Magnetic Properties ofCoordination Compounds
Chapter 20
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Review of the Previous Lecture
1. Discussed the two sources of electronic spectroscopy in coordination compounds
2. Reviewed the rules that govern electronic excitation Correlated extinction coefficient (ϵ) with how “allowed” an electronic transition is
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Magnetochemistry
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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.
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2. Magnetic property of Coordination Compounds
Depends on:
a. Metal and oxidation state: d orbital e- configuration
b. Coordination # of the metal
c. Coordination field induced by the ligands
In 1845 Michael Faraday noticed that different compounds behaved differently in amagnetic field.
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3. Classification of magnetic material
Diamagnetic
Paramagnetic
Ferromagnetic
Anti-ferromagnetic
Ferrimagnetic
Superconductors
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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 field @constant temperature.
Diamagnetic compounds are slightly repelled and decrease linearly (b) with thestrength of the external field.
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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 externalmagnetic field
χ is related to the magnetic moment, μ, of compounds
μ = 2.828 (χMT)1/2 in Bohr magneton (μB)
χM = Molar χ (cm3/mol)
T = Temp (K)
μB = 9.27 x 10-24 JT-1 (Joules/Tesla)
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3A. Diamagnetism vs Paramagnetism
The magnetic moment can be determined for an atom or ion based on the quantumnumbers 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
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3A. Diamagnetism vs Paramagnetism
Paramagnetic
Diamagnetic
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 constantT
Compete with random thermal motion
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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
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3C. Spin Crossover
A transition from low to high-spin configuration for d4, d5, d6, and d7 compounds canoccur due to:
Change in pressure
Change in temperature
The change in the value of μ may be gradual or abrupt.
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Temperature-dependent spin crossover
[Fe(phen)2(NCS-N)2][Fe(btz)2(NCS-N)2]
Abrupt changeSlowchangeS = 0
S = 2
Fe(II) complexes; d6
t2g
eg
t2g
eg
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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 oneanother. Think about these compounds as having magnetic domains.
Typical paramagnetic species Ferromagnetic species
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3DI. Ferromagnets
Unpaired e- align to parallel to each other in the absence of an external magnetic field inlarge 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 enhancedparamagnetism
Ferromagnetic species Ferromagnetic speciesin external field
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3DI. Ferromagnets
The enhanced paramagnetism is felt up to the Curie temp,Tc
χM = CT – θ θ is the Weiss constant
Above Tc, thermal energy overcomes the alignment andnormal paramagnetic behavior prevails
At absolute zero, alignment is complete and spontaneous.Magnetization has its largest possible value.
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3DII. Ferrimagnets
Have magnetic domains where some moments are systematically aligned to oppose othersbut there is a net resultant magnetic moment in the presense of an external field.
Ferrimagnetic speciesin external field
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Magnetite (Fe3O4) is ferrimagnetic
"Fe3O4ferrimagnetism" by Tem5psu - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons -http://commons.wikimedia.org/wiki/File:Fe3O4ferrimagnetism.png#/media/File:Fe3O4ferrimagnetism.png
Fe(III)2Fe(II)O4
Fe(III)in Oh site
Fe(III) in Td site
Fe(II)in Oh site
Net S = 2
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3DIII. Antiferromagnets
Néel proposed this concept (1930s)
Interacting magnetic dipoles on neighboring atoms tend to assume an antiparallelarrangement.
Net μ = 0
Antiferromagnetic species
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3DIII. Antiferromagnets
Antiferromagnetism occurs below the Néel temp, TN
Above TN, normal paramagnetism
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4. Measuring magnetism (or detecting changes)
A. 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
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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 tohigher frequency
Shift related to difference in χ
D. SQUID (Superconducting Quantum Interference Devices)
E. Electron paramagnetic resonance
F. Mössbauer spectroscopy