energy

d1 ≡≡≡≡ d6 d4 ≡≡≡≡ d9


D

or E

T2g or T2

Eg

ligand field strength

0 ∆∆d4, d9 tetrahedral

or T2

Eg or

d4, d9 octahedral

E

d1, d6 tetrahedral

T2g

d1, d6 octahedral

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A

ν / cm-1-

30 00020 00010 000

d1 oct [Ti(OH2)6]3+


2Eg �2T2g

2Eg

2T2g

2D∆∆∆∆

E

LF strength

Orgel diagram for d , d , d , d

0 ∆∆

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

∆∆∆∆

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Effect of a ligand field on Ground State Terms

GS

d1 d9 2D

d2 d8 3F 3P

d3 d7 4F 4P

d4 d6 5D

d5 6S

D T2(g) and E(g)

free ion ligand field

d S

GS

d1 d9 2D

d2 d8 3F 3P

d3 d7 4F 4P

d4 d6 5D

d5 6S

P

F

free ion ligand field

?

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Octahedral d2 complex

3P: high repulsion

In a LF, these orbitals are unaffected

3F: low repulsion

In a LF, orbitals in the t2g set go down in energy, orbitals in the eg set go up in energy

xz + z2

yz + z2

xy + x2-y2

3T1g

d2 ≅≅≅≅ d7

x2-y2 + z2

x2-y2 + yzx2-y2 + xzxy + z2

xy + xzxy + yzxz + yz

3A2g

3T2g

3T1g

eg eg singly degenerate, high energy

eg t2g triply degenerate, medium energy

t2g t2g triply degenerate, low energy

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Octahedral d3 complex

4P: high repulsion

In a LF, these orbitals are unaffected � 4T1g

4F: low repulsion

The order of energy levels is the opposite to that for a d2 ion �

d3 ≅≅≅≅ d8(same terms, different spin)

The order of energy levels is the opposite to that for a d2 ion �

4T1g

4T2g

4A2g

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Energy

Orgel diagram for d2, d3, d7, d8 ionsQuantum Mixing

F

PT1 or T1g

T1 or T1g

A2 or A2g

T1 or T1gT2 or T2g

Ligand field strength (Dq)

d2, d7 tetrahedral d2, d7 octahedral

d3, d8 octahedral d3, d8 tetrahedral

0

T2 or T2g

A2 or A2g

T1 or T1g

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P

Energy level diagram for oct d2, d7, tet d3, d8

15 B'15 B

15 B > 15 B'

x

10 Dq

T1(g)

A2(g)

F

x

10 Dq

6 Dq

2 Dq

T1(g)

T2(g)

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Calculating B' and x d7 octahedral complex

4A2g

4T1g

10 Dq

x

15 B'

A

[Co(H2O)6]2+

νννν2

νννν3

25 000 20 000 15 000 10 000

νννν1νννν2

νννν3

v / cm-1

4T1g

4T2g

6 Dq

2 Dq

x

νννν1

25 000 20 000 15 000 10 000

νννν1 = 8 000 cm-1

νννν2 = 16 000 cm-1

νννν3 = 19 400 cm-1

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F

P

Energy


A2 or A2g

T1 or T1gT2 or T2g

T1 or T1gT1 or T1g




0

T2 or T2g

A2 or A2g

T1 or T1g

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Px 10 Dq

T1(g)

A2(g)

Energy level diagram for oct d2, d7, tet d3, d8νννν1: x + 8 Dq

νννν2: 2 x + 6 Dq + 15 B'

νννν3: x + 18 Dq

νννν2

νννν3

νννν1: T2(g) � T1(g)

νννν2: T1(g)(P) � T1(g)

νννν3: A2(g) � T1(g)

F

15 B'15 B

x

6 Dq

2 Dq

T1(g)

T2(g)

νννν1

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Transition Metal Complexes

Housecroft and Sharpe, p. 20

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Metal d orbitals – octahedral metal complexes

M = metal

L = ligand


t2g orbitals – point between the ligands – π symmetry

eg orbitals – point at the ligands – σ symmetry

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UV-visible spectra of transition metal complexes

Transition metal complexes commonly exhibit UV-visible spectra containing both weak “d-d” (ε < 100) and strong “charge-transfer” (ε > 1000) bands which are characteristic of the nature of both the metal and the ligand(s)of both the metal and the ligand(s)

Shriver, Langford & Atkins (2nd ed), p. 582Usama El-Ayaan

Selection Rules

In electronic spectroscopy there are three selection rules which determine whether or not transitions are formally allowed:

