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19.3 Spectroscopic and Magnetic Properties of Coordination Compounds

3 min read•june 25, 2024

explains how metal ions interact with surrounding ligands in . It's crucial for understanding why these compounds have specific colors, magnetic properties, and reactivities.

The theory focuses on how ligands split the metal's d-orbitals into different energy levels. This splitting affects electron distribution, determining whether a complex is high-spin or low-spin, which impacts its magnetic behavior and color.

Crystal Field Theory and Coordination Compounds

Principles of crystal field theory

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  • (CFT) models the electronic structure and properties of by focusing on the interaction between the metal ion and ligands
  • Ligands are treated as point charges that create an electrostatic field around the metal ion, interacting with its and causing them to split into different energy levels
  • CFT explains various properties of coordination compounds such as color, absorption spectra, magnetic behavior, stability, and reactivity

Geometry and d-orbital splitting

  • The geometry of a coordination compound determines the splitting pattern of the metal ion's d orbitals
  • have six ligands arranged around the metal ion, splitting the d orbitals into two energy levels: (higher energy) and (lower energy) with an energy difference of Δo\Delta_o
  • have four ligands arranged around the metal ion, splitting the d orbitals into (lower energy) and (higher energy) with an energy difference of Δt\Delta_t, which is approximately 4/9 of Δo\Delta_o for the same metal ion and ligands
  • have four ligands arranged around the metal ion, with a d orbital splitting similar to octahedral complexes but with a larger energy difference between the split levels
  • The can cause further distortion of the geometry, leading to additional splitting of d orbitals in certain complexes

Electron distribution in d orbitals

  • The electron distribution in split d orbitals depends on the number of d electrons in the metal ion and the strength of the ligand field
  • have a weak ligand field (smaller Δo\Delta_o or Δt\Delta_t), and electrons occupy orbitals according to , maximizing the number of unpaired electrons
  • have a strong ligand field (larger Δo\Delta_o or Δt\Delta_t), and electrons pair up in lower energy orbitals before occupying higher energy orbitals
  • Examples of electron distribution in octahedral complexes:
    1. d3d^3 (): t2g3t_{2g}^3 (high-spin) or t2g3t_{2g}^3 (low-spin)
    2. d6d^6 (): t2g4eg2t_{2g}^4 e_g^2 (high-spin) or t2g6t_{2g}^6 (low-spin)
    3. d8d^8 (): t2g6eg2t_{2g}^6 e_g^2 (high-spin) or t2g6eg2t_{2g}^6 e_g^2 (low-spin)

Spectroscopic and Magnetic Properties

Properties of coordination compounds

  • Color and absorption spectra:
    • The color of a coordination compound is determined by the energy of the absorbed light when an electron is promoted from a lower energy d orbital to a higher energy d orbital
    • The wavelength of the absorbed light corresponds to the energy difference between the split d orbitals (Δo\Delta_o or Δt\Delta_t)
    • Complexes with larger Δo\Delta_o or Δt\Delta_t absorb higher energy (shorter wavelength) light and appear blue or green, while complexes with smaller Δo\Delta_o or Δt\Delta_t absorb lower energy (longer wavelength) light and appear red or yellow
    • in coordination compounds are governed by , which determine the allowed transitions based on symmetry and spin considerations
  • Magnetic properties:
    • The magnetic behavior of a coordination compound depends on the number of unpaired electrons in the split d orbitals
    • complexes have unpaired electrons and are attracted to magnetic fields, while complexes have no unpaired electrons and are weakly repelled by magnetic fields
    • High-spin complexes have more unpaired electrons and are more paramagnetic than
    • The magnetic moment (μ\mu) of a complex can be calculated using the spin-only formula: μ=n(n+2)\mu = \sqrt{n(n+2)} μB\mu_B, where nn is the number of unpaired electrons and μB\mu_B is the
    • Some complexes can exhibit , where they transition between high-spin and low-spin states under certain conditions
  • ranks ligands based on their ability to split d orbitals (ligand field strength), with (, ) causing a larger splitting and favoring low-spin complexes, while (, ) cause a smaller splitting and favor high-spin complexes

Advanced Theoretical Approaches

  • extends crystal field theory by incorporating molecular orbital theory to provide a more comprehensive understanding of bonding and electronic structure in coordination compounds

Key Terms to Review (42)

