Colour Flashcards
What determines the colour we see and the intensity?
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
What happens when a photon is absorbed?
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
Describe the visible spectra of d block ions
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
Why does the complex have colour?
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
What colours do we see?
The colour observed is complementary to the colour absorbed
Red- green
Orange- blue
Yellow- violet
What is the wavelength equation?
Wavelength= hc/ ^o
Because ^o equals energy of the photon
What is the beer lambert law?
A= logIo/I = E x c x l
UV vis spectroscopy
How can you calculate ^o?
Energy of transition
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)
What determines what colour is observed
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
How can complexes with the same metal with the same oxidative state shown different colours?
This is because ^ also depends on the ligands
How do different ligands determine colour for a metal with the same oxidative state?
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
What factors affect CFSE?
- coordination geometry
- number of ligands
- nature of ligands
- nature of metal ion
What are examples of strong field, intermediate and weak field ligands?
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)
What is the cobalt thiocyanante test (Scott test)?
Cocaine + [Co(SCN)2(H20)4] + 2HCl -> [CoCl2[SCN)2]2-
Pink to blue
Octahedral to tetrahedral
What determines intensity of colour?
Intensity is related to E and the value for E is determined by selection rules
Larger E means more intense colour
In what way is probability of transition governed by selection rules ?
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)
What is la porte selection rule?
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
Why is the colour of [MnO4]- more intense?
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
What is gerade?
Orbitals that have a centre of symmetry within a molecule that itself has a centre of symmetry are gerade (g)
Why are d orbitals in an octahedral field labelled g?
D orbital posses a centre of symmetry and octahedral complexes possess a centre of symmetry
What is parity selection rules?
The rule states that g-> g and u-> u transitions are FORBIDDEN
g-> u transitions are ALLOWED
Why is transition in octahedral complexes forbidden by parity rule but not tetrahedral?
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
Why are tetrahedral complexes more intensely coloured than octahedral complexes?
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
Why is weak colour still observed even if the transition is d-d eg
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
What is the spin state selection rule?
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
Relate intensity and colour to the UV vis spectrum for what bands we will see
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)
What is the charge transfer process
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
What the colour of compounds containing M caused by?
Charge transfer process as they have no d electrons so the colour cannot be caused by d- d transitions
What happens to the absorption band during ligand to metal transfer?
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
What happens to the band as the metal ion becomes more oxidising?
The band moves to lower energy as the metal ion becomes more oxidising
What are strong colours caused by in d5 complexes?
All d-d transitions are spin forbidden and symmetry forbidden
This means any strong colours must be due to charge transfer
What happens to the band as the ligand becomes more reducing?
The band moves to lower energy as the ligand becomes more reducing
