🧶Inorganic Chemistry I Unit 11 – Organometallic Chemistry: Structures & Bonds

Key Concepts

• Organometallic compounds contain at least one metal-carbon bond, which can be ionic or covalent in nature
• Ligands are molecules or ions that bind to a central metal atom, forming a coordination complex
  • Common ligands include carbon monoxide (CO), cyclopentadienyl (Cp), and phosphines (PR3)
• The 18-electron rule states that stable organometallic compounds tend to have 18 valence electrons around the metal center, analogous to the octet rule for main group elements
• Backbonding occurs when metal d-electrons donate into the empty π* orbitals of ligands, strengthening the metal-ligand bond
• Hapticity (η) denotes the number of contiguous atoms in a ligand that are bonded to the metal center (e.g., η5-cyclopentadienyl)
• Oxidation states of metals in organometallic compounds can vary widely, depending on the ligands and the electronic configuration of the metal
• Organometallic complexes can undergo various reactions, such as ligand substitution, oxidative addition, and reductive elimination

Electron Counting and Formal Charge

• Electron counting involves determining the total number of valence electrons around the metal center, which helps predict the stability and reactivity of the complex
• The number of valence electrons is calculated by summing the electrons from the metal, ligands, and overall charge of the complex
  • Metal electrons = group number (for transition metals) or valence electrons (for main group metals)
  • Ligand electrons = 2 for each X-type ligand, 2 for each L-type ligand, and 0 for each Z-type ligand
• Formal charge is the charge assigned to an atom in a molecule, assuming that electrons in a bond are shared equally between the atoms
• To calculate formal charge, use the formula: FC = [# valence electrons in free atom] - [# non-bonding electrons] - 1/2[# bonding electrons]
• Formal charges help determine the most stable resonance structure and the likely site of reactivity in a complex
• The sum of formal charges in a molecule or ion should equal the overall charge of the species

Metal-Ligand Bonding

• Metal-ligand bonding involves the interaction between the metal's d-orbitals and the ligand's orbitals, which can be σ (sigma) or π (pi) in nature
• Sigma (σ) bonding occurs when there is a direct overlap between the metal and ligand orbitals along the bonding axis
  • Examples include the overlap of a metal's d$_{z^2}$ orbital with a ligand's s or p$_z$ orbital
• Pi (π) bonding involves the sideways overlap of orbitals, which can be either bonding or antibonding (π*)
  • Metal d$_{xz}$, d$_{yz}$, d$_{x^2-y^2}$, and d$_{xy}$ orbitals can participate in π bonding
• Ligands can be classified as σ-donors, π-donors, or π-acceptors based on their ability to engage in different types of bonding
  • Carbon monoxide (CO) is a strong π-acceptor due to its empty π* orbitals
• The strength of the metal-ligand bond depends on factors such as the metal's oxidation state, the ligand's donor/acceptor properties, and the presence of backbonding

Common Organometallic Structures

• Organometallic complexes can adopt various geometries, depending on the number and type of ligands and the metal's electronic configuration
• Linear complexes have two ligands arranged in a straight line with the metal center, with an angle of 180° between the ligands
  • Examples include [Ag(NH3)2]+ and [Au(Cl)2]-
• Trigonal planar complexes have three ligands arranged in a plane with the metal center, with angles of 120° between the ligands
  • An example is [HgMe3]-
• Tetrahedral complexes have four ligands arranged in a tetrahedron around the metal center, with angles of 109.5° between the ligands
  • Examples include [Zn(CH3)4] and [Cd(SMe)4]
• Square planar complexes have four ligands arranged in a square plane around the metal center, with 90° angles between the ligands
  • Examples include [Pt(NH3)2Cl2] and [Pd(PPh3)2Cl2]
• Octahedral complexes have six ligands arranged in an octahedron around the metal center, with 90° angles between the ligands
  • Examples include [Co(NH3)6]3+ and [Ru(bpy)3]2+
• Sandwich compounds consist of a metal center "sandwiched" between two planar aromatic ligands, such as cyclopentadienyl (Cp) or benzene
  • Ferrocene, [Fe(C5H5)2], is a classic example of a sandwich compound