$ ext{Delta}_t$: $ ext{Delta}_t$ is a change in time, often used in the context of spectroscopic and magnetic properties of coordination compounds. It represents the time interval or duration of a specific event or process within these systems.
$ abla_o$: $ abla_o$ is a key term in the context of spectroscopic and magnetic properties of coordination compounds. It represents the energy difference between the ground state and the first excited state of a transition metal complex, which is a crucial factor in determining the complex's electronic and magnetic behavior.
$e_g$: $e_g$ is the energy of the ground state electronic configuration of an atom or ion. It represents the lowest possible energy level that an electron can occupy in the electronic structure of a particular system. This term is crucial in understanding the spectroscopic and magnetic properties of coordination compounds, as the ground state energy plays a key role in determining the behavior and characteristics of these materials.
$e$: $e$ is a mathematical constant that is the base of the natural logarithm. It is an irrational number, meaning its decimal representation never repeats or terminates, and it is approximately equal to 2.71828. $e$ is a fundamental constant that appears in many areas of mathematics, science, and engineering, including the fields of spectroscopy and magnetism in coordination compounds.
$t_{2g}$: $t_{2g}$ refers to the set of three degenerate d-orbitals in a transition metal complex that have lower energy compared to the $e_{g}$ set of d-orbitals. This term is particularly important in the context of understanding the spectroscopic and magnetic properties of coordination compounds.
$t_2$: $t_2$ is a term that refers to a specific type of molecular orbital in coordination compounds. It is one of the three t-type molecular orbitals that can be formed from the interaction between the central metal atom's d-orbitals and the ligand orbitals in an octahedral complex.
Acid-ionization constant, Ka: The acid-ionization constant, $K_a$, quantifies the strength of a weak acid in solution. It is the equilibrium constant for the dissociation of an acid into its conjugate base and a proton.
Bohr magneton: The Bohr magneton is a fundamental unit of magnetic moment, which is a measure of the magnetic properties of an atom or subatomic particle. It is named after the Danish physicist Niels Bohr and is used to describe the magnetic behavior of atoms, particularly in the context of coordination compounds and their spectroscopic and magnetic properties.
Br^-: Br^- is the bromide ion, which is the negatively charged form of the bromine atom. It is an important species in the context of spectroscopic and magnetic properties of coordination compounds.
CN^-: The cyanide ion, CN^-, is a polyatomic ion consisting of a carbon atom triple-bonded to a nitrogen atom. It is an important ligand in coordination chemistry and has unique spectroscopic and magnetic properties.
CO: CO, or carbon monoxide, is a colorless, odorless, and highly toxic gas that is produced during the incomplete combustion of carbon-based fuels. This term is particularly relevant in the context of understanding the occurrence and preparation of representative metals, the occurrence, preparation, and properties of phosphorus, as well as the spectroscopic and magnetic properties of coordination compounds.
Coordination compounds: Coordination compounds are complex molecules consisting of a central metal atom or ion bonded to surrounding molecules or anions, called ligands. These compounds exhibit unique chemical and physical properties due to the interactions between the central metal and its ligands.
Coordination Compounds: Coordination compounds are a class of chemical compounds that consist of a central metal atom or ion surrounded by a number of ligands, which are molecules or ions that are attached to the central metal. These compounds are an important aspect of transition metal chemistry and exhibit unique spectroscopic and magnetic properties.
Cr^3+: Cr^3+ is the trivalent chromium cation, which is a common oxidation state of the chromium element. This species is particularly relevant in the context of spectroscopic and magnetic properties of coordination compounds.
Crystal field splitting: Crystal field splitting refers to the energy difference between d-orbitals in a transition metal complex when ligands approach and interact with the central metal ion. This interaction causes the degeneracy of the d-orbitals to break, leading to different energy levels.
Crystal field theory: Crystal Field Theory (CFT) describes the breaking of degeneracies of electronic orbitals in transition metal complexes due to the presence of ligands. It explains how the arrangement of ligands around a central metal ion affects the energy levels and properties like color and magnetism.
Crystal Field Theory: Crystal Field Theory is a model that explains the electronic structure and bonding in coordination complexes formed between transition metal ions and surrounding ligands. It provides a framework for understanding the spectroscopic and magnetic properties of these complexes.
D Orbitals: d orbitals are a set of five atomic orbitals that are part of the electron configuration of atoms. They are characterized by their unique shape and energy levels, and play a crucial role in the spectroscopic and magnetic properties of coordination compounds.
Diamagnetic: Diamagnetic substances are materials that create an opposing magnetic field when exposed to an external magnetic field, causing a repulsive effect. These materials have all their electrons paired, leading to no net magnetic moment.
Diamagnetic: Diamagnetic materials are substances that have no unpaired electrons in their atomic or molecular structure, resulting in a weak, opposing magnetic field when placed in an external magnetic field. This property is crucial in understanding the magnetic behavior of materials, particularly in the context of molecular orbital theory and the spectroscopic and magnetic properties of coordination compounds.
Electronic Transitions: Electronic transitions refer to the process by which an electron in an atom or molecule moves from one energy level or orbital to another. This change in energy state is accompanied by the absorption or emission of a photon, which carries the energy difference between the two levels.
Fe^2+: Fe^2+ refers to the ferrous ion, which is the divalent oxidation state of the iron (Fe) atom. This ionic form of iron is an important component in various coordination compounds and plays a crucial role in the spectroscopic and magnetic properties of these complexes.
