🧶Inorganic Chemistry I Unit 10 – Coordination Compounds: Spectra & Magnetism

Key Concepts

• Coordination compounds consist of a central metal atom or ion surrounded by ligands, which are ions or molecules that donate electron pairs to the metal
• Electronic spectra of complexes arise from electronic transitions between different energy levels of the d or f orbitals of the metal ion
• Crystal field theory explains the splitting of d orbitals in transition metal complexes due to the electrostatic field generated by the ligands
• Magnetic properties of coordination compounds depend on the number of unpaired electrons in the d orbitals of the metal ion
• Spectroscopic techniques, such as UV-visible and electron paramagnetic resonance (EPR) spectroscopy, provide information about the electronic structure and magnetic properties of complexes
• Applications of coordination compounds include catalysis, bioinorganic chemistry, and materials science
• Common pitfalls in understanding coordination chemistry include confusing crystal field theory with molecular orbital theory and neglecting the role of spin-orbit coupling in electronic spectra

Coordination Compound Basics

• Coordination compounds have a central metal atom or ion bonded to surrounding ligands, which can be neutral molecules (H2O, NH3) or anions (Cl-, CN-)
• The coordination number of the metal ion represents the number of ligands directly bonded to it, with common coordination numbers being 4 (tetrahedral or square planar) and 6 (octahedral)
• Ligands are classified as monodentate (one donor atom), bidentate (two donor atoms), or polydentate (multiple donor atoms) based on the number of atoms that bond to the metal
• The geometry of the complex depends on the coordination number and the nature of the ligands, with common geometries being tetrahedral, square planar, and octahedral
• Isomerism in coordination compounds can occur in various forms:
  • Structural isomers have the same formula but different bonding arrangements (linkage isomers, coordination isomers)
  • Stereoisomers have the same bonding arrangement but different spatial orientations (geometric isomers, optical isomers)
• The stability of coordination compounds is determined by the metal-ligand bond strength, which is influenced by factors such as the size and charge of the metal ion, the basicity of the ligands, and the chelate effect

Electronic Spectra of Complexes

• Electronic spectra of complexes result from transitions between different electronic energy levels of the metal ion, primarily involving d or f orbitals
• The absorption of light in the visible or ultraviolet region causes electrons to be promoted from lower to higher energy levels, resulting in characteristic absorption bands
• The energy and intensity of the absorption bands depend on the metal ion, its oxidation state, and the nature of the ligands
• Selection rules govern the allowed and forbidden electronic transitions based on the symmetry and spin of the orbitals involved
  • Laporte selection rule: Transitions between orbitals of the same parity (g → g or u → u) are forbidden, while transitions between orbitals of different parity (g → u or u → g) are allowed
  • Spin selection rule: Transitions between states with different spin multiplicities are forbidden (ΔS = 0)
• The spectrochemical series ranks ligands based on their ability to split the d orbitals of the metal ion, with strong-field ligands (CN-, CO) causing a larger splitting than weak-field ligands (I-, Br-)
• Charge transfer bands in the electronic spectra arise from transitions between molecular orbitals involving both the metal and the ligands, often resulting in intense absorption bands

Crystal Field Theory

• Crystal field theory (CFT) describes the splitting of d orbitals in transition metal complexes due to the electrostatic field generated by the ligands
• The five d orbitals are split into two sets of orbitals with different energies: the eg set (dx2-y2, dz2) and the t2g set (dxy, dxz, dyz)
• The magnitude of the d orbital splitting (Δ) depends on the strength of the ligand field, with strong-field ligands causing a larger splitting than weak-field ligands
• The electron configuration of the metal ion in the complex determines the ground state term symbol, which describes the overall spin and orbital angular momentum of the system
• High-spin and low-spin complexes arise from the relative magnitude of the d orbital splitting compared to the pairing energy:
  • In high-spin complexes, the d orbital splitting is smaller than the pairing energy, resulting in the maximum number of unpaired electrons
  • In low-spin complexes, the d orbital splitting is larger than the pairing energy, resulting in paired electrons in the lower energy orbitals
• The spectrochemical series and the spin state of the complex influence the color and magnetic properties of the compound