1. Spin selection rule: ∆S = 0

allowed transitions: singlet → singlet or triplet → tripletforbidden transitions: singlet → triplet or triplet → singletforbidden transitions: singlet → triplet or triplet → singlet

Changes in spin multiplicity are forbidden

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Selection rules

2. Laporte selection rule: there must be a change in the parity (symmetry) of the complex

Laporte-allowed transitions: g → uLaporte-forbidden transitions: g → g or u → u

Transition metal complexes get around this rule through M-L vibrations, which temporarily removes the center of symmetry

g stands for gerade – compound with a center of symmetryu stands for ungerade – compound without a center of symmetry

3. Selection rule of ∆ℓ = ± 1 (ℓ is the azimuthal or orbital quantum number, where ℓ = 0 (s orbital), 1 (p orbital), 2 (d orbital), etc.)

allowed transitions: s → p, p → d, d → f, etc.forbidden transitions: s → s, d → d, p → f, etc.

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Crystal field theory – d orbitals split into two sets

eg

t2g


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Crystal Field Splitting


To promote an electron from the lower d orbital to the higher d orbital requires energy equal to ∆oct, the “crystal field splitting energy”Usama El-Ayaan

Spectrochemical Series

Spectrochemical Series (the effect of the ligand set on the magnitude of ∆o)

I- < Br- < Cl- < F- < O2- < OH- < H2O < NH3 < NO2- < CN- < PR3 < CO

σ-donor ligands

π-donor ligands increase the energy of the t2g orbitals

π-acceptor ligands decrease the energy of the t2g orbitals

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Visible spectra of octahedral Ni(II) complexes

[Ni(NH3)6]2+

1000 300 nm

[Ni(H2O)6]2+


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d3d2

excited

Shriver, Langford & Atkins

ground state

excited states

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UV-visible spectrum of a Chromium(III) complex

Spin-allowed transitions

Quartet → Quartet

Spin-forbidden transitions

Shriver, Langford & Atkins (2nd ed), p. 582

Spin-forbidden transitions

Quartet → Doublet

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d-d transitions in d3 metal complexes

For a d3 metal ion, such as Cr3+

there is one electron in each of the three t2g orbitals, while the eg orbitals are empty

eg

d3


t2g

eg

t2g

eg

d-d transition

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Excitation of d electrons

y

z z

y

Electron goes from the dxyorbital into a

Electron goes from the dxzorbital into a d orbital


x x

dz2 orbital

*requires a change in planes (xy to z)

dz2 orbital

*much less relocation of the electron

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Tanabe-Sugano diagram – d2 ion

Tanabe-Sugano diagram for a d2

ion, such as V2+

3T1g → 3T2g

3T1g

3T1g(F) → 3T1g(P)

3T1g → 3A2g


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Jahn-Teller distortions

e.g. [Ti(H2O)6]3+ d1 electronic configuration

When a set of degenerate orbitals are unevenly filled, then a perturbation of the molecular structure will occur so that the degeneracy is removed

eg

t2g

dz2

dxz, dyz

dxy

dx2-y2

ν1 ν2d

Usama El-Ayaan

[Ti(H2O)6]3+ visible spectrum

ν1ν2

500 400600

[Ti(H2O)6]3+

λ, nm


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Visible spectrum of [Mn(H2O)6]2+ (1 M solution)

Mn(H2O)62+ is a “high-spin” d5 ion (t2g

3 eg2) and

therefore all d-d transitions are spin-forbidden

All of the transitions have ε values of << 1, (very weak)

Brisdon, p. Usama El-Ayaan

Charge-Transfer

Ligand-to-Metal Charge Transfer

(LMCT)


Metal-to-Ligand Charge Transfer

(MLCT)

Charge transfer bands are allowed transitions, with εvalues > 10,000

Usama El-Ayaan

Ligand-to-Metal Charge Transfer

• Occurs with metals in high oxidation states (few d electrons) complexed by ligands which are good electron-donors

O

MnO

O

O

Permanganate anion

Manganese(VII) has no d electrons, while the O2- (oxide) ligands donate lone pairs of electrons to form double bonds

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UV-Visible spectrum of MnO4-

Lπ → M(t2g)

Lπ → M(eg)

Usama El-Ayaan

Metal-to-Ligand Charge Transfer (MLCT)

• Occurs with metals in low oxidation states (lots of d electrons) complexed by ligands which are good electron-acceptors

