Transition Metal Complexes Electronic Spectra 2

Size: px

Start display at page:

Download "Transition Metal Complexes Electronic Spectra 2"

Muriel Caldwell
6 years ago
Views:

1 Transition Metal Complexes Electronic Spectra 2

2 Electronic Spectra of Transition Metal Complexes Cr[(NH 3 ) 6 ] 3+ d 3 complex Molecular Term Symbols Quartet states Doublet state

3 Different Ways of Transitions a) d z 2 d xy Creates more repulsion b) d z 2 d xz Creates less repulsion

4 Correlation of Terms of Free Ion and O h Complexes Atomic Term S P D F G Number of States Terms in O h Symmetry A 1g (no splitting) T 1g (no splitting) T 2g + E g T 1g + T 2g + A 2g A 1g + E g + T 1g + T 2g

5 Correlation of Terms of Free Ion and O h d 1 and d 2 Complexes Orgel Diagrams

6 Tanabe-Sugano Diagram of d 2 Configuration

7 Tanabe-Sugano Diagrams For a given C/B value A plot of energy E (in terms of B) vs. ligand field splitting o (in terms of B) E = energy relative to the ground-state term (i.e. ground state term energy = zero) As o increases, electrons tend to pair up, if possible (i.e. change in spin multiplicity) Electronic transition occurs from the ground state to the next excited states with the same multiplicity (spin selection rule) Help on Tanabe-Sugano diagrams

8 As the strength of the interaction changes, states of the same spin degeneracy (multiplicity) and symmetry CANNOT cross. Non-crossing Rule

9 Determine the o and B using Tanabe- Sugano Diagram 28500/21500 ~ 1.32 at 0 /B ~ B = B = 657 cm -1 0 /B = = cm Ratio = 1.32

10 Nephelauxetic Effect Nephelauxetic : cloud expanding B is a measure of electronic repulsion B(complex) < B(free ion) B(complex)/B(free ion) < 1 Example: B for [Cr(NH 3 ) 6 ] 3+ = 657 cm -1 B for Cr 3+ free ion ~ 1027 cm -1 Electronic repulsion decreases as molecular orbitals are delocalized over the ligands away from the metal Nephelauxetic Series = B(complex)/B(free ion) small : large nephelauxetic effect, large delocalization, high covalent character (soft ligands) For a given metal center, ligands can be arranged in decreasing order of : F - > H 2 O > NH 3 > CN -, Cl - > Br -

11 Intensities of Transitions Electronic Transition: interaction of electric field component E of electromagnetic radiation with electron Beer s Law: absorbance A = log I o /I = bc c = concentration, M b = path length, cm = molar extinction coefficient, M -1 cm -1 Probability of Transition transition moment µ fi µ fi = f * µ i d f : final state i : initial state µ : - er electric dipole moment operator Intensity of absorption µ 2 fi Allowed Transition µ fi 0 Forbidden Transition µ fi = 0

12 Spin Selection Rule The electromagnetic field of the incident radiation cannot change the relative orientation of the spins of electrons in a complex S = 0, spin-allowed transitions transition between states of same spin multiplicity S 0, spin-forbidden transitions transition between states of different spin multiplicity

13 Laporte Selection Rule In a centrosymmetric molecule or ion (with symmetry element i ), the only allowed transitions are those accompanied by a change in parity (u g, g u) Laporte (Symmetry) Allowed g u, u g Laporte (Symmetry) Forbidden g x g, u x u d orbitals have g character in O h all d-d transitions are Laporte forbidden µ = - er : u function d orbital : g function µ fi = f * µ i d = g x u x g = u = 0 In T d, no i. Laporte rule is silent.

14 Intensities of Spectroscopic Bands in 3d Complexes Transition max (M -1 cm -1 ) Spin-forbidden (and Laporte forbidden) < 1 Laporte-forbidden (spin allowed) Laporte-allowed ~ 500 Symmetry allowed (charge transfer)

15 Relaxation of Laporte Selection Rules Depart from perfect symmetry Ligand Geometric Distortion Vibronic coupling Mixing of asymmetric vibrations More intense absorption bands than normal Laporte forbidden transitions

16 Charge Transfer (CT) Transitions Move of electrons between metal and ligand orbitals Very high intensity LMCT: ligand to metal MLCT: metal to ligand

17 Ligand to Metal Charge Transfer (LMCT) d (M) p (L) transitions are both spin and symmetry allowed and therefore are usually have much higher intensity than d-d transitions.

18 d (M) p (L) LMCT of [Cr(NH 3 ) 5 X] 2+ X - weaker field ligand than NH 3 0 reduced Symmetry reduced, O h C 4v energy level splitted LMCT energy : M Cl > M Br > M I

19 Comparison of [Cr(NH 3 ) 6 ] 3+ and [Cr(NH 3 ) 5 X] 2+

20 d 0 Oxo Ions [MO x ] y- d (M) p (O) Charge Transfer LMCT energy [MnO 4 ] - (purple) < [TcO 4 ] - < [ReO 4 ] - (white) [CrO 4 ] 2- (yellow) < [MoO 4 ] 2- < [WO 4 ] 2- (white) [WS 4 ] 2- (red) < [WO 4 ] 2- (white) d (1 st row T.M.) lower than d (3 rd row T.M.) in same group p (E) higher down the same group p (O) lower than p (S)

21 Effect of M and L on LMCT 3 rd row T.M. 2 nd row T.M. 1 st row T.M. d M d L S p p O

22 Optical Electrnegativities Optical Electrnegativities variation in position of LMCT bands = ligand metal 0 0 = 3.0 X 10 4 cm -1 Metal O h T d Ligand Cr(III) F Co(III) l.s. 2.3 Cl Ni(II) Br Co(II) I Rh(III) l.s. 2.3 H 2 O 3.5 Mo(V) 2.1 NH 3 3.3

23 Metal to Ligand Charge Transfer (MLCT) For metal ions in low oxidation state (d low in energy) For ligands with low-lying * orbitals, especially aromatic ligands (e.g. diimine ligands such as bipy and phen)

24 Charge Transfer (CT) Transitions Move of electrons between metal and ligand orbitals Very high intensity LMCT: ligand to metal MLCT: metal to ligand

25 Luminescence Ruby: Cr 3+ in alumina Fluorescence S =0 Phosphorescence S 0

26 Phosphorescence of [Ru(bipy)3]2+

27 Spectra of f-block Complexes Free-ion limit f-orbitals are deep inside atoms. Ligand show little effects Sharp transitions Pr 3+ (aq), f2 # of f La 3+ Ce 3+ Pr 3+ Nd 3+ Pm 3+ Sm 3+ Eu 3+ Gd 3+ color colorless colorless colorless Green red pink yellow Pink # of f Lu 3+ Yb 3+ Tm 3+ Er 3+ Ho 3+ Dy 3+ Tb 3+

28 Circular Dichroism Spectra CD spectra can be observed for chrial complexes, it can be used to infer the absolute configuration of enantiomers

6.2. Introduction to Spectroscopic states and term symbols

Chemistry 3820 Lecture Notes Dr. M. Gerken Page62 6.2. Introduction to Spectroscopic states and term symbols From the number of absorption bands we have already seen that usually more d-d transitions are