Kuliah Spec 3 Tanabe Spectroscopi

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

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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.


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

Δ is the splitting energy

The effect of ligand on splitting energy

The color of [Ti(H2O)6]3+


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 spectrochemical series

•For a given ligand, the color depends on the oxidation state of the metal ion.

•For a given metal ion, the color depends on the ligand.

I- < Cl- < F- < OH- < H2O < SCN- < NH3 < en < NO2- < CN- < CO

WEAKER FIELD STRONGER FIELD

LARGER ΔSMALLER Δ

LONGER λ SHORTER λ


High-spin and low-spin complex ions of Mn2+


Orbital occupancy for high- and low-spin complexes of d4 through d7 metal ions

high spin: weak-field

ligand

low spin: strong-field

ligand

high spin: weak-field

ligand

low spin: strong-field

ligand


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

λ / nm (wavelength)

200 700

visible

Absorption

~14 000 50 00025 000

UVvisible

ν / cm-1 (frequency)−

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

10104

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, Δoct or Δtet 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

[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

λ / nm

This complex is has a light purple colour

in solution because it absorbs green light

λmax = 510 nm

Absorption of light

eg

t2g

Δo

hν

d-d transition

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

20 300 cm-1

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

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

Limitations of ligand field theory

LFT assumes there is no inter-electron repulsion

[Ni(OH2)6]2+ = d8 ion

2+

Ni

A

3 absorption bands

eg

t2g

Repulsion between electrons in d-orbitals has an effect on the energy of the whole ion

15 00025 000ν / cm-1−

Electron-electron repulsiond2 ion

eg

t2g

xy xz yz

z2 x2-y2 eg

t2g

xy xz yz

z2 x2-y2

xz + z2

lobes overlap, large electron repulsion

x

z

xy + z2

lobes far apart, small electron repulsion

x

z

y y

These two electron configurations do not have the same energy

A

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

Degenerate electronic ground state: T or E

Non-degenerate ground state: A

- 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 ε 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

Tanabe-Sugano diagram for d2 ions

E/B

Δ/B

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

ν1 = 17 800 cm-1 visible


ν3 = obscured by CT transition in UV

10 000

ε

30 000ν / cm-1−

10

20 000

5

25 700 = 1.4417 800

Δ/B = 32

ν3 = 2.1ν1 = 2.1 x 17 800

∴ ν3 = 37 000 cm-1

= 32

E/B

Δ/B = 32

ν1 = 17 800 cm-1

ν2 = 25 700 cm-1

ν1

ν2E/B = 43 cm-1

E/B = 30 cm-1

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

B = 600 cm-1

Δo / B = 32

Δo = 19 200 cm-1

Tanabe-Sugano diagram for d3 ions

E/B

Δ/B

[Cr(H2O)6]3+: Three spin allowed transitionsν1 = 17 400 cm-1 visible


ν3 = obscured by CT transition

24 500 = 1.4117 400

Δ/B = 24

ν3 = 2.1ν1 = 2.1 x 17 400

∴ ν3 = 36 500 cm-1

= 24

Calculating ν3

E/B

Δ/B

ν1 = 17 400 cm-1

ν2 = 24 500 cm-1

= 24

E/B = 34 cm-1

E/B = 24 cm-1

When ν1 = E =17 400 cm-1

E/B = 24

so B = 725 cm-1

When ν2 = E =24 500 cm-1

E/B = 34

so B = 725 cm-1

If Δ/B = 24

Δ = 24 x 725 = 17 400 cm-1

Tanabe-Sugano diagrams

E/B

Δ/B

2T2g

4A1g, 4E

4T2g

4T1g

4T2g

4T1g

2A1g

4T2g

2T2g

6A1g

2Eg

4A2g, 2T1g

2T1g

2A1g

4EgAll terms includedGround state assigned to E = 0Higher levels drawn relative to GSEnergy in terms of BHigh-spin and low-spin configurations

Critical value of Δ

d5

WEAK FIELD STRONG FIELD

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 colourless

dark orange

Zn2+ d10 ion white

extremely purplebright orange

d0 and d10 ions

white

whiteorange

dark brown

Charge Transfer Transitions

Charge Transfer Transitions

Ligand-to-metal charge transferLMCT transitions

Metal-to-ligand charge transferMLCT transitions

MdLπ

Lσ

Lπ∗

t2g*

eg*

d-d transitions

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

Kuliah Spec 3 Tanabe Spectroscopi

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