ESR spectra of Metal complexes

1
APPLICATIONS OF ESR TO METAL COMPLEXES

• V.SANTHANAM
• DEPARTMENT OF CHEMISTRY
• SCSVMV

2
METAL COMPLEXES A SURVEY

• Metal complexes are important- Diverse biological
  roles
• Griffiths and Owen proved the M-L covalency by
  taking complexes (NH4)2IrCl6 and Na2IrCl6
• The hyperfine splitting by Chloride ligands
  showed the covalent nature of M-L bond

3

• Proved the back donation (pi-bonding) concept
• With the ESR data they were able to calculate ?,?
  and ? of metal ions and the extent of
  delocalization
• In metal complexes the above said parameters were
  having lower values than the free metal ions.

4
THINGS TO BE CONSIDERED

• Nature of the metal
• Number of ligands
• Geometry
• No of d electrons
• Ground term of the ion

5

• Electronic degeneracy
• Inherent magnetic field
• Nature of sample
• Energy gap between g.s and e.s
• Experimental temperature

6
NATURE OF THE METAL ION

• Since d metal ions have 5 d orbitals situations
  are complicated
• But the spectra are informative
• In 4d and 5d series L-S / j-j coupling is strong
  making the ESR hard to interpret

7

• Crystal field is not affecting the 4f and 5f e-
  so the ESR spectra of lanthanides and actinides
  are quite simple.
• If ion contains more than one unpaired e- ZFS may
  be operative

8
GEOMETRY OF THE COMPLEX

• Ligands and their arrangement CFS
• CFS in turn affect the electronic levels hence
  the ESR transitions
• The relative magnitude of CFS and L-S coupling is
  giving three situations.

9

• If the complex ion is having cubic symmetry
  (octahedral or cubic) g is isotropic
• Complexes with at least one axis of symmetry show
  two g values
• Ions with no symmetry element will show three
  values for g.

10
SYSTEM WITH AN AXIS OF SYMMETRY NO SYMMETRY
11

• Symmetry of the complex ion- important why?
• ESR is recorded in frozen solutions
• Spins are locked
• Lack of symmetry influences the applied field
  considerably.

12

• Spin Hamiltonian of an unpaired e- if it is
  present in a cubic field is
• H g ß Hx.Sx Hy.Sy Hz.Sz
• If the system lacks a spherical symmetry and
  possess at least one axis ( Distorted Oh,SP or
  symmetric tops) then
• H ß gxx Hx.Sx gyy Hy.Sy gzz Hz.Sz
• Usually symmetry axis coincides with the Z axis
  and H is applied along Z axis then
• gxx gyy gL gzz g

13

• If crystal axis is not coinciding with Z axis
• The sample is rotated about three mutually
  perpendicular axis and g is measured.
• g is got by one of the following relations
• for rotation about
• X axis - g2 gyy2Cos2? 2gyz2 Cos2? Sin2? gzz2
  Sin2?
• Y axis - g2 gzz2Cos2? 2gzx2 Cos2? Sin2? gxx2
  Sin2?
• Z axis - g2 gyy2Cos2? 2gxy2 Cos2? Sin2? gyy2
  Sin2?

14
NUMBER OF d ELECTRONS

• Magnetically active nucleus cause hyperfine
  splitting.
• If more than one unpaired e- present in the ion,
  more no of transitions possible leads to fine
  structure in ESR spectrum.
• Here we have to consider two things
• Zero field splitting due to dipolar
  interaction
• Kramers degeneracy

15
ZERO FIELD SPLITTING

• Considering a system with two unpaired e-s
• Three combinations possible
• In absence of external field all three states are
  having equal energy
• With external field three levels are no longer
  with same energy.
• Two transitions possible both with same energy

S 1
?E2
S 0
?E1
S -1
H ? 0 ZFS 0
16

• SPLITTING OF ELECTRONIC LEVELS EVEN IN ABSENCE OF
  EXTERNAL MAGNETIC FIELD IS CALLED ZERO FIELD
  SPLITTING (ZFS)
• The splitting may be assisted by distortion and
  L-S coupling also.