Reaction Mechanisms

• Organometallic complexes can undergo various types of reactions, each with its own distinct mechanism
• Ligand substitution reactions involve the replacement of one ligand by another, without changing the oxidation state of the metal
  • Associative substitution (A) occurs when the incoming ligand binds to the metal before the leaving ligand departs, forming a higher-coordinate intermediate
  • Dissociative substitution (D) occurs when the leaving ligand departs before the incoming ligand binds, forming a lower-coordinate intermediate
• Oxidative addition reactions involve the addition of a small molecule (e.g., H2, HX, or RX) to the metal center, increasing its oxidation state by two units
  • The metal's coordination number also increases by two
• Reductive elimination is the reverse of oxidative addition, where two ligands on the metal center combine to form a new molecule, reducing the metal's oxidation state and coordination number by two units
• Insertion reactions occur when a small molecule (e.g., CO or alkene) inserts into a metal-ligand bond, forming a new ligand
  • Examples include the insertion of CO into a metal-alkyl bond to form an acyl complex
• β-hydride elimination involves the formation of a metal-hydride complex and an alkene from a metal-alkyl complex, via the transfer of a β-hydrogen to the metal center

Spectroscopic Analysis

• Spectroscopic techniques are essential for characterizing organometallic compounds and elucidating their structures and properties
• Nuclear Magnetic Resonance (NMR) spectroscopy provides information about the chemical environment of specific nuclei (e.g., 1H, 13C, 31P) in a complex
  • Chemical shifts, coupling constants, and peak multiplicities can help identify ligands and their coordination modes
• Infrared (IR) spectroscopy is useful for detecting the presence of certain functional groups, such as carbonyl (C≡O) or cyclopentadienyl (C5H5) ligands
  • The position and intensity of the absorption bands can provide information about the bonding and coordination environment of the ligands
• UV-Visible spectroscopy can help determine the electronic transitions and d-orbital splitting in organometallic complexes
  • The position and intensity of the absorption bands are influenced by the metal, its oxidation state, and the ligands present
• Mass spectrometry (MS) can provide information about the molecular mass and fragmentation patterns of organometallic compounds
  • Techniques such as Electron Ionization (EI) and Electrospray Ionization (ESI) are commonly used in organometallic chemistry

Applications in Synthesis

• Organometallic compounds play a crucial role in organic synthesis, catalyzing a wide range of reactions and enabling the formation of complex molecules
• Cross-coupling reactions, such as the Suzuki, Negishi, and Heck reactions, involve the formation of new carbon-carbon bonds using organometallic reagents
  • These reactions typically employ palladium or nickel catalysts and have broad applications in the synthesis of pharmaceuticals, natural products, and materials
• Hydrogenation reactions use organometallic catalysts (e.g., Wilkinson's catalyst, [RhCl(PPh3)3]) to add hydrogen across unsaturated bonds, such as alkenes or alkynes
  • Asymmetric hydrogenation, using chiral organometallic catalysts, can produce optically active compounds with high enantioselectivity
• Olefin metathesis, catalyzed by ruthenium or molybdenum complexes (e.g., Grubbs' catalyst), involves the redistribution of carbon-carbon double bonds in alkenes
  • This reaction has applications in the synthesis of polymers, natural products, and pharmaceuticals
• Carbonylation reactions, such as hydroformylation and the Monsanto process, use organometallic catalysts to introduce carbonyl groups into organic molecules
  • These reactions are important in the industrial production of aldehydes, carboxylic acids, and esters
• C-H activation, catalyzed by organometallic complexes, allows the direct functionalization of otherwise inert C-H bonds, enabling the synthesis of complex molecules from simple starting materials

Practice Problems and Review

• Practice drawing the structures of organometallic compounds, including the metal center, ligands, and coordination geometry
• Assign formal charges and count the total number of valence electrons for a given organometallic complex
• Identify the type of metal-ligand bonding (σ, π, or backbonding) in a given complex based on the metal and ligand properties
• Predict the products of ligand substitution, oxidative addition, reductive elimination, and insertion reactions for a given organometallic complex
• Interpret NMR, IR, UV-Visible, and mass spectra to characterize organometallic compounds and determine their structures
• Propose a catalytic cycle for a given organometallic reaction, such as cross-coupling, hydrogenation, or olefin metathesis
• Solve problems involving the synthesis of organic molecules using organometallic reagents and catalysts
• Review the key concepts, bonding theories, and reaction mechanisms covered in the unit, and practice applying them to new examples and problems