Electronic Spectra of Complexes - radchem.nevada.edu

Electronic Spectra of Complexes

• Interpret electronic spectra of coordination compounds
❧ Correlate with bonding
❧ Orbital filling and electronic transitions
➠ Electron-electron repulsion
❧ Application of MO theory

• Spectroscopic terms
• Complex transitions

❧ Ligand field
❧ Charge transfer
❧ Selection rules

13-1

Electronic Spectra

• Cr(NH3)63+
❧ d3
❧ Weak low energy
transition
➠ Spin forbidden
❧ 2 stronger transitions
➠ Spin allowed
* ttr2gaannsidtieogns
➘ Lower
energy to
higher
energy
❧ CT at higher energy
➠ Ligand to metal
transition

13-2

Spectroscopic terms

• Electronic designation
❧ Configuration does not provide all information
➠ Does not contain angular or spin configuration
* 2p2
➘ p x, y, or z; 1, 0, -1;ml
➘ Spin pair or unpaired; -1/2, +1/2; ms
➘ Describes mircostate
➘ Energy of different microstate can vary due to
interelectronic repulsion
➘ Same energy levels are called terms
❧ 3d and 4f
➠ Electron spin important in energy, then orbital
* Total spin S and total orbital L
➘ Russell-Saunders coupling
❧ In heavier atoms (4d, 5d and 5f)
➠ spin-orbital coupling
* Total angular momentum j
➘ j-j coupling

13-3

Spectroscopic terms 13-4

• For 2 electrons
❧ S=s1+s2, s1+s2-1, …| s1-s2 |
❧ L=l1+l2, l1+l2-1, … | l1-l2 |
➠ S or L cannot be negative

• d2
❧ s=1/2
➠ S=1, 0
❧ l=2
➠ L=4, 3, 2, 1, 0

• d3
❧ Combine l3 and s3 with L and S from 2 electron system

• Angular momentum
❧ ML
➠ L, L-1,,-L
* 2L+1
❧ MS
➠ S, S-1,,,-S
* 2S+1

• For a given microstate, ML and Ms is sum of each electron state
❧ (0+, -1-); MS=0 and ML=-1

Spectroscopic terms

• L=S, P, D, F, G…..
• S=2S+1

❧ Written as SL
➠ Term symbol

• 3P
❧ L=1, S=1
➠ Triplet state

• What is the term symbol for s1p1
❧ s=0, p=1, L=1=P, S=0 or 1
➠ 1P and 3P

• For p1d1
❧ L=1+2, F; S=0 or 1
➠ 1F and 3F

13-5

Term symbols

• d2 configuration
❧ Pauli principle and
Hund’s rules limit
configurations
❧ Largest ML is both
electrons in l=2, spin
paired
➠ L=4, S=0, 1G
* 1x9 states
❧ L=3, S=1,0,-1 3F
➠ 3x7 states
❧ L=2, S=0, 1D
➠ 1x5 states
❧ L=1, S=1,0,-1, 3P
❧ L=0, 1S

13-6

Term symbols

• Lowest energy identified by Hund’s rules

❧ Lowest energy with parallel spin

➠ Triple state

* 3F and 3P states

➘ Highest MS value most stable
➠ Greater L, lower energy

* 3F

❧ Order ➘ Highest L with highest MS state

➠ Predicted; 3F<3P<1G<1D<1S

➠ From spectroscopy; 3F<1D<3P<1G<1S

• Ground state term for Cr3+

❧ 3d3

➠ 3 electrons with same spin, S=3/2; MS=4
➠ Each electron in different orbital

* L=2+1+0=3; F; 4F

• Ground state of Mn2+

❧ 3d5

➠ 5 electrons with same spin, 5/2, MS=6
❧ All must occupy different orbital

➠ L=2+1+0-1-2=0 13-7

* 6S

Term

• Racah parameter
❧ Electron-electron
repulsion
❧ Parameter based on
combination of 3
terms
➠ A, B, and C
❧ All positive
➠ 1S=A+14B+7C
➠ 1G=A+4B+2C
➠ 1D=A-3B+2C
➠ 3P=A+7B
➠ 3F=A-8B

