Coordinate Compounds - Magnetic Moment

Definition: Magnetic moment arises from the movement of electrons in an atom or ion.
The magnetic moment in coordination compounds arises due to the presence of unpaired electrons.
It is an important property in determining the nature of coordination compounds.

Factors affecting Magnetic Moment

The magnetic moment is influenced by the number of unpaired electrons present.
The presence of unpaired electrons makes the compound paramagnetic.
If all electrons are paired, the compound is diamagnetic.
Magnetic moment is also influenced by the spin of electrons.

Calculation of Magnetic Moment

The magnetic moment (μ) can be calculated using the formula: μ = √(n(n+2))BM
n is the number of unpaired electrons and BM is the Bohr magneton.
Example: If a coordination compound has 2 unpaired electrons, the magnetic moment would be μ = √(2(2+2))BM = √(8)BM.

Classification of Coordination Compounds based on Magnetic Moment

Low Spin Complex: Have a small number of unpaired electrons.
High Spin Complex: Have a large number of unpaired electrons.
Spin Crossover Complex: Exhibits change in spin with temperature or external stimulus.

Examples of Low Spin Complexes

[Fe(CN)6]4-: Iron(II) hexacyanoferrate(II) has 0 unpaired electrons, making it diamagnetic.
[Co(NH3)6]3+: Cobalt(III) hexaammine has 3 unpaired electrons, making it paramagnetic.

Examples of High Spin Complexes

[Fe(H2O)6]3+: Iron(III) hexaaqua complex has 5 unpaired electrons, making it paramagnetic.
[Ni(CN)4]2-: Nickel(II) tetracyano complex has 2 unpaired electrons, making it paramagnetic.

Examples of Spin Crossover Complexes

[Fe(H2O)6]2+: Iron(II) hexaaqua complex undergoes a spin change with temperature, showing both paramagnetic and diamagnetic behavior.

Magnetic Moment and Ligand Field Theory

Ligands have an effect on the magnetic properties of coordination compounds.
Strong field ligands result in low spin complexes with fewer unpaired electrons.
Weak field ligands result in high spin complexes with more unpaired electrons.

Factors Affecting Ligand Field Strength

Charge: Ligands with higher charge have a stronger ligand field.
Size: Smaller ligands generate a stronger ligand field.
Electronic Configuration: Ligands with a majority of nonbonding electrons generate a stronger ligand field.

Examples of Strong and Weak Field Ligands

Strong Field Ligands: CN-, CO, NO2-, SCN-
Weak Field Ligands: F-, Cl-, OH-, H2O

Coordination Compounds - Magnetic Moment

Magnetic moment is an important property of coordination compounds.
It arises due to the presence of unpaired electrons.
It is influenced by factors such as the number of unpaired electrons and the spin of electrons.
The magnetic moment can be calculated using the formula: μ = √(n(n+2))BM.
It can be used to classify coordination compounds into low spin, high spin, and spin crossover complexes.

Factors Affecting Magnetic Moment

Number of Unpaired Electrons: Determines the magnitude of the magnetic moment.
Spin of Electrons: Results in different orientations and contributes to the total magnetic moment.
Presence of Unpaired Electrons: Makes the compound paramagnetic.
Absence of Unpaired Electrons: Makes the compound diamagnetic.

Calculation of Magnetic Moment

Magnetic moment (μ) can be calculated using the formula: μ = √(n(n+2))BM.
Where n is the number of unpaired electrons and BM is the Bohr magneton constant.
Example: A complex with 4 unpaired electrons will have a magnetic moment of μ = √(4(4+2))BM = √(24)BM.

Classification of Coordination Compounds based on Magnetic Moment

Low Spin Complex: Has a small number of unpaired electrons.
High Spin Complex: Has a large number of unpaired electrons.
Spin Crossover Complex: Exhibits a change in spin with temperature or external stimulus.
Magnetic moment provides information about the nature of the coordination compound.

Examples of Low Spin Complexes

[Fe(CN)6]4-: Iron(II) hexacyanoferrate(II) has 0 unpaired electrons, making it diamagnetic.
[Co(NH3)6]3+: Cobalt(III) hexaammine complex has 3 unpaired electrons, making it paramagnetic.
These complexes demonstrate different magnetic behaviors due to the presence of unpaired electrons.

Examples of High Spin Complexes

[Fe(H2O)6]3+: Iron(III) hexaaqua complex has 5 unpaired electrons, making it paramagnetic.
[Ni(CN)4]2-: Nickel(II) tetracyano complex has 2 unpaired electrons, making it paramagnetic.
The presence of unpaired electrons in these complexes enhances their paramagnetic properties.

