Electronic spectra of TM complexes

Question 1

Which d block metal complexes are coloured

Answer 1

1. Characteristic feature of species with ground state electronic configurations
2. Except d0 and d10

Question 2

What is responsible for the colours of complexes

Answer 2

1. Electronic transitions give rise to these colours in the visible and uv regions of the em spectrum

Question 3

What is the wavelength range of visible and uv spectra

Answer 3

1. 200-740 nm

Question 4

When can we see the colours of complexes and give example if we an object appears pinkish

Answer 4

1. Colours in transition metal complexes arise when the visible light reaching our eyes has some of the wavelengths removed
2. e.g when an object appears pinkish it is loosely because the complementary wavelengths in the colour wheel (green) are strongly absorbed by the material

Question 5

Why would a complex appar yellow

Answer 5

1. Because it absorbs more of the other spectral colours in its electronic transitions than it does yellow wavelengths/light

Question 6

What does the energy of the light absorbed correspond to

Answer 6

1. The excitation of an electron from a t2g orbital to an eg orbital

Question 7

What are the transition types possible for TM complexes

Answer 7

1. Ligand spectra
2. Counter-ion spectra
3. Charge transfer spectra
4. CF transitions

Question 8

Describe the ligand spectra

Answer 8

1. Ligands like water and organic molecules have absorption bands that are normally found in the uv region but may impinge on the visible

Question 9

Describe the counter-ion spectra

Answer 9

1. A complex ion must have a counter-ion

2. Some counter-ions show intense absorptions in the UV portion of the spectrum

Question 10

Describe the charge transfer spectra

Answer 10

1. These arise from transitions in which an electron is moved between orbitals that are mostly ligand in character and orbitals that are mostly metal in character or vice versa

Question 11

Describe CF transitions

Answer 11

1. CF transitions arise between d orbitals that have been split in the crystal or ligand field environment
2. d-d transitions

Question 12

How many peaks would be observed in the UV-vis spectrum of an octahedral d9 ion

Answer 12

1. A single transition is observed so one peak

Question 13

What happens in an UV-vis experiment

Answer 13

1. The energy or wavelength of light is varied
2. when a photon of the right energy is absorbed such that hv=Do, the electron is promoted to the higher energy eg subset

Question 14

Why are transitions so broad e.g. for [Ti(OH2)6]3+

Answer 14

1. In this real complex, the metal ion and 6 ligands vibrate continuously
2. One such vibration involves the bonds lengthening and shortening together- breathing mode
3. When the ligands are drawn closer to the metal, the repulsions experienced by the d electrons are increased and so is Do
4. Hence eg t2g transition energy will increase
5. These vibrations take place a lot more slowly than the d-d transitions
6. Thus, an incident light beam on the sample effectively takes a snapshot of loads of stationary complexes in every part of the vibrational cycle
7. This contributes to the fact that d-d absorption bands are broad

Question 15

What order do ligands follow when looking at wavelength in UV-vis spectra

Answer 15

1. Ligands in same order as in SCS for ligands

Question 16

What is the rule of average environment

Answer 16

1. Ascribed to Jorgensen
2. States that the position of lambda max for a Ti3+ containing complex of the formula, [TiXnL3]n+/- will be midway between that for [TiX6]3- and [TiL6]3+ provided that all 3 complexes have the same stereochemistry

Question 17

How would you calculate frequency for [TiXnL3]n+/-

Answer 17

1. Calculate average Do from wavelengths

2. V= c/lambda

Question 18

What can the rule of average environment be used for

Answer 18

1. Used to estimate the composition of unknown species in solution

Question 19

What is something to look out for with the rule of average environment

Answer 19

1. Complexes containing a mixture of ligands often have low symmetry- far from ideal octahedral geometry
2. This causes considerable band splitting in the spectra and the rule becomes inapplicable

Question 20

What observations can be made from spectrochemical series for ligands and metals

Answer 20

1. Decreasing size of donor halides increases Do
2. Neutral NH3 is bigger than for negative halide ions
3. O2-, OH-, H2O have roughly equal Do despite charge difference
4. Do values increase rapidly with oxidation state of the metal
5. Do rapidly increases down a group or triad

Question 21

Explain whether decreasing size of donor halides increases Do makes sense

Answer 21

1. As ligand size decreases Do increases

2. Seems reasonable for an electrostatic point charge model- M-X bond length falls

Question 22

Explain whether Do being larger for Neutral NH3 than for negative halide ions makes sense

Answer 22

1. Doesn’t make sense
2. Electrostatic interaction between negative ligand and a positive metal should be stronger than between a metal ion and a neutral ligand

Question 23

Explain whether O2-, OH-, H2O having a roughly equal Do despite charge difference makes sense

Answer 23

1. Doesn’t make sense

Question 24

Explain if Do values increasing rapidly with oxidation state of the metal makes sense

Answer 24

1. Higher charged cations have a smaller radii and hence shorter M-L distances and so bond lengths decrease a little
2. Reasonable

Question 25

Explain whether Do rapidly increasing down a group or triad makes sense

Answer 25

1. Difficult to rationalise using a 100% electrostatic model

2. Can make argument based on expanded orbitals (and more nodes) and better M-L overlap

Question 26

Does CFT rationalise the spectrochemical series

Answer 26

1. Fails to rationalise the series for ligands
2. Only partially rationalises the series for metal species
3. Doesn’t mean it is wrong- just Do probably doesn’t have electrostatic origin