17

• When there is a strong dipolar interaction the 1
  level is raised in energy Dipolar shift (D)
• This dipolar shift reduces the gap between S -1
  and S 0 state
• Now the two transitions do not have same energy
• Results in two lines

18

• EFFECT OF DIPOLAR SHIFT

Ms 1
D
Ms 1

?E1 ?E2
Ms 1,0
Ms 0
Ms 0
D
Ms -1
ZFS 0
19
KRAMERS THEOREM

• Systems with even no. of unpaired e-s will
  contain a state with S 0
• But in the case of odd e- s no state with S 0
  since Ms ½
• In such cases even after ZFS the spin states with
  opposite Ms values remain degenerate which is
  called Kramers degeneracy

20

• The levels are called Kramers doublets
• IN ANY SYSTEM WITH ODD NUMBER OF UNPAIRED e-s
  THE ZFS LEAVES THE GROUND STATE AT LEAST TWO FOLD
  DEGENERATE

21
EFFECT OF ZFS ON Mn(II)
5/2
5/2
3/2
6S
1/2

3/2
1/2
- 1/2
- 3/2
FREE ION
ZFS AND RESULTING KRAMERS DOUBLETS
- 5/2
22
CONSEQUENCES OF ZFS

• In some cases ZFS magnitude is very high than the
  splitting by external field.
• Then transitions require very high energy
• Some times only one or no transitions occur.
• Examples V3 and Co2

23
EFFECTIVE SPIN STATE - Co(II)

• Co(II) in cubic field has a ground term of
  4F.Since it is a d8 system it have 3/2 and 1/2
  levels.
• ZFS splits the levels by 200 cm-1
• Since the energy gap is higher only the
  transition -1/2
• to 1/2 is seen.
• So it appears as if Co(II) has only one unpaired
  e- (Effective spin S ½)

24
3/2
3/2
- 3/2
3/2, 1/2
200 cm-1
1/2
1/2
ONLY OBS.TRANSITION
-1/2
25
BREAK DOWN OF SELECTION RULE

• In some cases like V(III) the magnitude of ZFS
  very high.
• It exceeds the normal energy range of ESR
  transitions
• Normal transitions occur with ?Ms 1 . But its
  energy exceeds the microwave region
• Then the transition from -1 to 1 levels with ?Ms
  2 occurs ,which is a forbidden one

26
1
FORBIDDEN TRANSITION
Ms 1
-1
Ms 0, 1
NOT OCURRING
Ms 0
0
27
MIXING OF STATES

• The magnitude of ZFS can be taken as originating
  from CFS.
• But orbitally singlet state 6S is not split by
  the crystal field even then Mn(II) shows a small
  amount of ZFS.
• This is attributed to the mixing of g.s and e.s
  because of L-S coupling

28

• The spin spin interaction is negligible.
• But for triplet states spin spin terms are
  important and they are solely responsible for ZFS
• Naphthalene trapped in durene in diluted state
  shows two lines as if it has ZFS.
• Since there is no crystal field or L-S coupling
  this is attributed to spin spin interaction of
  the pe- s in the excited triplet state

29
ESR AND JAHN-TELLER DISTORTION

• Jahn Teller theorem
• Any non-linear electronically degenerate
  system is unstable, hence it will undergo
  distortion to reduce the symmetry, remove the
  degeneracy and hence increase its stability.
• But this theorem does not predict the type of
  distortion
• Because of J-T distortion the electronic levels
  are split and hence the number of ESR lines may
  increase or decrease.

30
FACTORS AFFECTING THE g-VALUES

• Operating frequency of the instrument
• Concentration of unpaired e-
• Ground term of the metal ion present
• Direction and temperature of measurement
• Lack of symmetry
• Inherent magnetic field in the crystals
• Jahn Teller distortion
• ZFS

31
SUSTAINING EFFECT

• The g value for a gaseous atom or ion for which
  L-S coupling is applicable is given by
• g 1 J(J1) S(S 1) L(L1) / 2J(J1)
• For halogen atoms the g values calculated and
  experimental are equal.
• But for metal ions it varies from 0.2 -8

32

• The reason is the orbital motion of the e- are
  strongly perturbed by the crystal field.
• Hence the L value is partially or completely
  quenched
• In addition to this ZFS and J-T distortion may
  also remove the degeneracy

33

• The spin angular momentum S of e- tries to couple
  with the L
• This partially retains the orbital degeneracy
• The crystal field tries to quench the L value and
  S tries to restore it
• This phenomenon is called sustaining effect

