Colour

Question 1

What determines the colour we see and the intensity?

Answer 1

Colour depends on magnitude of the energy gap ^o. Higher ^o complexes will absorb higher energy colour- we see the complementary colour
Intensity depends on selection rules- the more rules that are broken, the weaker the colour

Question 2

What happens when a photon is absorbed?

Answer 2

The absorption of a photon promotes the electron from the t2g orbital to the eg orbital

^o the energy gap corresponds to the energy of a visible light proton

Question 3

Describe the visible spectra of d block ions

Answer 3

Usually broad because the transition occurs into one of a number of vibrational states of the electronically EXCITED state
Single band is really a number of overlapping peaks

Question 4

Why does the complex have colour?

Answer 4

It absorbs light at a specific wavelength (longer wavelengths are lower in energy) in the visible light region of the spectrum
Wavelengths that are not absorbs are transmitted and observed
Causes d to d transitions

Question 5

What colours do we see?

Answer 5

The colour observed is complementary to the colour absorbed
Red- green
Orange- blue
Yellow- violet

Question 6

What is the wavelength equation?

Answer 6

Wavelength= hc/ ^o

Because ^o equals energy of the photon

Question 7

What is the beer lambert law?

Answer 7

A= logIo/I = E x c x l

UV vis spectroscopy

Question 8

How can you calculate ^o?

Energy of transition

Answer 8

The energy of the transition corresponds to ^o therefore ^o can be calculated

1) convert vmax to energy by x 0.012KJ mol-1
2) multiply by CFSE using d orbital configuration (t2g= -0.4 and eg= 0.6)

Question 9

What determines what colour is observed

Answer 9

Colour is determined by ^
A large ^ means high energy light is absorbed
A small ^ means lower energy light is absorbed

Red- yellow- orange- blue- green-violet
Green- violet- blue- orange- red, yellow

Question 10

How can complexes with the same metal with the same oxidative state shown different colours?

Answer 10

This is because ^ also depends on the ligands

Question 11

How do different ligands determine colour for a metal with the same oxidative state?

Answer 11

Weak field ligands form long, weaker M-L bonds and so ^o is smaller and lower light is absorbed
Strong field ligands form short strong ML bonds and so ^o is smaller and higher energy light is absorbed

Question 12

What factors affect CFSE?

Answer 12

• coordination geometry
• number of ligands
• nature of ligands
• nature of metal ion

Question 13

What are examples of strong field, intermediate and weak field ligands?

Answer 13

I-, Br-, S2-, SCN-, Cl-, F-, Oh-, CH3CO2-, H2O, SCN-, py, NH3, en, bipy, NO2-, CN-, CO

The ones on the left and weak field (small ^o), the ones in the middle are intermediate and the ones on the right and strong field (large ^o)

Question 14

What is the cobalt thiocyanante test (Scott test)?

Answer 14

Cocaine + [Co(SCN)2(H20)4] + 2HCl -> [CoCl2[SCN)2]2-

Pink to blue
Octahedral to tetrahedral

Question 15

What determines intensity of colour?

Answer 15

Intensity is related to E and the value for E is determined by selection rules

Larger E means more intense colour

Question 16

In what way is probability of transition governed by selection rules ?

Answer 16

If a transition is allowed by selection rules it has a high probability of happening (strong colour)
If a transition is forbidden by selection rules, it has a low probability of happening (pale colour)

Question 17

What is la porte selection rule?

Answer 17

In an allowed transition, the quantum number l must change by +1 or -1 (movement between adjacent orbitals)

S: l=0
P: l= 1
D: l=2
F: l=3

Question 18

Why is the colour of [MnO4]- more intense?

Answer 18

The colour arises from a charge transfer process which not forbidden by la Porte
Other compounds colour arises from d to d transitions which is forbidden

Question 19

What is gerade?

Answer 19

Orbitals that have a centre of symmetry within a molecule that itself has a centre of symmetry are gerade (g)

Question 20

Why are d orbitals in an octahedral field labelled g?

Answer 20

D orbital posses a centre of symmetry and octahedral complexes possess a centre of symmetry

Question 21

What is parity selection rules?

Answer 21

The rule states that g-> g and u-> u transitions are FORBIDDEN
g-> u transitions are ALLOWED

Question 22

Why is transition in octahedral complexes forbidden by parity rule but not tetrahedral?

Answer 22

In octahedral, the transition eg t2 is not affected by the rule as they don’t have g and u labels since they don’t have a centre of symmetry

Question 23

Why are tetrahedral complexes more intensely coloured than octahedral complexes?

Answer 23

Tetrahedral complexes do not break the parity selection rule whereas octahedral do
This means that d-d bands in spectra are much stronger and so tetrahedral complexes are more intensely coloured

Question 24

Why is weak colour still observed even if the transition is d-d eg

Answer 24

This is because of vibrations in the molecule
This means the complex may be slight asymmetrical and not perfectly octahedral
This allowed weak colour to be observed

Question 25

What is the spin state selection rule?

Answer 25

This states that the probability of a transition occurring in which the spin is changing, is very small
The spin of an electron cannot change during an electron transition

Question 26

Relate intensity and colour to the UV vis spectrum for what bands we will see

Answer 26

The probability of the transition (selection rules) controls the intensity of the band
^o controls the position of the band in the spectrum ( which colour is absorbed)

Question 27

What is the charge transfer process

Answer 27

The electronic transition is from an orbital on a ligand to an empty d orbital on the metal

It can be ligand to metal, metal to ligand and metal to metal

Question 28

What the colour of compounds containing M caused by?

Answer 28

Charge transfer process as they have no d electrons so the colour cannot be caused by d- d transitions

Question 29

What happens to the absorption band during ligand to metal transfer?

Answer 29

Ligand metal transfer is a momentary oxidation of the ligand and reduction of the metal
The metal ion becomes more oxidising and the ligand becomes more reducing
This means the absorption band should move to lower energy

Question 30

What happens to the band as the metal ion becomes more oxidising?

Answer 30

The band moves to lower energy as the metal ion becomes more oxidising

Question 31

What are strong colours caused by in d5 complexes?

Answer 31

All d-d transitions are spin forbidden and symmetry forbidden
This means any strong colours must be due to charge transfer

Question 32

What happens to the band as the ligand becomes more reducing?

Answer 32

The band moves to lower energy as the ligand becomes more reducing