Examples of Spin Crossover Complexes

[Fe(H2O)6]2+: Iron(II) hexaaqua complex undergoes a spin change with temperature.
It exhibits both paramagnetic and diamagnetic behavior depending on the temperature.
This spin crossover behavior is observed in certain coordination compounds.

Magnetic Moment and Ligand Field Theory

Ligands have an impact on the magnetic properties of coordination compounds.
Strong field ligands result in low spin complexes with fewer unpaired electrons.
Weak field ligands result in high spin complexes with more unpaired electrons.
Ligand field strength is influenced by charge, size, and electronic configuration of the ligand.

Factors Affecting Ligand Field Strength

Charge: Ligands with higher charge generate a stronger ligand field.
Size: Smaller ligands generate a stronger ligand field.
Electronic Configuration: Ligands with a majority of nonbonding electrons generate a stronger ligand field.
Ligand field strength affects the magnetic properties of coordination compounds.

Examples of Strong and Weak Field Ligands

Strong Field Ligands: CN-, CO, NO2-, SCN-
- These ligands generate a strong ligand field and result in low spin complexes.
Weak Field Ligands: F-, Cl-, OH-, H2O
- These ligands generate a weak ligand field and result in high spin complexes.
The choice of ligands can affect the magnetic behavior of coordination compounds.

Spin-Only Formula (S.O.F)

The spin-only formula is a simplified way to calculate the magnetic moment of a coordination compound.
It is given by the formula: μ = √(n(n+2)) BM, where n is the number of unpaired electrons and BM is the Bohr magneton.
The spin-only formula assumes that all electrons have the same spin and neglects orbital angular momentum contributions.

Example Calculation using Spin-Only Formula

Consider the complex [Fe(CN)6]4- with iron in the +2 oxidation state.
Iron(II) has 6 d electrons, and CN- is a strong field ligand.
Using ligand field theory, we find that it has all electrons paired.
Thus, the number of unpaired electrons (n) is 0.
Plugging the values into the spin-only formula, we get μ = √(0(0+2))BM = 0 BM.

Effect of Temperature on Magnetic Moment

The magnetic moment of a coordination compound can change with temperature.
At low temperatures, most complexes are in the low spin state due to the energy required to overcome the ligand field.
As the temperature increases, there is a transition from low spin to high spin state due to thermal energy.
This phenomenon is known as spin crossover.

Factors Affecting Spin Crossover Temperature

Ligand Field Strength: Stronger ligand field ligands result in higher spin crossover temperatures.
Geometric Factors: Different geometries can affect the spin crossover temperature.
Other Factors: Pressure, external magnetic field, and solvent can also influence the spin crossover temperature.

Example of Spin Crossover Complex

[Fe(H2O)6]2+: Iron(II) hexaaqua complex undergoes a spin crossover transition at a specific temperature.
Below the spin crossover temperature, it is in the low spin state with 0 unpaired electrons.
Above the spin crossover temperature, it is in the high spin state with 4 unpaired electrons.
This transition can be observed by changes in magnetic properties.

Magnetic Susceptibility

Magnetic susceptibility (χ) is a measure of a material’s ability to become magnetized in an applied magnetic field.
For paramagnetic compounds, χ > 0; for diamagnetic compounds, χ < 0.
Magnetic susceptibility is related to the magnetic moment by the Curie’s Law: χ = C/T, where C is the Curie constant and T is the temperature.

Magnetic Susceptibility and Temperature

The magnetic susceptibility of a material changes with temperature due to the thermal energy overcoming magnetic ordering.
For paramagnetic materials, the susceptibility increases with increasing temperature.
For diamagnetic materials, the susceptibility decreases with increasing temperature.

Curie’s Law

Curie’s Law states that the magnetic susceptibility (χ) of a paramagnetic material is inversely proportional to the temperature (T).
Mathematically, it can be written as: χ = C/T, where C is the Curie constant.
This law holds for materials that follow the Curie-Weiss behavior and have a high temperature paramagnetic phase.

Examples of Curie’s Law

Consider a paramagnetic material with a Curie constant (C) equal to 1.
At T = 300K, the magnetic susceptibility (χ) will be 1/300 = 0.0033.
At T = 500K, the magnetic susceptibility (χ) will be 1/500 = 0.0020.
Thus, as the temperature increases, the magnetic susceptibility decreases according to Curie’s Law.

Summary

Magnetic moment is a property of coordination compounds determined by the presence of unpaired electrons.
The magnetic moment can be calculated using the spin-only formula: μ = √(n(n+2))BM.
Low spin complexes have fewer unpaired electrons, while high spin complexes have more unpaired electrons.
Spin crossover complexes demonstrate a change in spin with temperature.
Ligand field strength and other factors influence the magnetic properties of coordination compounds.
Magnetic susceptibility and Curie’s Law explain the behavior of paramagnetic and diamagnetic materials.