💍Inorganic Chemistry II Unit 1 – Coordination Chemistry

Key Concepts and Definitions

• 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
• The coordination number represents the number of ligands directly bonded to the central metal atom or ion, with common coordination numbers being 2, 4, and 6
• Ligands can be 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 chelate effect describes the enhanced stability of complexes with polydentate ligands compared to those with monodentate ligands due to entropic factors
• Isomerism in coordination compounds can occur in various forms, such as structural isomers (different connectivity of atoms) and stereoisomers (same connectivity but different spatial arrangements)
  • Structural isomers include linkage isomers and coordination isomers
  • Stereoisomers include geometrical isomers and optical isomers
• The spectrochemical series ranks ligands based on their ability to split the d-orbital energy levels of the metal, with strong-field ligands causing a larger splitting than weak-field ligands
• The crystal field stabilization energy (CFSE) represents the energy stabilization of a complex due to the splitting of d-orbital energies by the ligand field

Coordination Compounds: Structure and Bonding

• Coordination compounds exhibit a variety of geometries depending on the coordination number and ligand arrangement, such as linear (2), tetrahedral (4), square planar (4), and octahedral (6)
• The metal-ligand bond in coordination compounds is typically a coordinate covalent bond, where the ligand donates both electrons to form the bond
• The 18-electron rule states that stable complexes tend to have 18 valence electrons, which can be used to predict the stability and reactivity of coordination compounds
• Ligands can exert both sigma (σ) and pi (π) bonding interactions with the metal, depending on their electronic structure and orbital overlap
  • Sigma bonding involves the donation of electron density from the ligand to the metal through a single covalent bond
  • Pi bonding can occur through back-donation of electron density from the metal to empty π* orbitals on the ligand (π-backbonding) or donation from filled π orbitals on the ligand to the metal (π-donation)
• The Jahn-Teller effect describes the distortion of certain high-symmetry geometries (octahedral and tetrahedral) to remove orbital degeneracy and lower the overall energy of the complex
• Trans effect refers to the ability of a ligand to weaken the bond trans (opposite) to it in a square planar or octahedral complex, influencing the reactivity and substitution patterns

Ligand Types and Properties

• Ligands can be classified based on their charge as neutral ligands (e.g., $NH_3$, $H_2O$) or anionic ligands (e.g., $Cl^-$, $CN^-$)
• The denticity of a ligand refers to the number of donor atoms it possesses, with common examples being monodentate (e.g., $NH_3$, $Cl^-$), bidentate (e.g., ethylenediamine, acetylacetonate), and hexadentate (e.g., EDTA)
• Ambidentate ligands, such as thiocyanate ($SCN^-$), can bond to the metal through different donor atoms (S or N), leading to linkage isomerism
• Ligands can also be categorized as hard or soft based on their polarizability and preference for bonding with certain metal ions (hard-hard or soft-soft interactions)
  • Hard ligands (e.g., $F^-$, $OH^-$) have low polarizability and prefer to bond with hard metal ions (e.g., $Al^{3+}$, $Ti^{4+}$)
  • Soft ligands (e.g., $I^-$, $PR_3$) have high polarizability and prefer to bond with soft metal ions (e.g., $Pt^{2+}$, $Hg^{2+}$)
• The steric properties of ligands, such as their size and shape, can influence the geometry and reactivity of coordination compounds
• Ligands with strong σ-donor and π-acceptor abilities (e.g., $CO$, $CN^-$) tend to form stable, low-spin complexes with metals in low oxidation states

