1. Magnetic Properties of Coordination Compounds
Chapter 20

2. Review of the Previous Lecture
Crystal Field Theory
Molecular Orbital Theory

3. Magnetochemistry

4. 1. Magnetism
The term “magnetism” is derived from Magnesia, the name of a region in Asia where Lodestone (pictured below) was mined.
Lodestone: A natural magnetic iron ore.

5. 2. Magnetic property of Coordination Compounds
Depends on:
Metal and oxidation state: d orbital e- configuration
Coordination # of the metal
Coordination field induced by the ligands
In 1845 Michael Faraday noticed that different compounds behaved differently in a magnetic field.

6. The Spectrochemical Series
I- < Br - < [NCS]- < Cl- < F- < [OH]- < [ox]2- ~ H2O < [NCS]- < NH3 < en < [CN]- ~ CO
π donors
σ donors
π acceptors
Weak field ligands
Strong field ligands
Ligands increasing Δoct
Small Δ
High spin
π donors
Large Δ
Low spin
π acceptors

7. 3. Classification of magnetic material
Diamagnetic
Paramagnetic
Ferromagnetic
Anti-ferromagnetic
Ferrimagnetic
Superconductors

8. 3A. Diamagnetism vs Paramagnetism
In an external magnetic field:
Paramagnetic compounds are attracted into the field. The magnetization (M) increases linearly (a) with the strength of the externally applied magnetic constant temperature.
Diamagnetic compounds are slightly repelled and decrease linearly (b) with the strength of the external field.

9. 3A. Diamagnetism vs Paramagnetism
A measure of the magnetism of a material is called the magnetic susceptibility, χ
χ , magnetization of a compound (ability to become a magnet) in an external magnetic field
χ is related to the magnetic moment, μ, of compounds
μ = (χMT)1/2 in Bohr magneton (μB)
χM = Molar χ (cm3/mol)
T = Temp (K)
μB = 9.27 x JT-1 (Joules/Tesla)

10. 3A. Diamagnetism vs Paramagnetism
The magnetic moment can be determined for an atom or ion based on the quantum numbers S (spin) and L (orbital angular momentum).
μ S+L = g ([S(S+1)] + [0.25L(L+1)])0.5
g = 2 μB
For most complexes of the first transition metal series, the spin-only moment is sufficient:
μ S = g ([S(S+1)])0.5 = ([4S(S+1)])0.5 μB = ([n(n+2)])0.5 μB

11. 3A. Diamagnetism vs Paramagnetism
Diamagnetic compounds:
Induced magnetic moment opposes applied magnetic field
χM ~ 10-6 to 10-3 cm3/mol; No temperature dependence
Paramagnetic compounds:
Increase in T, decrease effect of paramagnetism
Curie’s Law: χM = C ; C is Curie’s constant T
Compete with random thermal motion
Paramagnetic
Diamagnetic

12. 3B. Strong vs Weak Paramagnetism
Strong Paramagnetism
Temperature Dependent
Observed for compounds of transition metals
Weak Paramagnetism
Relatively independent of T
Tend to be observed for nonmetallic material
χM for paramagnetic compounds can range from 10-4 to 102 cm3/mol

13. 3C. Spin Crossover
A transition from low to high-spin configuration for d4, d5, d6, and d7 compounds can occur due to:
Change in pressure
Change in temperature
The change in the value of μ may be gradual or abrupt.

14. Temperature-dependent spin crossover
Fe(II) complexes; d6
[Fe(btz)2(NCS-N)2]
[Fe(phen)2(NCS-N)2] eg
t2g
S = 2
Slow
change
Abrupt change eg
S = 0
t2g

15. 3D. Coupled Interactions
When paramagnetic species are very close together or are separated by a species (i.e. ligand) that can transmit magnetic interactions, the metal centers may interact (couple) with one another. Think about these compounds as having magnetic domains.
Typical paramagnetic species
Ferromagnetic species

16. 3DI. Ferromagnets Ferromagnetic species Ferromagnetic species
Unpaired e- align to parallel to each other in the absence of an external magnetic field in large regions known as magnetic domains. These domains are randomly oriented giving
Net μ = 0
Application of external field leads to the alignment of the domains and greatly enhanced paramagnetism
Ferromagnetic species
Ferromagnetic species
in external field

17. 3DI. Ferromagnets
The enhanced paramagnetism is felt up to the Curie temp, Tc
χM = C
T – θ θ is the Weiss constant
Above Tc, thermal energy overcomes the alignment and normal paramagnetic behavior prevails
At absolute zero, alignment is complete and spontaneous. Magnetization has its largest possible value.

18. 3DII. Ferrimagnets Ferrimagnetic species in external field
Have magnetic domains where some moments are systematically aligned to oppose others but there is a net resultant magnetic moment in the presense of an external field.
Ferrimagnetic species
in external field

19. Magnetite (Fe3O4) is ferrimagnetic
Fe(III) in Td site
Fe(III)2Fe(II)O4
Fe(III)
in Oh site
Fe(II)
in Oh site
Net S = 2
"Fe3O4ferrimagnetism" by Tem5psu - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons -

20. 3DIII. Antiferromagnets
Néel proposed this concept (1930s)
Interacting magnetic dipoles on neighboring atoms tend to assume an antiparallel arrangement.
Net μ = 0
Antiferromagnetic species

21. 3DIII. Antiferromagnets
Antiferromagnetism occurs below the Néel temp, TN
Above TN, normal paramagnetism

22. 4. Measuring magnetism (or detecting changes)
Gouy method
Sample suspended in homogenous magnetic field
Sample weighed in and out of field
Weight difference related to χ and field strength
B. Faraday method
Sample suspended in inhomogenous field
Has gradient (∫H/∫χ)
Compare standard and sample
Small sample size required

23. 4. Measuring magnetism (or detecting changes)
C. NMR method (Evan’s)
Paramagnetic sample measured along with reference diamagnetic (inert sample)
Paramagnetic sample will shift the NMR signal of the reference to higher frequency
Shift related to difference in χ
D. SQUID (Superconducting Quantum Interference Devices)
E. Electron paramagnetic resonance
F. Mössbauer spectroscopy