13-8

Ligand Field Transitions

• Cr(NH3)63+
❧ Oh configuration, d3
➠ t2g3
❧ Transition
➠ t2g2eg1Å t2g3
* Near 400 nm
➘ Two peaks
❧ Molecular term symbols based
on multiplicity and orbital
➠ e4Tne2grÅgy4)Aa2ng d(h4iTgh1geÅr 4A2g
* Superscript denotes
S=3/2
* Orbitals from
character tables

• Weak and strong fields
❧ Ignore electron repulsion in
strong field
❧ Ligands of increasing field
strength effect terms

13-9

Ligand field

• Tanabe-Sugano
diagrams
❧ Symmetry and
ligand field strength
➠ Energy and
Racah term B
❧ States of same
symmetry avoid
crossing
❧ Permits evaluation
of orbitals
➠ At 0 energy
lowest term

13-10

Charge transfer bands

• High energy absorbance
❧ Energy greater than d-d
transition
➠ Electron moves between
orbitals
* Metal to ligand
* Ligand to metal
➠ Sensitive to solvent

• LMCT
❧ High oxidation state metal ion
❧ Lone pair ligand donor

• MLCT
❧ Low lying pi, aromatic
❧ Low oxidation state metal
➠ High d orbital energy

13-11

Selection rules

• Allowed and forbidden transitions
❧ ∆S=0 is allowed
➠ Otherwise spin forbidden
* More likely in heavier atoms
➘ 4d and 5d
❧ Transitions must involve electronic dipole
change to absorb light
➠ d transition become more allowed with
asymmetric vibrations
➠ f sharp due to shielding

13-12

Transition metals

13-13

Absorbance Actinide transitions

5 13-14

Pu6+(835 nm)
4 Pu4+ (489 nm)

Normal
3 Heavy

Light

2

1

0
400 500 600 700 800

Wavelength (nm)

Figure 2: UV-vis spectra of organic phases for 13M
HNO3 system

Energy diagram

13-15

Excitation

• Rates of absorption are high (fs)
❧ Fluorescence emission has a longer lifetime (ns)

• Relaxation
❧ Vibration
❧ Internal conversion
➠ Overlap vibration levels
❧ External conversion
➠ Quenching with solvent
❧ Intersystem crossing
➠ Spin flip

• Quantum Yield
❧ Ratio of molecules that luminence to total excited
molecules
❧ Mainly n->π∗ or π ->π∗

13-16

Fluorescence

• Increase fluorescence
❧ Aromatic groups
❧ Rigidity

• decrease fluorescence
❧ Temperature
increase
❧ Heavy atoms in
solvent
❧ Dissolved O2

• pH can change species
• Self absorption at high

concentration

13-17

Emission and Excitation

13-18

Circular Dichroism

• Chiral complexes
❧ Different
interactions for
polarized photons
➠ Plot difference
in absorbance
for polarized
light

13-19

Electron Paramagnetic resonance

B0=0 B0≠0
me=+1/2

∆E=geβeB0=hν

me=-1/2

Energy levels split in the
presence of external field

13-20

Electron-Nuclear Hyperfine Interaction

B0=0 B0≠0 Hyperfine interaction
me=+1/2
me=-1/2 me=+1/2 mI
me=-1/2 +1
0
-1

-1
0
+1

• Electron-nuclear interaction further split the energy levels
• Transitions are allowed between Dme=1, DmI=0.
• For an I=1 nucleus, 3 allowed transitions.

13-21

EPR spectrum

Absorbance
spectrum

1st derivative
spectrum

mI=-1 mI=0 mI=+1

Magnetic field 13-22