Crystal Field Theory

• Crystal field theory (CFT) describes the splitting of d-orbital energies in transition metal complexes due to the electrostatic interaction between the metal and the ligands
• In an octahedral field, the d-orbitals split into two sets: the lower energy $t_{2g}$ orbitals ($d_{xy}$, $d_{xz}$, $d_{yz}$) and the higher energy $e_g$ orbitals ($d_{z^2}$, $d_{x^2-y^2}$)
  • The energy difference between the $t_{2g}$ and $e_g$ orbitals is denoted as $Δ_o$ (octahedral crystal field splitting energy)
• In a tetrahedral field, the d-orbital splitting is inverted, with the $e$ orbitals being lower in energy than the $t_2$ orbitals
  • The energy difference between the $e$ and $t_2$ orbitals is denoted as $Δ_t$ (tetrahedral crystal field splitting energy), which is approximately 4/9 of $Δ_o$
• The magnitude of the crystal field splitting depends on the nature of the ligands, with strong-field ligands (e.g., $CO$, $CN^-$) causing a larger splitting than weak-field ligands (e.g., $I^-$, $Br^-$)
• The electron configuration of the metal ion in a complex can be high-spin (maximum number of unpaired electrons) or low-spin (minimum number of unpaired electrons), depending on the relative magnitudes of the crystal field splitting and the pairing energy
• The spectrochemical series ranks ligands based on their ability to split the d-orbitals: $I^- < Br^- < S^{2-} < Cl^- < F^- < OH^- < H_2O < NH_3 < en < NO_2^- < CN^- < CO$
• Crystal field stabilization energy (CFSE) is the energy difference between the electronic configuration in the complex and the hypothetical configuration in a spherical field, and it contributes to the stability and properties of the complex

Molecular Orbital Theory in Coordination Complexes

• Molecular orbital theory (MOT) provides a more comprehensive description of bonding in coordination compounds by considering the overlap of metal and ligand orbitals
• In octahedral complexes, the metal d-orbitals and ligand orbitals combine to form bonding and antibonding molecular orbitals
  • The $t_{2g}$ orbitals of the metal interact with the ligand π orbitals to form π-bonding and π*-antibonding orbitals
  • The $e_g$ orbitals of the metal interact with the ligand σ orbitals to form σ-bonding and σ*-antibonding orbitals
• The relative energies of the molecular orbitals depend on the metal, its oxidation state, and the nature of the ligands
• Ligand-to-metal charge transfer (LMCT) transitions involve the excitation of electrons from filled ligand orbitals to empty metal orbitals, resulting in intense absorption bands in the UV-visible spectrum
• Metal-to-ligand charge transfer (MLCT) transitions involve the excitation of electrons from filled metal orbitals to empty ligand orbitals, and they are commonly observed in complexes with π-acceptor ligands (e.g., $CO$, $CN^-$)
• The electronic spectra of coordination compounds can be interpreted using MOT, with the observed transitions corresponding to excitations between different molecular orbitals
• MOT can also explain the magnetic properties of coordination compounds, such as the presence of unpaired electrons and the resulting paramagnetism or diamagnetism

Spectroscopic and Magnetic Properties

• UV-visible spectroscopy is used to study the electronic transitions in coordination compounds, providing information about the ligand field splitting and the nature of the metal-ligand bonding
  • The position and intensity of absorption bands depend on the metal, its oxidation state, and the ligands
  • The selection rules for electronic transitions (e.g., Laporte rule, spin multiplicity rule) govern the allowed and forbidden transitions
• The crystal field splitting energy ($Δ$) can be determined from the UV-visible spectrum by analyzing the position of the d-d transitions
• Charge transfer bands, such as LMCT and MLCT, appear at higher energies than d-d transitions and are often more intense
• Infrared (IR) spectroscopy is used to identify the presence of certain ligands and to study the bonding in coordination compounds
  • The positions of characteristic vibrational bands (e.g., $CO$ stretching, $NH$ stretching) can provide information about the coordination mode and the strength of the metal-ligand bond
• Magnetic susceptibility measurements can determine the number of unpaired electrons in a complex and provide insights into its electronic structure
  • Diamagnetic compounds have no unpaired electrons and are repelled by a magnetic field
  • Paramagnetic compounds have one or more unpaired electrons and are attracted to a magnetic field
• The magnetic moment ($μ_eff$) of a complex can be calculated from its magnetic susceptibility and compared to the spin-only value expected for the given number of unpaired electrons
• Deviations from the spin-only value can indicate the presence of orbital angular momentum or magnetic exchange interactions between metal centers

