Magnetic Properties ofCoordination Compounds

Chapter 20

Magnetic Properties ofCoordination Compounds

Chapter 20

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Review of the Previous Lecture

1. Discussed different bonding theories to explore metal ligand bonding in coordinationcompounds

2. Explored electronic transitions in coordination compounds using the differentbonding theories

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In 1845 Michael Faraday noticed that different compounds behaved differently in amagnetic field.

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Magnetochemistry

We will have to wait to see how the MCU reboots the Xmen.

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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 the d orbital electron configuration as defined by:

a. Metal and oxidation state

b. Coordination # of the metal

c. Coordination field induced by the ligands (Spectrochemical series)

I- < Br - < [NCS]- < Cl- < F- < [OH]- < [ox]2- ~ H2O < [NCS]- < NH3 < en < [CN]- ~ CO

Weak field ligands Strong field ligandsLigands increasing Δoct

Small Δ High spin π donors

Large Δ Low spin π acceptors

σ donors π donors π acceptors

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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)])0.5μB = ([n(n+2)])0.5μB

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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- Compete with random thermal motion

Curie’s Law: χM = C ; C is Curie’s constantT

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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 electrons align 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 an external field leads to the alignment of the domains and greatly enhanced

paramagnetism

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.

The temperature dependence of the magnetic susceptibility of ferrimagnets exhibits a profilecomparable to a ferromagnet but less pronounced.

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

S = 5/2 S = -5/2 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 diamagnetic reference(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 (very well-defined for Fe-based compounds)