UV-VIS Spectroscopy

Transition Metal

Compounds

Part 2 of

Spectroscopic Methods in Inorganic Chemistry

Introduction

d1 VIS Spectra

d1 Spectra 2

Composite Colors

d-d spectra and MO theory:

3A2g →3T2g

3A2g →1Eg

υ, cm-1

UV

[Ni(NH3)6]2+

visible infrared

The electronic spectra of d-block complexes:

The features of electronic spectra that we need to be

able to master are:

1) naming of electronic states and d-d transitions,

e.g.3A2g, or 3A2g→1Eg

2) Explanation of relative intensities of bands in the

spectra of complexes of d-block metal ions. (The

Laporte and spin selection rules)

3) calculation of the crystal field splitting parameters

from energies of d-d bands

Naming of electronic states:

In names of electronic states, e.g. 4A2g, the labels A, E,

and T, stand for non-degenerate, doubly degenerate, and

triply degenerate, while the numeric superscript stands

for the multiplicity of the state, which is the number of

unpaired electrons plus one. Note that the electronic

states can be ground states (states of lowest energy) or

excited states:

4A2g

t2g

eg

Multiplicity =

3 unpaired electrons + 1

= 4

Non-degenerate

ground state =

‘A’

g = gerade

energy

eg eg eg

t2g t2g 6A2g

3T2g 1A2g

Non-degenerate triply degenerate non-degenerate

Multiplicity

= 5 + 1

energy

t2g

Naming of electronic states (contd.):

NOTE: In determining degeneracy, one can re-arrange the electrons, but

the number of unpaired electrons must stay the same, and the number

of electrons in each of the eg and t2g levels must stay the same.

Multiplicity

= 2 + 1

Multiplicity

= 0 + 1

eg eg eg

t2g t2g

5Eg 5T2g

2Eg

eg eg eg

t2g t2g

3A2g 1Eg

3T2g

Naming of electronic states (contd.):

t2g

t2g

ground state excited state excited state

ground state excited state ground state

energy

Electronic Transitions

Example d2 complex

Electronic transitions for Ni2+

eg eg

eg eg

t2g t2g

t2g t2g

3A2g →3T2g

3A2g →1Eg

3A2g 3T2g

3A2g 1Eg

ground state excited state

visible infrared UV

green 3A2g →3T2g

3A2g →1Eg

[Ni(H2O)6]2+

The electronic spectrum of [Ni(H2O)6]2+:

λ,

The complex looks green, because it absorbs only weakly at 500 nm,

the wavelength of green light.

On the previous slide we saw the two bands due to the 3A2g →

3T2g and 3A2g →1Eg transitions. The band at λ =

1180 nm which is the 3A2g →3T2g transition shown below,

corresponds to Δ for the complex. This is usually

expressed as Δ in cm-1 = (1/λ(nm)) x 107 = 8500 cm-1.

The electronic spectrum of [Ni(H2O)6]2+:

eg eg

t2g t2g

3A2g →3T2g 3A2g 3T2g Δ

= Δ

= 8500

cm-1

Note the weak band at 620 nm that corresponds to the 3A2g →

1Eg transition. The electron that is excited moves

within the eg level, so that the energy does not involve Δ,

but depends on the value of P, the spin-pairing energy.

The point of interest is why this band is so weak, as

discussed on the next slide.

The electronic spectrum of [Ni(H2O)6]2+:

eg eg

t2g t2g

3A2g →1Eg 3A2g 1Eg Δ

= 16100

cm-1

The electronic spectrum of [Ni(H2O)6]2+:

The two peaks at higher energy resemble the 3A2g→3T2g transition, but

involve differences in magnetic quantum numbers of the d-orbitals,

and are labeled as 3A2g→3T1g(F) and 3A2g→

3T1g(P) to reflect this:

3A2g →3T2g

3A2g →3T1g(F)

3A2g →3T1g(P)

3A2g →1Eg

λ,

[Ni(H2O)6]2+

The Selection rules for electronic transitions

There are three levels of intensity of the bands that we observe in the spectra of complexes of metal ions. These are governed by two selection rules, the Laporte selection rule, and the spin selection rule. The Laporte selection rule reflects the fact that for light to interact with a molecule and be absorbed, there should be a change in dipole moment. When a transition is ‘forbidden’, it means that the transition does not lead to a change in dipole moment.

The Laporte Selection rule: This states that transitions where there is no change in parity are forbidden:

g→g u→u g→u u→g forbidden allowed

Selection Rules

All transitions within the d-shell, such as 3A2g→3T2g are

Laporte forbidden, because they are g→g. Thus, the

intensity of the d-d transitions that give d-block metal

ions their colors are not very intense. Charge transfer

bands frequently involve p→d or d→p transitions, and so

are Laporte-allowed and therefore very intense.

The Spin Selection rule: This states that transitions that

involve a change in multiplicity (or number of unpaired

electrons) are forbidden. This accounts for why

transitions within the d-shell such as 3A2g→1Eg that

involve a change of multiplicity are much weaker than

those such as 3A2g→3T2g that do not.

The Selection rules for electronic transitions

The Selection rules for electronic transitions

3A2g →3T2g

Charge-transfer band – Laporte and spin allowed – very intense

[Ni(H2O)6]2+ a

b c

3A2g →1Eg Laporte and spin forbidden – very weak

a, b, and c, Laporte

forbidden, spin

allowed, inter-

mediate intensity

The three types of bands present in e.g. [Ni(H2O)6]2+ are:

1) Laporte-allowed plus spin allowed charge transfer

bands of very high intensity

2) Laporte-forbidden plus spin-allowed d→d transitions

(e.g. 3A2g→3T2g) of moderate intensity

3) Laporte forbidden plus spin-forbidden d→d transitions

(3A2g→1Eg) of very low intensity.

