Glossary

10.1 Electronic Spectra of Coordination Compounds

3 min read•august 9, 2024

reveal their unique colors and properties. These spectra arise from transitions between energy levels in metal complexes. Understanding these transitions helps explain the vibrant hues of transition metal compounds.

This topic connects to the broader chapter by exploring how spectroscopy probes electronic structure. It delves into selection rules, theoretical frameworks, and tools that explain and predict the spectra of coordination compounds.

Electronic Transitions

Absorption Spectroscopy and Transitions

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  • Absorption spectroscopy measures the absorption of light by a sample as a function of wavelength
  • occur between d orbitals of a transition metal ion in a coordination compound
    • Involve promotion of electrons within the d orbitals
    • Result in characteristic colors of many transition metal complexes
  • involve electron movement between metal and ligand
    • (MLCT) transfers electron from metal to ligand
    • (LMCT) transfers electron from ligand to metal
    • Generally more intense than d-d transitions

Selection Rules and Their Implications

  • Selection rules determine which electronic transitions are allowed or forbidden
  • states transitions between states of the same parity are forbidden
    • Applies to centrosymmetric molecules
    • d-d transitions are formally forbidden by this rule
    • Rule can be relaxed by (coupling between electronic and vibrational states)
  • prohibits transitions between states with different spin multiplicities
    • Transitions involving a change in spin state (ΔS≠0\Delta S \neq 0) are forbidden
    • Can be relaxed by spin-orbit coupling, especially in heavier elements

Theoretical Frameworks

Crystal Field and Ligand Field Theories

  • describes the breaking of d-orbital degeneracy in transition metal complexes
    • Treats ligands as point charges
    • Explains splitting of d orbitals into and eg sets in octahedral complexes
  • extends crystal field theory by incorporating molecular orbital theory
    • Accounts for covalent bonding between metal and ligands
    • Provides more accurate description of electronic structure and spectra
  • Both theories help predict and interpret electronic spectra of coordination compounds

Advanced Analytical Tools

  • graphically represent electronic energy levels of transition metal ions
    • x-axis represents (Δo/B\Delta_o/B)
    • y-axis represents (E/BE/B)
    • Used to predict and interpret electronic spectra of octahedral complexes
  • describes geometric distortion in non-linear molecules with degenerate electronic states
    • Common in octahedral complexes of certain d-electron configurations (d9, high-spin d4, low-spin d7)
    • Results in elongation or compression along one axis, lifting orbital degeneracy
    • Affects electronic spectra by splitting absorption bands

Spectroscopic Properties

Color and Spectrochemical Series

  • Color of coordination compounds results from absorption of visible light
    • Complementary color to absorbed wavelength is observed
    • Intensity of color relates to strength of d-d transitions
  • ranks ligands based on their ability to cause d-orbital splitting
    • (I-, Br-, Cl-) cause small splitting
    • (CO, CN-, NO2-) cause large splitting
    • Affects color by changing energy of d-d transitions

Spectroscopic Term Symbols and Their Significance

  • describe electronic states of atoms or molecules
    • Format: 2S+1LJ^{2S+1}L_J for atoms, 2S+1Γ^{2S+1}\Gamma for molecules
    • S represents total spin angular momentum
    • L represents total orbital angular momentum (S, P, D, F, etc. for atoms)
    • J represents total angular momentum (for atoms)
    • Γ\Gamma represents irreducible representation of point group (for molecules)
  • Used to label electronic states and transitions in spectroscopy
  • Help predict and interpret selection rules and spectral features

Key Terms to Review (20)

