13-1

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

13-2

Electronic Spectra• Cr(NH3)6

3+

❧ d3

❧ Weak low energy transition➠ Spin forbidden

❧ 2 stronger transitions➠ Spin allowed

* t2g and egtransitions

➘ Lower energy to higher energy

❧ CT at higher energy➠ Ligand to metal

transition

13-3

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

13-4

Spectroscopic terms• 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

13-5

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

13-6

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

13-7

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

➘ Highest L with highest MS state❧ Order

➠ 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* 6S

13-8

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

13-9

Ligand Field Transitions• Cr(NH3)6

3+

❧ Oh configuration, d3

➠ t2g3

❧ Transition➠ t2g

2eg1 t2g

3

* Near 400 nm➘ Two peaks

❧ Molecular term symbols based on multiplicity and orbital

➠ 4T2g4A2g (higher

energy) and 4T1g4A2g

* 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

13-10

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

13-11

Charge transfer bands• High energy absorbance

❧ Energy greater than d-dtransition➠ 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

13-12

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

13-13

Transition metals

13-14

Actinide transitions

400 500 600 700 8000

1

2

3

4

5

Abs

orba

nce

Wavelength (nm)

Normal Heavy Light

Pu4+ (489 nm)

Pu6+(835 nm)

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

13-15

Energy diagram

13-16

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∗

13-17

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

13-18

Emission and Excitation

13-19

Circular Dichroism

• Chiral complexes❧ Different

interactions for polarized photons➠ Plot difference

in absorbance for polarized light

13-20

Electron Paramagnetic resonance

B0=0 B0≠0

me=+1/2

Energy levels split in the presence of external field

∆E=geβeB0=hν

me=-1/2

13-21

Electron-Nuclear Hyperfine Interaction

me=+1/2

me=-1/2

B0=0 B0≠0

mI

-1

me=+1/2

+10

+1

-10

me=-1/2

Hyperfine interaction

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

13-22

EPR spectrum

Absorbancespectrum

1st derivativespectrum

mI=0 mI=+1mI=-1

Magnetic field