💍Inorganic Chemistry II Unit 3 – Organometallic Chemistry

Key Concepts and Definitions

• Organometallic compounds contain at least one metal-carbon bond, where the carbon is part of an organic group
• Ligands are ions or molecules that bind to a central metal atom to form a coordination complex
  • Common ligands in organometallic chemistry include carbon monoxide (CO), cyclopentadienyl (Cp), and phosphines (PR3)
• Hapticity ($\eta$) denotes the number of contiguous atoms in a ligand that are bonded to the metal center
  • Example: $\eta^5$-cyclopentadienyl (Cp) ligand binds to the metal through all five carbon atoms
• 18-electron rule states that stable organometallic compounds tend to have 18 valence electrons, similar to the octet rule for main group elements
• Oxidative addition is a process where a molecule (A-B) adds to a metal complex, increasing the oxidation state of the metal by two units
• Reductive elimination is the reverse of oxidative addition, where two ligands on a metal center combine to form a new molecule, reducing the metal's oxidation state by two units

Bonding in Organometallic Compounds

• Organometallic compounds exhibit a variety of bonding modes between the metal and organic ligands
• Sigma ($\sigma$) bonds involve the overlap of a filled ligand orbital with an empty metal orbital, resulting in a single covalent bond
  • Example: methyl group (CH3) bonding to a metal center through a $\sigma$ bond
• Pi ($\pi$) bonds form when a filled metal d orbital overlaps with an empty ligand p orbital, creating a multiple bond
  • Example: metal-olefin complexes, where the C=C double bond of an alkene interacts with the metal through $\pi$ bonding
• Agostic interactions are intramolecular interactions between a metal center and a C-H bond, stabilizing the complex
• Backbonding occurs when a filled metal d orbital donates electron density to an empty ligand $\pi^*$ orbital, strengthening the metal-ligand bond
  • Commonly observed in metal carbonyl complexes, where the metal donates electron density to the antibonding orbitals of CO
• Metallocenes are organometallic compounds with two cyclopentadienyl (Cp) ligands sandwiching a metal center, such as ferrocene (Fe(C5H5)2)

Types of Organometallic Compounds

• Metal carbonyls contain carbon monoxide (CO) ligands bonded to a metal center
  • Example: nickel tetracarbonyl (Ni(CO)4), a highly toxic compound used in the Mond process for purifying nickel
• Metal alkyls and aryls feature metal-carbon $\sigma$ bonds with sp3 or sp2 hybridized carbon atoms, respectively
  • Example: trimethylaluminum (Al(CH3)3), a pyrophoric compound used in organic synthesis
• Metal alkene and alkyne complexes involve $\pi$ bonding between unsaturated hydrocarbons and metal centers
  • Example: Zeise's salt (K[PtCl3(C2H4)]), the first organometallic compound discovered, containing an ethylene ligand
• Metallocenes are composed of two cyclopentadienyl (Cp) ligands bound to a metal in a sandwich-like structure
  • Ferrocene (Fe(C5H5)2) is the most well-known metallocene, with applications in catalysis and materials science
• Metal carbenes contain a divalent carbon atom (CR2) bonded to a metal center, often used as catalysts in olefin metathesis reactions
• Metal hydrides have a hydrogen atom directly bonded to a metal, playing crucial roles in catalytic hydrogenation and hydrogen storage

Synthesis Methods

• Direct reaction involves the combination of a metal salt or complex with an organic ligand to form an organometallic compound
  • Example: synthesis of methylmercury chloride (CH3HgCl) by reacting mercury(II) chloride with methylmagnesium chloride
• Ligand exchange is a process where one ligand in a coordination complex is replaced by another, often used to modify the properties of organometallic compounds
  • Phosphine ligands can displace carbon monoxide in metal carbonyls, altering their reactivity
• Oxidative addition is a key step in many catalytic cycles, involving the addition of a molecule (A-B) to a metal complex, increasing the metal's oxidation state
  • Example: addition of hydrogen gas to Vaska's complex (trans-IrCl(CO)(PPh3)2) to form a dihydride complex
• Transmetalation is the transfer of an organic group from one metal to another, commonly employed in cross-coupling reactions
  • In the Suzuki coupling, a boronic acid transfers its organic group to a palladium catalyst
• Metalation involves the formation of a metal-carbon bond by deprotonation of a C-H bond, often using strong bases like organolithium or Grignard reagents
• Reduction of metal salts with strong reducing agents (e.g., sodium naphthalenide) can generate low-valent organometallic species
  • Collman's reagent (Na2Fe(CO)4) is prepared by reducing iron(II) chloride with sodium in the presence of carbon monoxide

