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/notes/ch7lec2.ppt
7.3 Optical Spectra and Magnetics
Topics:
• The absorption spectra • The spectrochemical Sequences • High-spin and Low-spin complexes
J 12
1 (1) 2 ( 2 )
r12
2
2
d 1d 2 state 1 state 2 state 3 state 4
1 s (1)
2
2 s(2)
2
r12
(1) * (1) d ( 2 ) * ( 2 ) d , 1 2 * * (1) (1) d 1 ( 2 ) ( 2 ) d 2 , d 1d 2 * * (1) (1) d 1 ( 2 ) ( 2 ) d 2 , (1) * (1) d ( 2 ) * ( 2 ) d , 1 2
e
eh 2mec
eh 4 m e c
9 .2 7 8 1 0
-2 1
erg /g au ss
e=4.803×10-10 esu, ħ=1.055e-27 erg· m=9.109 ×10-28 s, g, c=2.998×1010 cm/s.
The High-Spin and Lower-Spin
[Co(CN)6] 3+
470, 340
310
Blue, ultraviolet
Ultraviolet
Yellow-orange
Pale Yellow
The High-Spin and Lower-Spin
Magnetics Spin Quantum Number Effective Magnetic Moment Paramagnetic susceptibility
D-3K
4 5 6 7
D+6J-6K
6J-3K
2D+10J-10K
2D+15J-10K 2D+21J-11K
10J-4K
15J-6K D+21J-9K
2(D-3K)
2(D-2K) D-2K
For d4 and d5 become low-spin when D > 3K For d6 and d7 become low-spin when D > 2K Therefore, there will be more low-spin complexes with d6 and d7 configurations.
I < Br < S < Cl < F < O < N < C
The Spectrochemical Sequences
Example
Complex Ion Wavelength of Color of Light Color of Complex light absorbed Absorbed [CoF6] 3+ 700 (nm) Red Green
1 s (1)
2
2 s(2)
2
r12
d 1d 2
The High-Spin and Lower-Spin
What is Epairing ?
K 12 1 (1) 2 ( 2 )
* *
1 r12
1 ( 2 ) 2 (1) d 1 d 2
state 1 state 2 state 3 state 4
The High-Spin and Lower-Spin
Magneton
S.I. Unit
e
eh 2me
eh 4 m e
9 .2 7 7 1 0
-2 4
J/T
e=1.602e-19 ×10-10c, ħ=1.055e-34 J· m=9.109 ×10-31 s, kg.
Gaussian Unit
Weak Field:
2: 3T (- 3 d 1g 5
),
3T ( 1 2g 5
),
3A ( 6 2g 5
)
DE (3T1g DE (3T1g
3T ) 2g 3A ) 2g
=(4/5)D =(9/5)D
So the wavelength of the second band should be about half of the first band. That is to say, the frequency should be much higher than 26 000 cm-1. Accordingly, it is assigned to 3T 3T (P), strong Field. 1g 1g
(1) * (1) d ( 2 ) * ( 2 ) d , 1 2 * * (1) (1) d 1 ( 2 ) ( 2 ) d 2 , * * 1 1 s (1) 2 s ( 2 ) 1 s ( 2 ) 2 s (1) d 1 d 2 * * r12 (1) (1) d 1 ( 2 ) ( 2 ) d 2 , * (1) * (1) * d (2) (2)d 2 , 1 1, 1, * * 1 1 s (1) 2 s ( 2 ) 1 s ( 2 ) 2 s (1) d 1 d 2 r12 0, 0, state 1 state 2 state 3 state 4
[Co(C2O4)3] 3+
[Co(H2O)6] 3+ [Co(NH3)6] 3+
600, 420
600, 400 475, 340
Yellow, violet
Yellow, violet Blue, violet
Dark green
Blue-green Yellow-orange
[Co(en)3] 3+
The High-Spin and Lower-Spin
What is Epairing?
eg
t2g
D
The electron occupation depends on two competitive factors a) to stay in the lower orbital t2g. b) to stay in different orbitals with the same spin.
d2 configuration, assignment of absorption spectra Strong Field Weak Field The questions is, which one is more reasonable?
3T 1g 3T 1g 3T (P), 1g 3A , 2g
m eff [ 4 S ( S 1)]
1/ 2
meff
1/ 2
S
e [ n ( n 2 )]
e
e
eh 2me
eh 4 m e
Number of unpaired electrons
Bohr magneton
The High-Spin and Lower-Spin
Magnetics
N 0m
2 e ff
3 kT m 0
N0 : Avogadro’s Number, 6.0221367 x 1023 K : Boltzmann Constant, 1.38066 x 10-23 J· -1 K m0 : Magnetic Permeability of free space 4 x 10-7 J·2 ·C-2 · -1 s m
[VCl3[N(CH3)2]2
d2 High-Spin
C3V or D3h
[Ni(CN)5]3-
பைடு நூலகம்
d8 Low-Spin
Sometimes D3h, sometimes C4V, Depends on its chemical environment
CN CN NC Ni CN CN
NC Ni NC CN CN CN
Absorption spectra of typical transition metal complexes
d-d band
Charge-transfer band
The Absorption Spectra
1S
1A 1 1A 1 1E 3A 2
e2
1E
1G
1T 1 1T 2 1A 1 3T 1 1E 1T 2 3A
High-Spin and Low-Spin
The High-Spin and Lower-Spin
What is Epairing ?
2s 1s State 1
2s 1s State 2
2s
1s
2s
State 3
1s
State 4
The High-Spin and Lower-Spin
What is Epairing ?
The lower band is assigned to 3T1g
3T 2g
However, the higher band is assigned either to 3T 3T (P), Strong Field 1g 1g 3T 3A , Weak Field 1g 2g
The Absorption Spectra
Smaller D Lager D
High spin Lower spin