The Intensity of bands in complexes of d-block ions:

The MO view of electronic transitions in an

octahedral complex

t1u*

a1g*

eg*

t2g

t1u

eg

4p

4s

a1g

3d

t2g→t1u*

M→L Charge transfer

Laporte and spin

allowed

t1u→t2g

L→M Charge transfer

Laporte and spin

allowed

t2g→eg

d→d transition

Laporte forbidden

Spin-allowed or

forbidden

The eg level in CFT

is an eg* in MO

In CFT we consider

only the eg and t2g

levels, which are a

portion of the over-

all MO diagram

σ-donor orbitals

of six ligands

Charge-Transfer Peaks

Explanation and example for MnO4-

There are two mechanisms that allow ‘forbidden’

electronic transitions to become somewhat ‘allowed’.

These are:

1) Mixing of states: The states in a complex are never

pure, and so some of the symmetry properties of

neighboring states become mixed into those of the

states involved in a ‘forbidden’ transition.

2) Vibronic Coupling: Electronic states are always

coupled to vibrational states. The vibrational states may

be of opposite parity to the electronic states, and so help

overcome the Laporte selection rule.

Why do we see ‘forbidden’ transitions at all?

Mixing of states: Comparison of [Ni(H2O)6]2+ and [Ni(en)3]

2+:

[Ni(H2O)6]2+

[Ni(en)3]2+

3A2g →3T2g

3A2g →3T2g(F)

The spin-forbidden 3A2g →1Eg is close to the spin-allowed

3A2g →3T2g(F) and ‘borrows’ intensity by mixing of states

The spin-forbidden 3A2g →1Eg is not close

to any spin allowed band and is very weak

3A2g →1Eg

Note: The two spectra are

drawn on the same graph

for ease of comparison.

Electronic transitions are coupled to vibrations of various

symmetries, and the latter may impart opposite parity to

an electronic state and so help overcome the Laporte

selection rule:

Vibronic coupling:

electronic ground

state is ‘g’

electronic excited

state is ‘g’

g→g transition

is forbidden

g→(g+u) transition

is allowed

energy

coupled vibration

υ4’ is ‘u’

Electronic transitions, as seen

in the spectra of complexes of

Ni(II) shown above, are always

very broad because they are

coupled to vibrations. The

transitions are thus from ground

states plus several vibrational

states to excited states plus

several vibrational states (υ1, υ2, υ3),

so the ‘electronic’ band is actually

a composite of electronic plus

vibrational transitions.

υ5

υ3

υ1

υ5’

υ3’

υ1’

Symmetry of vibrational states, and their

coupling to electronic states:

T1u

symmetry

vibration

A1g

symmetry

vibration

(symbols have same meaning for

vibrations: A = non-degenerate,

T = triply degenerate, g = gerade,

u = ungerade, etc.)

The band one sees in the

UV-visible spectrum is the

sum of bands due to transitions

to coupled electronic (E) and

vibrational energy levels (υ1, υ2, υ3)

observed

spectrum

E E- υ1

E- υ2

E- υ3

E + υ1’

E + υ2’

E + υ3’

The spectra of high-spin d5 ions:

6A2g →4T2g

energy

For high-spin d5 ions all possible d-d transitions are spin-forbidden. As a

result, the bands in spectra of high-spin complexes of Mn(II) and Fe(III)

are very weak, and the compounds are nearly colorless. Below is shown

a d-d transition for a high-spin d5 ion, showing that it is spin-forbidden.

eg eg

t2g t2g

Complexes of Gd(III) are colorless, while those of other lanthanide

M(III) ions are colored, except for La(III) and Lu(III). Why is this?

Square Planar Complexes

Orbitals and Transitions

The spectra of complexes of tetrahedral

metal ions:

A tetrahedron has no center of symmetry, and so orbitals in

such symmetry cannot be gerade. Hence the d-levels in a

tetrahedral complex are e and t2, with no ‘g’ for gerade.

This largely overcomes the Laporte selection rules, so that

tetrahedral complexes tend to be very intense in color. Thus,

we see that dissolving CoCl2 in water produces a pale pink

solution of [Co(H2O)6]2+, but in alcohol tetrahedral

[CoCl2(CH3CH2OH)2] forms, which is a very intense blue

color. This remarkable difference in the spectra of

octahedral and tetrahedral complexes is seen on the next

slide:

The spectra of octahedral [Co(H2O)6]2+ and

tetrahedral [CoCl4]2- ions:

[CoCl4]2-

[Co(H2O)6]2+

The spectra at left

show the very intense

d-d bands in the blue

tetrahedral complex

[CoCl4]2-, as compared

with the much weaker

band in the pink

octahedral complex

[Co(H2O)6]2+. This

difference arises

because the Td com-

plex has no center of

symmetry, helping to

overcome the g→g

Laporte selection rule.

Tanabe-Sugano Diagrams

Free ion

terms

Spin allowed

transitions

Example d2

Calculate o

Energy ratio from the

peaks

Find ratio in the

diagram

=> o/B value

From the E/B and

the o/B value:

find B and o o/B = 30 and E/B = 28

=> o = 30 * B = 30 * E/28 = 30 * 17200/28

18500 cm-1

Exercise: Cr3+

Estimate the wavenumbers of the 2 peaks and calculate o

from the Tanabe Sugano diagram

Tanabe Sugano for d3

Estimate /B and E/B from

the Energy-relation of the 2

peaks in the spectrum.

From there you can calculate

the parameter B and from

there the splitting energy o