Electronic Spectra of Transition MetalComplexes
•Cr[(NH3)6]3+ d3 complex
Molecular Term SymbolsQuartet states
Doublet state
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
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
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)
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
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