Reactivity and Mechanisms

• Organometallic compounds participate in a wide range of reactions, often serving as catalysts or intermediates
• Insertion reactions involve the insertion of a molecule (e.g., CO, alkenes) into a metal-ligand bond
  • Example: migratory insertion of carbon monoxide into a metal-alkyl bond, a key step in hydroformylation catalysis
• $\beta$-hydride elimination is a common decomposition pathway for metal alkyls, resulting in the formation of a metal hydride and an alkene
  • This process is often undesirable in catalytic reactions but can be harnessed for alkene synthesis
• Reductive elimination is the reverse of oxidative addition, where two ligands on a metal center combine to form a new molecule, reducing the metal's oxidation state
  • Example: reductive elimination of an alkyl halide from a palladium(II) complex in the Heck reaction
• Nucleophilic attack on coordinated ligands can lead to the formation of new C-C or C-heteroatom bonds
  • Grignard reagents can add to metal carbonyl complexes, forming acyl anion equivalents
• Cyclometalation involves the formation of a metallacycle by C-H activation of a ligand, often observed in directed C-H functionalization reactions
• Oxidative coupling can join two organic fragments through a metal-mediated process, such as the Ullmann coupling of aryl halides

Catalytic Applications

• Organometallic compounds are widely used as catalysts in industrial processes and organic synthesis
• Hydroformylation converts alkenes, carbon monoxide, and hydrogen into aldehydes using cobalt or rhodium catalysts
  • Example: the Oxo process for producing butyraldehyde from propylene
• Olefin metathesis rearranges carbon-carbon double bonds using ruthenium or molybdenum carbene catalysts
  • Grubbs catalysts (e.g., (PCy3)2Cl2Ru=CHPh) are widely used in ring-opening metathesis polymerization (ROMP) and ring-closing metathesis (RCM)
• Cross-coupling reactions form new C-C bonds by coupling organic halides or pseudohalides with organometallic reagents, often using palladium catalysts
  • Examples include Suzuki, Negishi, and Sonogashira couplings
• Asymmetric catalysis employs chiral organometallic complexes to selectively produce one enantiomer of a product
  • Example: Noyori's BINAP-Ru catalyst for asymmetric hydrogenation of ketones and olefins
• C-H activation allows the direct functionalization of unactivated C-H bonds, enabling more efficient synthesis of complex molecules
  • Palladium and rhodium complexes are commonly used in directed C-H activation reactions

Characterization Techniques

• Nuclear Magnetic Resonance (NMR) spectroscopy provides information about the chemical environment of nuclei in organometallic compounds
  • 1H, 13C, and 31P NMR are routinely used to characterize ligands and metal-ligand interactions
• Infrared (IR) spectroscopy is particularly useful for identifying metal carbonyl complexes, as CO stretching frequencies are sensitive to the metal's electronic environment
  • The number and pattern of CO stretches can help determine the geometry of the complex
• Single-crystal X-ray diffraction (XRD) allows for the determination of the solid-state structure of organometallic compounds
  • XRD provides information on bond lengths, angles, and the overall geometry of the complex
• Mass spectrometry (MS) can help determine the molecular formula and fragmentation patterns of organometallic species
  • Electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI) are commonly used ionization techniques
• Mössbauer spectroscopy is a technique specific to iron-containing compounds, providing information on the oxidation state and coordination environment of iron centers
• Electron paramagnetic resonance (EPR) spectroscopy is used to study organometallic complexes with unpaired electrons, such as paramagnetic metal centers or organic radicals

Real-World Applications and Current Research

• Organometallic compounds play crucial roles in various industrial processes, such as catalysis, materials science, and pharmaceutical synthesis
• Ziegler-Natta catalysts, based on titanium and aluminum alkyls, revolutionized the production of polyolefins (polyethylene and polypropylene)
  • These catalysts enable the synthesis of high-density, linear polymers with controlled properties
• Organometallic complexes are used in the development of organic light-emitting diodes (OLEDs) and photovoltaic cells
  • Iridium and platinum complexes are commonly employed as phosphorescent emitters in OLED displays
• Medicinal organometallic chemistry explores the use of organometallic compounds as therapeutic agents
  • Example: ferrocifen, an organometallic analog of the anticancer drug tamoxifen, showing promise in treating drug-resistant breast cancer
• Organometallic catalysts are being developed for the conversion of carbon dioxide (CO2) into value-added chemicals and fuels
  • Nickel and ruthenium complexes have been reported to catalyze the hydrogenation of CO2 to methanol or formic acid
• Research efforts are focused on developing more efficient and selective catalysts for C-H activation and functionalization
  • Directing groups and ligand design play key roles in achieving site-selectivity and enhancing reactivity
• Computational studies, such as density functional theory (DFT) calculations, are increasingly used to understand reaction mechanisms and guide the design of new organometallic catalysts