Magnetic Properties of Coordination Compounds Chapter 20

Review of the Previous Lecture Crystal Field Theory Molecular Orbital Theory

Magnetochemistry

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.

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.

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

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

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 the strength of the external field.

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 = 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)

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

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

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

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.

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

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

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

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.

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

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 - http://commons.wikimedia.org/wiki/File:Fe3O4ferrimagnetism.png#/media/File:Fe3O4ferrimagnetism.png

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

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

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

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