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

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slide2

Colors of representative compounds of the Period 4 transition metals

nickel(II) nitrate hexahydrate

sodium chromate

zinc sulfate heptahydrate

potassium ferricyanide

titanium oxide

scandium oxide

manganese(II) chloride tetrahydrate

copper(II) sulfate pentahydrate

vanadyl sulfate dihydrate

cobalt(II) chloride hexahydrate

slide3

Mn(II)

Mn(VI)

Mn(VII)

Mn(VII)

Cr(VI)

V(V)

Aqueous oxoanions of transition elements

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

slide4

Effects of the metal oxidation state and of ligand identity on color

[V(H2O)6]3+

[V(H2O)6]2+

[Cr(NH3)6]3+

[Cr(NH3)5Cl]2+

slide12

-

n / cm-1 (frequency)

What is electronic spectroscopy?

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

Absorption

Absorption

[Ru(bpy)3]2+

[Ni(H2O)6]2+

104

10

~14 000

25 000

50 000

200

400

700

visible

UV

UV

visible

l / nm (wavelength)

UV = higher energy transitions - between ligand orbitals

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

- between metal and ligand orbitals

slide13

Absorption maxima in a visible spectrum have three important characteristics

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

intensity

This depends on the "allowedness" of the transitions which is described by two selection rules

slide14

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

slide15

3+

Ti

Absorption of light

[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

A

This complex is has a light purple colour in solution because it absorbs green light

l / nm

lmax = 510 nm

slide16

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

ES

ES

eg

eg

hn

Do

GS

GS

t2g

t2g

d-d transition

complex in electronic

excited state (ES)

complex in electronic

Ground State (GS)

[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

slide17

d2 ion

Electron-electron repulsion

eg

eg

x2-y2

x2-y2

z2

z2

t2g

t2g

xy

xz

yz

xy

xz

yz

xy + z2

xz + z2

z

z

y

y

x

x

lobes overlap, large electron repulsion

lobes far apart, small electron repulsion

These two electron configurations do not have the same energy

slide18

MS = S ms

ML = S ml

ML - MS

>

>

Which is the Ground State?

3P

States of the same spin multiplicity

D E

3F

D E = 15 B

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

Relative strength of coupling interactions:

slide19

6 Dq

4 Dq

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

d1 d6

tetrahedral field free ion octahedral field

2Eg

2T2

2D

2E

2T2g

slide20

Energy level diagram for d1 ions in an Oh field

2Eg

Energy

D

2D

2T2g

ligand field strength, Doct

For d6 ions in an Oh field, the splitting is the same, but the multiplicity of the states is 5, ie5Eg and 5T2g

slide21

A

D

Orgel diagram for d1, d4, d6, d9

10 000

20 000

30 000

Eg or E

E

T2g or T2

D

-

n / cm-1

T2g or T2

D

Eg or E

d1, d6 octahedral

D

d1, d6 tetrahedral

D

0

d4, d9 octahedral

d4, d9 tetrahedral

LF strength

[Ti(OH2)6]3+

d1oct

2Eg

2Eg 2T2g

2D

2T2g

slide22

-

n / cm-1

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

Degenerate electronic ground state: T or E

Non-degenerate ground state: A

d34A2g

d5 (high spin) 6A1g

d6 (low spin) 1A1g

d83A2g

2B1g

A

2Eg

[Ti(H2O)6]3+, d1

2A1g

2T2g

10 000

20 000

30 000

slide23

Racah Parameters

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

[Co(H2O)6]2+

[CoCl4]2-

d7 octahedral complex

15 B' = 13 800 cm-1

B' = 920 cm-1

d7 tetrahedral complex

15 B' = 10 900 cm-1

B' = 727 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

slide24

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)

slide25

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

slide26

The Spectrochemical Series

eg

eg

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

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

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

D

D

t 2g

t 2g

weak field ligands

e.g. H2O

high spin complexes

strong field ligands

e.g. CN-

low spin complexes

The Spin Transition

slide27

WEAK FIELD

STRONG FIELD

d5

Tanabe-Sugano diagrams

4T2g

2A1g

E/B

4T1g

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

4Eg

4T2g

4A1g, 4E

2A1g

2T1g

2T2g

2Eg

Critical value of D

4A2g, 2T1g

4T2g

6A1g

4T1g

2T2g

D/B

slide28

10

e

5

30 000

20 000

10 000

D/B = 32

-

n / cm-1

n3 = 2.1n1 = 2.1 x 17 800

n3 = 37 000 cm-1

= 32

Tanabe-Sugano diagram for d2 ions

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

E/B

n1 = 17 800 cm-1 visible

n2 = 25 700 cm-1 visible

n3 = obscured by CT transition in UV

25 700 = 1.44

17 800

D/B

slide29

n2

E/B = 43 cm-1

n1

E/B = 30 cm-1

D/B

= 32

E/B

n1 = 17 800 cm-1

n2 = 25 700 cm-1

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

B = 600 cm-1

Do / B = 32

Do = 19 200 cm-1

slide30

24 500 = 1.41

17 400

n3 = 2.1n1 = 2.1 x 17 400

n3 = 36 500 cm-1

= 24

Tanabe-Sugano diagram for d3 ions

n1 = 17 400 cm-1 visible

n2 = 24 500 cm-1 visible

n3 = obscured by CT transition

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

E/B

D/B = 24

D/B

slide31

E/B = 34 cm-1

E/B = 24 cm-1

Calculating n3

n1 = 17 400 cm-1

n2 = 24 500 cm-1

E/B

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

D/B

= 24

slide32

Charge Transfer Transitions

d0 and d10 ions

d0 and d10 ion have no d-d transitions

white

Zn2+ d10 ion

TiF4 d0 ion

TiCl4 d0 ion

TiBr4 d0 ion

TiI4 d0 ion

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

slide33

d-d transitions

Metal-to-ligand charge transfer

MLCT transitions

Ligand-to-metal charge transfer

LMCT transitions

Charge Transfer Transitions

Lp*

eg*

t2g*

Md

Lp

Ls