Transition Metal ComplexesElectronic Spectra 2

Electronic Spectra of Transition MetalComplexes

•Cr[(NH3)6]3+ d3 complex

Molecular Term SymbolsQuartet states

Doublet state

Different Ways ofTransitions

a) dz2 dxy

Creates more repulsion

b) dz2 dxz

Creates less repulsion

Correlation of Terms of Free Ion andOh Complexes

A1g + Eg + T1g + T2g9G

T1g + T2g + A2g7F

T2g + Eg5D

T1g (no splitting)3P

A1g (no splitting)1S

Terms in OhSymmetry

Number ofStates

AtomicTerm

Correlation of Terms of Free Ion andOh d1 and d2 Complexes

-0.80

0.20

1.20

Orgel Diagrams

Tanabe-Sugano Diagram of d2

Configuration

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 (spinselection rule)

•Help on Tanabe-Sugano diagramshttp://wwwchem.uwimona.edu.jm:1104/courses/Tanabe-Sugano/

Non-crossing Rule•As the strength of the

interaction changes, statesof the same spindegeneracy (multiplicity)and symmetry CANNOTcross.

Determine the o and B using Tanabe-Sugano Diagram

28500/21500 ~ 1.32 at0 /B ~ 32.8

32.8B = 21550 B = 657 cm-1

0 /B = 32.8 0 = 21550 cm-1

28500 21550

32.8

Ratio = 1.32

32.8

Nephelauxetic Effect• Nephelauxetic : cloud expanding• B is a measure of electronic repulsion

B(complex) < B(free ion)B(complex)/B(free ion) < 1Example: B for [Cr(NH3)6]3+ = 657 cm-1

B for Cr3+ free ion ~ 1027 cm-1

• Electronic repulsion decreases as molecular orbitals aredelocalized over the ligands away from the metal

• Nephelauxetic Series= B(complex)/B(free ion)small : large nephelauxetic effect, large delocalization, highcovalent character (soft ligands)For a given metal center, ligands can be arranged in decreasingorder of

: F- > H2O > NH3 > CN-, Cl- > Br-

Intensities of Transitions•Electronic Transition:

interaction of electric field component E ofelectromagnetic radiation with electron

•Beer’s Law: absorbance A = log Io/I = bcc = concentration, M b = path length, cm= molar extinction coefficient, M-1cm-1

•Probability of Transition transition moment µfi

µfi = f* µ i df : final state i : initial stateµ : - er electric dipole moment operator

•Intensity of absorption µfi2

Allowed Transition µfi 0Forbidden Transition µfi = 0

Spin Selection Rule•The electromagnetic field of the incident radiation

cannot change the relative orientation of the spins ofelectrons in a complex

S = 0, spin-allowed transitionstransition between states of same spin multiplicity

S 0, spin-forbidden transitionstransition between states of different spin multiplicity

Laporte Selection Rule• In a centrosymmetric molecule or ion (with symmetry

element i ), the only allowed transitions are thoseaccompanied by a change in parity (u g, g u)Laporte (Symmetry) Allowed gu, ugLaporte (Symmetry) Forbidden gxg , uxu

•d orbitals have g character in Oh

all d-d transitions are Laporte forbidden•µ = - er : u function

d orbital : g functionµfi = f* µ i d

= g x u x g = u = 0• In Td, no i. Laporte rule is silent.

Intensities of SpectroscopicBands in 3d Complexes

Transition max (M-1cm-1)

Spin-forbidden (and Laporte forbidden) < 1Laporte-forbidden (spin allowed) 20 - 100Laporte-allowed ~ 500Symmetry allowed (charge transfer) 1000 - 50000

Relaxation of LaporteSelection Rules

•Depart from perfect symmetryLigandGeometric Distortion

•Vibronic couplingMixing of asymmetric vibrations

•More intense absorption bands thannormal Laporte forbidden transitions

Move of electronsbetween metal andligand orbitals

Very high intensity

LMCT: ligand to metalMLCT: metal to ligand

Charge Transfer (CT) Transitions

Ligand to Metal ChargeTransfer (LMCT)

•d(M)p(L) transitions are both spinand symmetry allowed and thereforeare usually have much higher intensitythan d-d transitions.

d(M)p(L) LMCT of [Cr(NH3)5X]2+

•X- weaker field ligand than NH3

0 reduced•Symmetry reduced, Oh C4v

energy level splitted•LMCT energy : M–Cl > M–Br > M–I

Comparison of[Cr(NH3)6]3+ and[Cr(NH3)5X]2+

d0 Oxo Ions [MOx]y-

d(M) p(O) Charge Transfer•LMCT energy

[MnO4]- (purple) < [TcO4]- < [ReO4]- (white)[CrO4]2- (yellow) < [MoO4]2- < [WO4]2- (white)[WS4]2- (red) < [WO4]2- (white)

d(1st row T.M.) lower than d(3rd row T.M.) in samegroupp(E) higher down the same group

p(O) lower than p(S)

Effect of M and L on LMCT

d

1st row T.M.

3rd row T.M.2nd row T.M.

pL

dM

p

S

O

Optical Electrnegativities

•Optical Electrnegativitiesvariation in position of LMCT bands= | ligand –metal | 0

0 = 3.0 X 104 cm-1

3.3NH32.1Mo(V)

3.5H2O2.3Rh(III) l.s.

3.02.5I-1.8 - 1.9Co(II)

3.32.8Br-2.0 - 2.1Ni(II)

3.43.0Cl-2.3Co(III) l.s.

4.43.9F-1.8 - 1.9Cr(III)

LigandTdOhMetal

Metal to Ligand ChargeTransfer (MLCT)

•For metal ions in low oxidation state (dlow in energy)

•For ligands with low-lying * orbitals,especially aromatic ligands (e.g. di-imine ligands such as bipy and phen)

Move of electronsbetween metal andligand orbitals

Very high intensity

LMCT: ligand to metalMLCT: metal to ligand

Charge Transfer (CT) Transitions

Luminescence

PhosphorescenceS 0

FluorescenceS =0Ruby:

Cr3+ in alumina

Phosphorescence of [Ru(bipy)3]2+

Spectra of f-block Complexes•Free-ion limit• f-orbitals are deep inside atoms.

Ligand show little effects•Sharp transitions

8Tb3+

9Dy3+

10Ho3+

11Er3+

12Tm3+

13Yb3+

14Lu3+

# of f

color-less

PinkyellowpinkredGreencolor-less

color-less

color

7Gd3+

6Eu3+

5Sm3+

4Pm3+

3Nd3+

2Pr3+

1Ce3+

0La3+

# of f

Pr3+(aq), f2

Circular Dichroism Spectra

•CD spectra can be observed for chrialcomplexes, it can be used to infer the absoluteconfiguration of enantiomers