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

is the splitting energy

The effect of ligand on splitting energy

Electronic Spectroscopy of Transition Metal Complexes

Chemistry 412 Experiment 1

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

nm (wavelength)

200 700

visible

Absorption

~14 000 50 00025 000

UVvisible

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, 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

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

/ nm

This complex is has a light purple colour

in solution because it absorbs green light

max = 510 nm

Absorption of light

eg

t2g

o

h

d-d transition

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

20 300 cm-1

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

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

yy

These two electron configurations do not have the same energy

3P

3F

E

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 = ms > ML = 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

2Eg

2T2g

2D

Energy

ligand field strength, oct

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

/ cm-1-

30 00020 00010 000

d1 oct [Ti(OH2)6]3+

E

LF strength

Orgel diagram for d1, d4, d6, d9

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

2Eg 2T2g

2Eg

2T2g

2D

A

/ 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,

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 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

Tanabe-Sugano diagrams

E/B

/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

d5

WEAK FIELD STRONG FIELD

Tanabe-Sugano diagram for d2 ions

E/B

/B

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

1 = 17 800 cm-1 visible

2 = 25 700 cm-1 visible

3 = obscured by CT transition in UV

10 000

30 000cm-1

10

20 000

5

25 700 = 1.44

17 800

/B = 32

3 = 2.11 = 2.1 x 17 800

3 = 37 000 cm-1

= 32

E/B

/B = 32

1 = 17 800 cm-1

2 = 25 700 cm-1

1

2E/B = 43 cm-1

E/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

Tanabe-Sugano diagram for d3 ions

E/B

/B

[Cr(H2O)6]3+: Three spin allowed transitions1 = 17 400 cm-1 visible

2 = 24 500 cm-1 visible

3 = obscured by CT transition

24 500 = 1.41

17 400

/B = 24

3 = 2.11 = 2.1 x 17 400

3 = 36 500 cm-1

= 24

Calculating 3

E/B

/B

1 = 17 400 cm-1

2 = 24 500 cm-1

= 24

E/B = 34 cm-1

E/B = 24 cm-1

When 1 = E =17 400 cm-1

E/B = 24

so B = 725 cm-1

When 2 = E =24 500 cm-1

E/B = 34

so B = 725 cm-1

If /B = 24

= 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 purple

bright 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

MdL

L

L

t2g*

eg*

d-d transitions