Lecture 7 Flashcards
(19 cards)
Why does a complex appear to have a colour?
- it absorbs light at specific wavelengths in the visible region of the spectrum
- wavelengths not absorbed are transmitted and observed
- from the ground state> light, causes d-d transition> to excited state
What are the complementary colours?
red- green
orange- blue
yellow- violet
Give the equation for wavelength
λ = hc/∆o
How can we measure ∆T or ∆o?
- UV vis spec to measure absorbance
A = log I0/I = є x c x l - Electronic spectra- the maximum at 20 300 cm-1 (492 nm) lies in the green part of the spectrum so the colour observed is red-violet
the energy of the absorbed light is used to
promote the d-electron from the t2g the eg level
the energy of the transition corresponds to ∆o. Thus, the energy of ∆o can be calculated
Whats the conversion of cm-1 to kJmol-1?
1cm-1 = 0.012 kJmol-1
Whats the equation for ∆o?
∆o = h . c . Na . v-
Describe the links between ∆o and bond strength
- as ∆o increases, field strength increases
- as field strength increases, bond length decreases, bond strength increases
Which factors influence magnitude of crystal field splitting?
- Nature of ligands
- Co ordination geometry and number of ligands
- Nature of metal ion
How does the nature of the ligands influence crystal field splitting?
Spectrochemical series;
(weak field)
I- < Br- < S2-< SCN- < Cl- < F- < OH- < CH3CO2- < H2O < SCN- < py < NH3 < en < bipy < NO2- < CN- < CO (strong field)
How does co ordination geometry and number of ligands influence crystal field splitting?
difference between octahedral and tetrahedral
e.g. ∆T = 4/9∆o
How does the nature of the metal ion influence crystal splitting?
Trend; Mn2+ < Ni2+ < Fe2+ < V2+ < Fe3+ < Co3+ < Mn4+
Higher oxidation states lead to larger splitting
Describe the Cobalt thiocyanate test (Scott test)
reagents; colour seen pink, field strength; H2O intermediate
products; colour seen blue, field strength; Cl- weak, ∆T= 4/9∆o
What determines colour intensity of TM compounds?
Transitions occur when complexes
interact with the oscillating field of electromagnetic radiation. This places restrictions on the symmetries
of the orbitals involved.
The probability of a transition occurring is governed by selection rules
Explain the selection rules
- Laporte selection rule
In an allowed transition, the quantum number l (angular orbital momentum quantum number) must change by +1 or -1
- s to p (l =0 to l=1); allowed
- p →d transitions (l = 1 to l = 2): allowed
- d → d transitions (l = 2 to l =2): forbidden - Parity selection rule (Symmetry selection rule)
The s- and d-orbitals possess a centre of
symmetry whilst the p-orbital does not:
Orbitals that have a centre of symmetry
within in a molecule that itself has a centre of
symmetry are: gerade (g)
Those that do not are called: ungerade (u)
- d-orbital= g
- s-orbital= g
- p-orbital= u
g to g and u to u transitions are forbidden in octahedral complexes - Spin selection rule
The probability of a transition occurring in which the spin is changing is really small
If in the excited state, an electron becomes paired its spin-forbidden. If not its spin allowed.
Spin forbidden transitions give very weak bands. The probability controls the intensity of the bands. The energy controls the position of the bands in the spectrum.
There are exceptions to the Parity selection rule. Why?
- We observe an absorption due to a d-d eg to tsg transition in [Ti(H2O)6]3+
This is due to vibrations within the molecule; at any one instant, the complex maybe
slightly asymmetric, not perfectly
octahedral. this allows a weak colour to be visible - The symmetry selection rule also doesnt apply to tetrahedral complexes since they dont have a centre of symmetry. d-d bands in spectra of tetrahedral complexes are much stronger
What are charge transfer transitions?
- allowed
- can always occur in transition metal complexes in addition to d-d transitions
- electrons move temporarily from ligand
orbitals to orbitals with metal character (or
vice versa) - the resulting charge transfer bands
are often extremely intense
What causes variation in colour with metal ions?
- metal ions chosen are all d0, so colour must be due to charge transfer
- V5+ in VO43- is white with UV absorbed
- Cr6+ in CrO42- is yellow with purple absorbed
- Mn7+ in MnO4- is purple with yellow absorbed
- band moves to lower energy as the metal ion becomes more oxidising
What causes variation in colour with ligands?
all d-d transitions are spin forbidden and symmetry forbidden. Any strong colours must be due to charge transfer
e.g. High spin Fe3+ complexes (d5)
How does molar absorption co influence colour intensity?
- as molar absorption coefficients get larger, intensity of colour increases as transitions are allowed
e.g. MnO4- є= 200
KMnO4 is a strong oxidising agent so we get spontaneous and permanent reduction of Mn7+ to Mn4+