Ruthenium(II) has 6 d e-

Housecroft and Sharpe, p. 208Ru(2,2’-bipyridine)3

2+

has 6 d e-

2,2’-bipyridine is a good π-acceptor ligand

Usama El-Ayaan

Visible spectrum of Ru(2,2’-bipyridine)32+

[Ru(bpy)3]2+ MLCT

λmax = 452 nm ε = 14,600 M-1 cm-1

Usama El-Ayaan

Coordination ChemistryElectronic Spectra of Metal ComplexesElectronic Spectra of Metal Complexes

Usama El-Ayaan

Electronic configurations of multi-electron atomsRussell-Saunders (or LS) coupling

For each 2p electronn = 1; l = 1

ml = -1, 0, +1ms = ± 1/2

For the multi-electron atomL = total orbital angular momentum quantum numberS= total spin angular momentum quantum number

Spin multiplicity = 2S+1

ML = ∑ml (-L,…0,…+L)ML = ∑ml (-L,…0,…+L)MS = ∑ms (S, S-1, …,0,…-S)

ML/MS define microstatesand L/S define states(collections of microstates)

Groups of microstates with the same energy are called terms

Usama El-Ayaan

before we did:

p2

ML & M S

MicrostateTable

States (S, P, D)Spin multiplicity

Terms3P, 1D, 1S

Ground state term3P

Usama El-Ayaan

For metal complexes we need to considerd1-d10

d2

3F, 3P, 1G, 1D, 1S

For 3 or more electrons, this is a long tedious process

But luckily this has been tabulated before…

Usama El-Ayaan

Transitions between electronic terms will give rise to spectra

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Selection rules(determine intensities)

Laporte rule

g → g forbidden (that is, d-d forbidden)

but g → u allowed (that is, d-p allowed)

Spin rule

Transitions between states of different multiplicities forbidden

Transitions between states of same multiplicities allowed

These rules are relaxed by molecular vibrations, and spin-orbit coupling

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Group theory analysis of term splitting

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High Spin Ground Statesdn Free ion GS Oct. complex Tet complex

d0 1S t2g0eg

0 e0t20

d1 2D t2g1eg

0 e1t20

d2 3F t2g2eg

0 e2t20

d3 4F t2g3eg

0 e2t21

d4 5D t2g3eg

1 e2t22

An e electron superimposed on a spherical

distribution energies reversed because

tetrahedral

d5 6S t2g3eg

2 e2t23

d6 5D t2g4eg

2 e3t23

d7 4F t2g5eg

2 e4t23

d8 3F t2g6eg

2 e4t24

d9 2D t2g6eg

3 e4t25

d10 1S t2g6eg

4 e4t26

Holes: dn = d10-n and neglecting spin dn = d5+n; same splitting but reversed energies because positive.

A t2 hole in d5, reversed energies,

reversed again relative to

octahedral since tet.

Holes in d5

and d10, reversingenergies relative to

d1

Expect oct d1 and d6 to behave same as tet d4 and d9

Expect oct d4 and d9 (holes), tet d1 and d6 to be reverse of oct d1Usama El-Ayaan

Energy

d1 ≡≡≡≡ d6 d4 ≡≡≡≡ d9


D

or E

T2g or T2

Eg

ligand field strength

0 ∆∆d4, d9 tetrahedral

or T2

Eg or

d4, d9 octahedral

E

d1, d6 tetrahedral

T2g

d1, d6 octahedral

Usama El-Ayaan

F

P

Energy


A2 or A2g

T1 or T1gT2 or T2g

T1 or T1gT1 or T1g




0

T2 or T2g

A2 or A2g

T1 or T1g

Usama El-Ayaan

d2

3F, 3P, 1G, 1D, 1S

Real complexesUsama El-Ayaan


d2

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Electronic transitions and spectra

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Other configurations

d1 d9

d3

d2 d8

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Other configurations

d3

The limit betweenhigh spin and low spin

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Determining ∆o from spectra

d1d9

One transition allowed of energy ∆o

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mixing

Determining ∆o from spectra

Lowest energy transition = ∆o

mixing

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Ground statemixing

E (T1g→A2g) - E (T1g→T2g) = ∆o

Usama El-Ayaan

The d5 case

All possible transitions forbiddenVery weak signals, faint color

Usama El-Ayaan

Some examples of spectra

Usama El-Ayaan

Charge transfer spectra

LMCT

Ligand character

Metal character

MLCTMetal character

Ligand character

Much more intense bandsUsama El-Ayaan

Documents

Inorganic Spectoscopy-UV-Vis