Reaction Mechanisms and Kinetics

• Ligand substitution reactions in coordination compounds can proceed through associative (A), dissociative (D), or interchange (I) mechanisms
  • Associative mechanism (A): The incoming ligand binds to the metal center before the leaving ligand dissociates, forming a higher-coordinate intermediate
  • Dissociative mechanism (D): The leaving ligand dissociates from the metal center before the incoming ligand binds, forming a lower-coordinate intermediate
  • Interchange mechanism (I): The incoming ligand begins to bind as the leaving ligand starts to dissociate, with no distinct intermediate
• The rate law for ligand substitution reactions depends on the mechanism and can be determined experimentally by monitoring the concentration of reactants or products over time
• The activation parameters (activation energy, activation enthalpy, and activation entropy) can be obtained from the temperature dependence of the rate constant using the Eyring equation
• Ligand substitution reactions can exhibit stereochemical changes, such as retention or inversion of configuration, depending on the mechanism and the nature of the ligands
• Electron transfer reactions in coordination compounds involve the transfer of electrons between metal centers or between a metal center and a ligand
  • Inner-sphere electron transfer occurs through a bridging ligand that facilitates the electron transfer between the metal centers
  • Outer-sphere electron transfer occurs without the formation of a bridging ligand, with the electron transferring through space or solvent
• The Marcus theory describes the factors that influence the rate of electron transfer reactions, such as the reorganization energy, the driving force, and the electronic coupling between the donor and acceptor
• Photochemical reactions in coordination compounds involve the excitation of electrons by light, leading to the formation of excited states and subsequent chemical transformations
  • Photosubstitution reactions involve the replacement of a ligand by another ligand or solvent molecule upon light absorption
  • Photoisomerization reactions result in the change of the spatial arrangement of ligands around the metal center, such as cis-trans isomerization

Applications and Real-World Examples

• Coordination compounds have numerous applications in various fields, such as catalysis, medicine, and materials science
• Transition metal complexes are widely used as homogeneous catalysts in organic synthesis and industrial processes
  • Wilkinson's catalyst ($RhCl(PPh_3)_3$) is used for the hydrogenation of alkenes and alkynes
  • Ziegler-Natta catalysts (e.g., $TiCl_4/Al(C_2H_5)_3$) are used for the polymerization of alkenes to produce polyethylene and polypropylene
• Coordination compounds find applications in medicine as diagnostic agents and therapeutic drugs
  • Cisplatin ($cis-[PtCl_2(NH_3)_2]$) is an anticancer drug that binds to DNA and induces apoptosis in cancer cells
  • Gadolinium(III) complexes (e.g., $Gd(DTPA)^{2-}$) are used as contrast agents in magnetic resonance imaging (MRI) due to their paramagnetic properties
• Coordination polymers and metal-organic frameworks (MOFs) are materials composed of metal ions or clusters connected by organic ligands, forming porous structures with high surface areas and tunable properties
  • MOFs have potential applications in gas storage, separation, catalysis, and sensing
• Supramolecular coordination complexes are self-assembled structures held together by non-covalent interactions, such as hydrogen bonding and π-π stacking
  • Supramolecular coordination cages and capsules can encapsulate and transport molecules, with potential applications in drug delivery and catalysis
• Coordination compounds are used in the design of functional materials, such as luminescent sensors, photochromic switches, and single-molecule magnets
  • Ruthenium(II) polypyridyl complexes (e.g., $[Ru(bpy)_3]^{2+}$) exhibit intense luminescence and find applications in solar cells and light-emitting devices
• Bioinorganic chemistry studies the role of metal ions and coordination compounds in biological systems
  • Hemoglobin contains an iron(II) porphyrin complex that reversibly binds oxygen for transport in the blood
  • Chlorophyll, a magnesium(II) porphyrin complex, is essential for photosynthesis in plants and algae