34

• Depending upon which effect dominate the L value
  deviates from the original value
• So L and hence J is not a good quantum number to
  denote the energy of e- hence the g value also

35
COMBINED EFFECT OF CFS AND L-S COUPLING

• Three cases arise depending upon the relative
  magnitudes of strength of crystal field and L-S
  coupling
• L-S coupling gtgtCFS
• CFS gt L-S coupling
• CFS gtgt L-S coupling

36
L-S COUPLING gtgtCFS

• When L is not affected much by CFS, then J is
  useful in determining the g value
• Example rare earth ions
• 4f e- buried inside so not affected, g falls in
  expected region
• All 4f and 5f give agreeing results other than
  Sm(III) and Eu(III)

37
CFS gt gtL-S COUPLING

• If CFS is large enough to break L-S coupling then
  J is not useful in determining g.
• Now the transitions are explained by the
  selection rule and not by g value
• The magnetic moment is given by
• µs n(n2) 1/2

38

• All 3d ions fall in this category.
• Systems with ground terms not affected by CFS ie
  L0 are not affected and the g value is close to
  2.0036
• There may be small deviations because of L-S
  coupling, spin spin interaction and gs and es
  mixing

39
CFS gtgt L-S COUPLING

• In strong fields L-S coupling is completely
  broken and L 0 which means there is covalent
  bonding.
• Applicable to 3d strong field , 4d and 5d series.
• In many cases MOT gives fair details than CFT.

40
Example1 Ni (II) in an Oh field

• For Ni(II) g calculation includes mixing of
  3A2g(g.s) and 3T2g(e.s)
• g 2 8?/10Dq
• For Ni (II) the g value is 2.25 hence 8?/10 Dq
  must be - 0.25
• From the electronic spectrum 10Dq for Ni(II) in
  an Oh field is known to be 8500 cm-1,? is -270
  cm-1

41

• For free Ni(II) ion the ? is about -324 cm-1 the
  decrease is attributed to the e.s ,g.s mixing
• This example shows how ? and 10Dq can affect the
  g value

42
Example2 Cu (II) in a tetragonal field

• Cu (II) a d9 system. Ground term 2D
• 2D 2Eg 2T2g ( CFS)
• Since Cu (II) is a d9 system it must undergo J-T
  distortion.
• So the Oh field becomes tetragonal.

43

• 2T2g 2Eg 2B2g (J-T distortion)
• 2Eg 2B1g 2A1g
• The unpaired e- is present in 2A1g
• on applying the magnetic field the spin levels
  are split and we get an ESR line.

44
Cu (II) in various fields
2Eg
(E3)
2T2g
2B2g
(E2)
2D
2B1g
(E1)
2Eg
1/2
2A1g
ESR
(E0)
- 1/2
Free ion Oh field Tetragonal field H
45

• The g value is given by
• g 2 8 ? / (E2 E0)
• g- 2 2 ? / (E3 E0)
• From electronic spectrum (E2 E0) and (E3 E0)
  can be calculated.
• From the above values ? can be calculated.

46

• It is seen that when splitting by distortion is
  high g value approaches 2
• If the distortion splitting is lower then
  resulting levels may mix with each other to give
  deviated g values.

47
d1 system ( Ti3, VO2)
The 2B2g may be further lowered by L-S coupling
which is not shown.
The energy gap is very less. vibrations mix these
levels so T1 is very low-leading to broad lines
2Eg
2D
2Eg

2T2g
1/2
?E
2B2g
ESR
- 1/2
Free ion Oh field Tetragonal field H
48
d2 systems ( V3 ,Cr4)
3A2g
3A2g
3F
3Eg
1
3T1g
1
3A2g
0
0
- 1
Free ion Oh field J-T Distortion
ZFS H
49
d3 systems ( Cr3)
4T1
4T2
3/2
3/2
4F
1/2
4A2
4 B2
1/2
- 1/2
3/2
Free ion Oh field J-T Distortion ZFS
H
50
d4- system (weak field)
5Eg (10)
5T2g (15)
5B2g (5)
2
5A2g (5)
1
5D (25)
5Eg (10)
2 (2)
1 (2)
5B1g (5)
0 (1)
0
-1
-2
51
THANK YOU