UV-vis spectra

Question 1

How are absorption band positions described

Answer 1

1. In terms of the wavelength corresponding to absorption maximum Amax

Question 2

What measures absorbance

Answer 2

1. Spectrophotometers measure A as wavelength vaires

Question 3

What is absorbance given by

Answer 3

1. A= log10(lo/l)
2. lo= intensity of the incident light on the sample
3. l= intensity of the emergent light

Question 4

What is the molar extinction coefficient

Answer 4

1. Is a measure of the intensity (strength) of a transition at Amax
2. Given by Beer-Lambert Law

Question 5

What is Beer-Lambert Law

Answer 5

1. emax= Amax/cl = molar extinction coefficient
2. c= concentration of the solution in mol L^-1
3. l= is the path length of the sample in cm

Question 6

What are the units of emax

Answer 6

1. Lmol^-1 cm^-1

Question 7

As the molar extinction coefficient increases what happens to the colour intensity

Answer 7

1. Increases

Question 8

What do values of emax range from

Answer 8

1. Form close to zero (weak absorption)

2. To >10000 L mol^-1 cm^-1 (intense strong absorption

Question 9

What rationalises the difference between tetrahedral and octahedral emax values

Answer 9

1. How allowed the electronic transitions are

Question 10

What are the 3 selection rules

Answer 10

1. The Spin Selection Rule
2. The Orbital (Laporte) Selection Rule
3. The Parity Rule

Question 11

What does it mean when a transition is allowed or not

Answer 11

1. If a transition is allowed, it has a high probability of occurring and e will be large
2. If a transition is forbidden by a selection rule, it has a low probability of occuring and e will be small

Question 12

State the Spin Selection Rule

Answer 12

1. DS = 0 for spin allowed transitions

Question 13

Describe the Spin Selection Rule

Answer 13

1. The spin of an electron cannot change during an electronic transition
2. Sum of all the metal d electron spin Ems when added together must be the same before and after the electronic transition

Question 14

State the Laporte (orbital) Selection Rule

Answer 14

1. Dl= +/-1 for orbital allowed transitions

Question 15

Describe the Laporte (orbital) selection Rule

Answer 15

1. l= secondary/orbital quantum number
2. l= 0 –> S, l=1 –> p, l=2 –> d
3. s p transitions are allowed
4. p d transitions are allowed
5. dd transitions are not allowed

Question 16

State the Parity Rule

Answer 16

1. In a molecule/ion with a centre of symmetry, a transition must involve parity change

Question 17

Describe the Parity Rule

Answer 17

1. g–>u = parity allowed

2. g–>g = parity forbidden

Question 18

Describe parity rule for octahedral complexes

Answer 18

1. g in symmetry labels for orbital sets (t2g, eg)

2. so transitions are parity forbidden

Question 19

Describe parity rule for tetrahedral complexes

Answer 19

1. no ‘g’ in symmetry label of orbital sets

2. Crystal field transitions- parity allowed

Question 20

Describe colour in [Mn(H2O)6]2+

Answer 20

1. Spin forbidden
2. orbital forbidden- d–>d
3. parity forbidden- octahedral so g–>g
4. Breaks all 3 rules so colour is very weak

Question 21

Describe colour in [Ti(H2O)6]3+

Answer 21

1. Spin allowed
2. orbital forbidden- d–>d
3. parity forbidden- octahedral so g–>g
4. Breaks 2 rules so colour is weak

Question 22

Describe colour in [CoCl4]2-

Answer 22

1. Spin allowed
2. orbital forbidden- d–>d
3. parity allowed- tetrahedral
4. Breaks all 1 rules so colour is stronger

Question 23

Describe colour in [TiCl6]2-

Answer 23

1. Spin allowed
2. orbital allowed- p–>d (from ligand)
3. parity allowed- electrons from ligand- not CFT
4. Breaks NO rules so colour is very strong

Question 24

What type of transfer occurs in [TiCl6]2-

Answer 24

1. Ligand to metal charge transfer (LMCT)

Question 25

Summarise results of rules

Answer 25

1. All d-d transitions are forbidden (orbital selection rule) 2. Tetrahedral complexes have more intense colours than octahedral ones (parity selection rules) 3. Colours arising from electrons that move from ligand orbitals onto the metal are most intense (fully allowed)- LMCT bands

Question 26

How can you diagnose geometry using Molar extinction coefficients

Answer 26

1. Octahedral, nearly octahedral and square-planar complexes: emax values in 1 or 2 figures 2. Tetrahedral complexes: emax values in the hundreds 3. Charge-transfer bands (MLCT or LMCT): emax values range in the thousands

Question 27

Why do d-d transitions occur

Answer 27

1. Octahedral complexes can undergo asymmetrical vibrations which destroy the centre of inversion 2. Complexes may not be perfectly octahedral- no real inversion centre 3. Orbitals can mix with each other- means transitions may not be 100% pure d-d in nature 4. A spin forbidden transition has more impact on reducing the colour intensity (emax) than an orbital forbidden transition

Question 28

Describe how [Co(en)2F2]+ emax varies with different structures

Answer 28

1. Cis complex has a larger emax than trans complex 2. Due to partial breaking of parity rule in cis complex 3. CFT does not recognise the cis-complex as breaking the parity rule- not sophisticated enough