Charge transfer transitions: Charge transfer transitions refer to electronic transitions where an electron moves between different chemical species, typically from a donor to an acceptor. This process is crucial in understanding the electronic spectra of coordination compounds, as it can lead to significant absorption features that provide insights into the nature of bonding and the environment around metal centers.
Colorimetry: Colorimetry is the scientific technique used to determine the concentration of colored compounds in solution by measuring the intensity of their color. This method is based on the principles of light absorption and is essential in analyzing various chemical substances, particularly in coordination chemistry where transition metal complexes exhibit vivid colors due to electronic transitions. The relationship between color intensity and concentration is utilized to quantify the amount of a specific compound in a sample.
Crystal field strength: Crystal field strength refers to the ability of ligands to split the degenerate d-orbitals of transition metal ions in a complex. This concept is essential in understanding the electronic spectra of coordination compounds, as the strength of the ligand field influences the energy differences between split d-orbitals, affecting the absorption of light and the color observed in these compounds.
Crystal field theory: Crystal field theory is a model that describes the electronic structure and properties of transition metal complexes by considering the interaction between the metal ion and surrounding ligands. It explains how the presence of ligands causes the degenerate d-orbitals of the metal to split into different energy levels, leading to various electronic configurations, magnetic properties, and colors in coordination compounds.
D-d transitions: d-d transitions refer to the electronic transitions between different d-orbitals in transition metal complexes. These transitions occur when an electron moves from one d-orbital to another, usually in the presence of a ligand field that splits the degenerate d-orbitals into distinct energy levels. This phenomenon is crucial for understanding the electronic spectra of coordination compounds and has significant applications in various areas of spectroscopy.
Electronic Spectra of Coordination Compounds: Electronic spectra of coordination compounds refer to the patterns of light absorption and emission that occur when electrons in a metal complex transition between different energy levels. These spectra provide insight into the electronic structure and behavior of coordination compounds, revealing information about the metal center, ligand interactions, and overall molecular geometry.
Energy of electronic states: The energy of electronic states refers to the specific energy levels that electrons can occupy within an atom or molecule, particularly in the context of coordination compounds. These energy levels are determined by factors such as the nature of the metal ion, the ligands involved, and the geometry of the complex. Understanding these energy levels is crucial for interpreting electronic spectra and the color properties of coordination compounds.
Jahn-Teller Effect: The Jahn-Teller effect is a phenomenon in coordination chemistry where non-linear molecular geometries become distorted due to the presence of degenerate electronic states. This distortion leads to a lowering of the overall energy of the complex, resulting in more stable structures and influencing electronic spectra. It occurs primarily in coordination compounds with uneven electron distribution, affecting their color and absorption properties.
Laporte Selection Rule: The Laporte selection rule is a principle that states that electronic transitions in coordination compounds are allowed only if there is a change in parity, meaning transitions can occur between states of different symmetry. This rule is particularly important in the context of d-d transitions in transition metal complexes, where the symmetry of the electronic wave functions determines whether the transition is allowed or forbidden.
Ligand field theory: Ligand field theory is a model that describes the electronic structure and bonding in coordination complexes by considering the effects of ligands on the metal center's d-orbitals. This theory expands upon crystal field theory by incorporating molecular orbital theory and emphasizing the role of covalent character in metal-ligand interactions, thereby enhancing our understanding of stability, spectra, and magnetic properties in these compounds.
Ligand-to-metal charge transfer: Ligand-to-metal charge transfer (LMCT) refers to the process where an electron is transferred from a ligand to a metal center in a coordination compound. This electronic transition plays a significant role in determining the color and electronic spectra of coordination complexes, as it involves changes in the oxidation states of both the metal and the ligand.
Metal-to-ligand charge transfer: Metal-to-ligand charge transfer (MLCT) refers to the electronic transition in coordination compounds where an electron is promoted from a metal's d-orbital to an empty or partially filled orbital of a ligand. This phenomenon is crucial in understanding the electronic spectra of coordination compounds as it highlights how the interactions between metal ions and ligands can affect their light absorption properties. MLCT can significantly influence the color and reactivity of these compounds, making it a key concept in the study of their behavior under various conditions.
Spectrochemical series: The spectrochemical series is a ranking of ligands based on their ability to split the degenerate d-orbitals in transition metal complexes. It helps predict the strength of interactions between ligands and metal ions, influencing properties such as color and magnetic behavior.
Spectroscopic Term Symbols: Spectroscopic term symbols are a way to describe the electronic states of atoms or ions in terms of their total angular momentum and spin. These symbols provide essential information about the energy levels of an atom, particularly in coordination compounds, by indicating the multiplicity and symmetry of the electronic configuration. The term symbols are derived from quantum mechanics and are crucial for interpreting electronic spectra, helping to predict the allowed transitions between energy levels.
Spin Selection Rule: The spin selection rule is a principle in quantum mechanics that states that transitions between states of different spin multiplicities are forbidden. This means that when electrons transition between energy levels, they must maintain the same total spin state, leading to selection rules that dictate which electronic transitions are allowed or forbidden in processes like spectroscopy. Understanding this rule helps explain the intensity and patterns observed in the electronic spectra of molecules, particularly in coordination compounds.
Strong field ligands: Strong field ligands are ligands that create a significant crystal field splitting when they coordinate to a metal ion, resulting in the stabilization of low-energy d-orbitals and promoting the pairing of electrons. These ligands influence the electronic structure of coordination compounds, leading to distinct properties and reactivity, especially evident in the behavior of transition metals under varying ligand fields.
T2g: The t2g term refers to a specific set of molecular orbitals in transition metal complexes, particularly related to the splitting of d-orbitals in an octahedral field. This splitting occurs due to the presence of ligands, which interact with the d-orbitals of the metal ion, causing them to become higher or lower in energy. The t2g orbitals consist of three lower-energy orbitals (dxy, dyz, and dzx), which can hold up to six electrons and play a key role in determining the electronic configuration and properties of coordination compounds.
Tanabe-Sugano Diagrams: Tanabe-Sugano diagrams are graphical representations that depict the energy levels of electronic states in transition metal complexes as a function of the ligand field strength. These diagrams are crucial for understanding the electronic spectra of coordination compounds, allowing chemists to predict and interpret the colors and spectral properties based on ligand interactions with the metal center.
Vibronic coupling: Vibronic coupling refers to the interaction between electronic and vibrational states in a molecule, which can significantly influence the electronic spectra of coordination compounds. This phenomenon arises from the simultaneous changes in electronic energy levels and nuclear geometry during molecular vibrations, leading to a mixing of states that impacts the absorption and emission characteristics observed in spectroscopy.
Weak field ligands: Weak field ligands are ligands that produce a small crystal field splitting energy (\Delta) in coordination complexes, resulting in a preference for high-spin electron configurations. These ligands typically have low electronegativity and larger sizes, leading to less interaction with the central metal ion. This characteristic has implications for various concepts, including electronic transitions, the stability of complexes, and their spectroscopic properties.
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