High-spin Complexes: High-spin complexes are a type of coordination compound where the metal center has the maximum possible number of unpaired electrons in its valence shell, resulting in a high total spin quantum number. This spin state is favored when the ligand field splitting energy is relatively weak compared to the pairing energy of the electrons.
Hund's Rule: Hund's rule is a fundamental principle in quantum mechanics that describes the preferred electron configuration of an atom or molecule. It states that when electrons occupy degenerate orbitals, they will singly occupy these orbitals with parallel spins before pairing up, in order to minimize electron-electron repulsion and maximize the total spin angular momentum of the system.
I^-: I^- is the iodide ion, which is a negatively charged ion of the element iodine. It is an important species in the context of spectroscopic and magnetic properties of coordination compounds, as it can serve as a ligand in metal complexes.
Jahn-Teller effect: The Jahn-Teller effect is a geometric distortion that occurs in certain molecules or ions with a particular electronic configuration, leading to a stabilization of the molecule's energy. This effect is particularly relevant in the context of spectroscopic and magnetic properties of coordination compounds.
Ligand Field Theory: Ligand field theory is a model used to describe the electronic structure and bonding in coordination compounds. It provides a framework for understanding the spectroscopic and magnetic properties of these complexes by considering the interactions between the metal center and the surrounding ligands.
Low-spin complexes: Low-spin complexes are coordination compounds where electrons prefer to pair up in lower energy orbitals before occupying higher energy ones, resulting from strong field ligands. These complexes often exhibit lower magnetic moments due to fewer unpaired electrons.
Low-spin complexes: Low-spin complexes are coordination compounds in which the pairing of electrons occurs before they occupy higher energy orbitals, leading to a lower number of unpaired electrons. This phenomenon typically arises in the presence of strong field ligands that exert significant crystal field splitting, favoring the occupation of lower energy d-orbitals. The result is that these complexes often exhibit distinct spectroscopic and magnetic properties, which are important for understanding their behavior in various chemical environments.
Ni^2+: Ni^2+ is the divalent nickel cation, which is a common metal ion found in coordination compounds. It has a +2 oxidation state and a closed-shell electron configuration, making it a diamagnetic species with interesting spectroscopic and magnetic properties.
Octahedral complexes: Octahedral complexes are coordination compounds in which a central metal ion is surrounded by six ligands arranged at the corners of an octahedron. This geometry plays a crucial role in determining the spectroscopic and magnetic properties of coordination compounds, as the arrangement of ligands influences crystal field splitting and the overall electronic configuration of the metal center.
Pairing energy (P): Pairing energy (P) is the energy required to pair two electrons in the same orbital. It is a key factor in determining the electronic configuration of coordination compounds.
Paramagnetic: Paramagnetic materials are substances that have a weak positive susceptibility to an applied magnetic field, meaning they are slightly attracted to magnetic fields. This property arises from the presence of unpaired electrons within the material's atoms or molecules.
Selection Rules: Selection rules are a set of guidelines that determine which electronic transitions in atoms and molecules are allowed or forbidden in spectroscopic and magnetic processes. These rules govern the probability of specific transitions occurring, and they are essential for understanding the spectroscopic and magnetic properties of coordination compounds.
Spectrochemical series: The spectrochemical series is an empirically derived list of ligands ordered by the magnitude of the crystal field splitting energy ($\Delta$) they produce when coordinating to a metal ion. Ligands at the high end of the series cause a larger splitting of d-orbital energies, resulting in stronger field effects.
Spectrochemical Series: The spectrochemical series is a ranking of ligands based on their ability to split the d-orbital energy levels in coordination complexes, which is directly related to the strength of the ligand field. This concept is crucial in understanding the coordination chemistry of transition metals and the spectroscopic and magnetic properties of coordination compounds.
Spin Crossover: Spin crossover is a phenomenon observed in certain transition metal complexes where the electronic configuration of the metal center can reversibly switch between a high-spin and a low-spin state, leading to changes in the magnetic and spectroscopic properties of the complex.
Square Planar Complexes: Square planar complexes are a type of coordination compound in which a central metal atom is bonded to four ligands arranged in a square planar geometry. This unique arrangement has significant implications for the spectroscopic and magnetic properties of these compounds.
Strong-field ligands: Strong-field ligands are ligands that create a significant splitting of the d-orbitals in a metal complex, leading to low-spin configurations when possible. They typically have high ligand field strength and can greatly affect the spectroscopic and magnetic properties of coordination compounds.
T2g orbitals: t2g orbitals are a set of three d-orbitals in an octahedral field that experience lower electrostatic repulsion compared to the eg orbitals. These orbitals include $d_{xy}$, $d_{xz}$, and $d_{yz}$.
Tetrahedral complexes: Tetrahedral complexes are coordination compounds where a central metal atom is surrounded by four ligands arranged at the corners of a tetrahedron. This geometry significantly influences their spectroscopic and magnetic properties, which can be analyzed to understand the behavior of these compounds in various environments.
Weak-field ligands: Weak-field ligands are ligands that produce a small splitting of the d-orbitals in a coordination complex. They typically lead to high-spin configurations in transition metal complexes.
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