Magnetic Properties

• Magnetic properties of coordination compounds depend on the number of unpaired electrons in the d orbitals of the metal ion
• Diamagnetic compounds have no unpaired electrons and are weakly repelled by a magnetic field, while paramagnetic compounds have one or more unpaired electrons and are attracted to a magnetic field
• The magnetic moment (μ) of a complex is related to the number of unpaired electrons (n) by the spin-only formula: $μ = \sqrt{n(n+2)}$ Bohr magnetons (BM)
• The experimental magnetic moment can be determined using techniques such as the Gouy method or the Faraday method
• Curie's law describes the inverse relationship between the magnetic susceptibility (χ) and the temperature (T) for paramagnetic compounds: $χ = \frac{C}{T}$, where C is the Curie constant
• Deviations from the spin-only formula can occur due to orbital contribution to the magnetic moment, which is more significant for complexes with heavy metal ions or low symmetry
• Magnetic properties can provide information about the oxidation state and the geometry of the complex, as well as the presence of magnetic interactions between metal centers in polynuclear complexes

Spectroscopic Techniques

• UV-visible spectroscopy measures the absorption of light in the ultraviolet and visible regions, providing information about the electronic transitions and the color of the complex
  • The position and intensity of the absorption bands depend on the metal ion, its oxidation state, and the nature of the ligands
  • The molar absorptivity (ε) is a measure of the intensity of the absorption band and is related to the oscillator strength of the transition
• Electron paramagnetic resonance (EPR) spectroscopy detects the transitions between the energy levels of unpaired electrons in the presence of an external magnetic field
  • EPR is sensitive to the local environment of the unpaired electrons and can provide information about the oxidation state, coordination geometry, and the nature of the ligands
  • The g-factor, hyperfine coupling constants, and zero-field splitting parameters can be extracted from the EPR spectrum and used to characterize the complex
• Mössbauer spectroscopy is a nuclear spectroscopic technique that measures the recoilless absorption and emission of gamma rays by certain nuclei (e.g., 57Fe) in a solid sample
  • Mössbauer spectroscopy provides information about the oxidation state, coordination environment, and electronic structure of the metal ion
  • The isomer shift and quadrupole splitting parameters are sensitive to the electron density and the electric field gradient at the nucleus, respectively
• Magnetic circular dichroism (MCD) spectroscopy measures the differential absorption of left and right circularly polarized light in the presence of a magnetic field
  • MCD is sensitive to the magnetic properties and the electronic structure of the complex and can provide information complementary to UV-visible and EPR spectroscopy

Applications and Examples

• Hemoglobin, a tetrameric protein containing iron(II) porphyrin complexes, is responsible for oxygen transport in the blood
  • The binding of oxygen to the iron center causes a change in the electronic structure and the geometry of the complex, resulting in a color change from purple (deoxyhemoglobin) to red (oxyhemoglobin)
• Chlorophyll, a magnesium porphyrin complex, is the primary pigment in photosynthesis, absorbing light in the visible region and converting it into chemical energy
• Vitamin B12, a cobalt(III) corrin complex, is a coenzyme involved in various metabolic reactions, such as the synthesis of red blood cells and the maintenance of the nervous system
• Cisplatin, a square planar platinum(II) complex, is a widely used anticancer drug that binds to DNA and induces apoptosis in cancer cells
• Transition metal complexes are used as catalysts in various industrial processes, such as the Haber-Bosch process for ammonia synthesis (iron catalyst) and the Monsanto process for acetic acid production (rhodium catalyst)
• Magnetic resonance imaging (MRI) contrast agents, such as gadolinium(III) complexes, enhance the contrast between different tissues by altering the relaxation times of water protons in the body
• Dye-sensitized solar cells (DSSCs) use ruthenium(II) polypyridyl complexes as photosensitizers to convert sunlight into electrical energy

Common Pitfalls and Tips

• Confusing crystal field theory with molecular orbital theory: CFT is a purely electrostatic model that considers only the d orbitals of the metal ion, while MOT takes into account the overlap and mixing of metal and ligand orbitals
• Neglecting the role of spin-orbit coupling in electronic spectra: Spin-orbit coupling can cause deviations from the spin selection rule and lead to additional transitions and splitting of the energy levels
• Forgetting to consider the symmetry of the complex when assigning the electronic transitions: The symmetry of the complex determines the allowed and forbidden transitions based on the Laporte selection rule
• Misinterpreting the magnetic properties of the complex: The experimental magnetic moment can be affected by factors such as temperature, diamagnetic corrections, and the presence of impurities
• Overlooking the possibility of charge transfer transitions in the electronic spectra: Charge transfer bands can be intense and may overlap with the d-d transitions, complicating the interpretation of the spectra
• Assuming that the color of the complex is solely determined by the d-d transitions: The color of the complex can also be influenced by charge transfer transitions and the presence of other chromophores in the ligands
• Relying too heavily on the spectrochemical series without considering the specific metal ion and its oxidation state: The relative strength of the ligands in the spectrochemical series can vary depending on the metal ion and its oxidation state