10.2 Magnetic Properties of Coordination Compounds

Coordination compounds showcase fascinating magnetic properties, ranging from paramagnetism to diamagnetism. These behaviors stem from unpaired electrons and their interactions with external magnetic fields, influencing the compound's magnetic susceptibility and moment.

Spin states play a crucial role in determining a complex's magnetic properties. High-spin and low-spin configurations, influenced by crystal field splitting, affect the number of unpaired electrons and, consequently, the magnetic behavior of coordination compounds.

Magnetic Behavior of Coordination Compounds

Types of Magnetic Behavior

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• Paramagnetism occurs when unpaired electrons in a material align with an external magnetic field
  • Results in a weak attraction to the magnetic field
  • Paramagnetic materials have a positive magnetic susceptibility
• Diamagnetism arises from the interaction of paired electrons with an external magnetic field
  • Causes a weak repulsion from the magnetic field
  • Diamagnetic materials have a negative magnetic susceptibility
• Magnetic susceptibility measures the degree of magnetization of a material in response to an applied magnetic field
  • Expressed as the ratio of magnetization to the strength of the applied field
  • Can be positive (paramagnetic) or negative (diamagnetic)

Temperature-Dependent Magnetic Phenomena

• Temperature-independent paramagnetism (TIP) manifests as a weak paramagnetic effect that does not vary with temperature
  • Observed in some transition metal complexes
  • Arises from mixing of ground and excited electronic states
• Curie law describes the relationship between magnetic susceptibility and temperature for paramagnetic materials
  • States that magnetic susceptibility is inversely proportional to temperature
  • Expressed mathematically as: $χ = C/T$
    • χ represents magnetic susceptibility
    • C denotes the Curie constant
    • T stands for absolute temperature

Spin States and Magnetic Moments

Fundamental Concepts of Magnetic Moments

• Spin-only magnetic moment calculates the magnetic moment considering only the spin angular momentum of unpaired electrons
  • Expressed as: $μ_s = √[n(n+2)] μ_B$
    • n represents the number of unpaired electrons
    • μ_B denotes the Bohr magneton
• Effective magnetic moment accounts for both spin and orbital contributions to the magnetic moment
  • Generally larger than the spin-only magnetic moment
  • Measured experimentally and compared to theoretical calculations

Spin Configurations in Coordination Complexes

• High-spin complexes form when the crystal field splitting energy is smaller than the electron pairing energy
  • Electrons occupy all available d orbitals before pairing
  • Results in a maximum number of unpaired electrons
  • Often observed in octahedral complexes with weak-field ligands (Cl⁻, H₂O)
• Low-spin complexes occur when the crystal field splitting energy exceeds the electron pairing energy
  • Electrons pair in lower-energy d orbitals before occupying higher-energy orbitals
  • Results in a minimum number of unpaired electrons
  • Commonly seen in octahedral complexes with strong-field ligands (CN⁻, CO)
• Spin crossover phenomenon involves the transition between high-spin and low-spin states
  • Can be induced by changes in temperature, pressure, or light
  • Observed in some iron(II) complexes (Fe²⁺)

Magnetic Ordering

Types of Magnetic Ordering

• Antiferromagnetism occurs when neighboring magnetic moments align in opposite directions
  • Results in zero net magnetization in the absence of an external field
  • Observed in materials like manganese oxide (MnO)
  • Characterized by a critical temperature called the Néel temperature
• Ferromagnetism arises when magnetic moments align parallel to each other
  • Produces a strong net magnetization even in the absence of an external field
  • Found in materials like iron (Fe), cobalt (Co), and nickel (Ni)
  • Exhibits a critical temperature known as the Curie temperature

Characteristics of Magnetic Ordering

• Both antiferromagnetism and ferromagnetism involve cooperative interactions between magnetic moments
• These phenomena typically occur at low temperatures and disappear above their respective critical temperatures
• Magnetic ordering can significantly influence the physical and chemical properties of materials

Advanced Magnetic Properties

Complex Magnetic Interactions

• Spin-orbit coupling describes the interaction between an electron's spin and its orbital angular momentum
  • Affects the magnetic properties of transition metal complexes
  • Can lead to deviations from spin-only magnetic moment predictions
  • Becomes more significant for heavier elements (lanthanides and actinides)

Experimental Techniques

• Gouy balance measures the magnetic susceptibility of materials
  • Utilizes the force experienced by a sample in an inhomogeneous magnetic field
  • Sample is placed in a glass tube suspended between the poles of an electromagnet
  • Change in the apparent weight of the sample is used to calculate magnetic susceptibility
  • Suitable for both solid and liquid samples