Organometallic compounds feature unique bonding between metals and carbon-containing ligands. This section dives into hapticity, which describes how many atoms in a ligand bind to a metal, and various bonding modes like π and σ interactions.

Understanding these bonding types is crucial for grasping organometallic chemistry. We'll explore common ligands, metal-carbon bonds, and specialized interactions like agostic bonds, connecting these concepts to the broader study of organometallic structures and bonding.

Hapticity and Bonding Types

Understanding Hapticity and Notation

• Hapticity describes the number of contiguous atoms in a ligand bound to a metal center
• η (eta) notation indicates the number of atoms in a ligand directly bonded to the metal
• Monohapto ligands (η1) bond to the metal through a single atom
• Polyhapto ligands (η2, η3, etc.) bond to the metal through multiple contiguous atoms
• η notation applies to both σ and π bonding modes

Bonding Modes in Organometallic Compounds

• π-bonding occurs when electrons are shared between a metal and a ligand's π-system
• π-complexes form when metal d-orbitals interact with ligand π-orbitals
• σ-bonding involves the direct overlap of a metal orbital with a ligand orbital
• σ-bonds in organometallic compounds typically form between a metal and carbon or hydrogen
• Mixed bonding modes can occur in some complexes (ferrocene exhibits both σ and π bonding)

Examples of Hapticity in Common Ligands

• Cyclopentadienyl (Cp) ligand can bond in η1 (sigma) or η5 (pi) modes
• Benzene can form η6 complexes with transition metals (chromium tricarbonyl)
• Allyl ligands often bond in an η3 fashion (allyl palladium chloride dimer)
• Ethylene forms η2 complexes through its π-bond (Zeise's salt)

Metal-Ligand Interactions

Types of Metal-Carbon Bonds

• Metal-carbon σ bonds form through direct overlap of metal and carbon orbitals
• Alkyl complexes exhibit strong M-C σ bonds (methyllithium, dimethylzinc)
• Metal-carbon π bonds involve the interaction of metal d-orbitals with ligand π-systems
• Carbene complexes feature a metal-carbon double bond (Schrock and Fischer carbenes)
• Carbyne complexes contain a metal-carbon triple bond

Specialized Metal-Ligand Interactions

• Agostic interactions involve the coordination of C-H bonds to a metal center
• Three-center two-electron bonds form in agostic interactions
• Bridging ligands connect two or more metal centers
• Carbonyl ligands (CO) bond to metals through σ-donation and π-backbonding
• Olefin complexes form through the interaction of the C=C π-bond with the metal

Bonding in Specific Organometallic Compounds

• Metal carbonyls exhibit strong metal-carbon bonds through synergistic bonding
• Ferrocene features η5 bonding of two cyclopentadienyl rings to iron
• Grignard reagents contain a polar covalent metal-carbon bond
• Wilkinson's catalyst includes both phosphine and chloride ligands bound to rhodium

Organometallic Compounds and Rules

Metallocene Compounds and Their Properties

• Metallocenes consist of a metal atom sandwiched between two cyclopentadienyl rings
• Ferrocene serves as the prototypical metallocene compound
• Metallocene structure provides stability and unique reactivity
• Bent metallocenes form when one Cp ring is replaced with other ligands
• Metallocene derivatives find applications in catalysis and materials science

The 18-Electron Rule and Its Applications

• 18-electron rule predicts stability in transition metal complexes
• Counts valence electrons from the metal and all ligands
• Stable complexes typically have 18 valence electrons (nickel tetracarbonyl)
• Exceptions exist for early transition metals and certain geometries
• Rule helps in predicting reactivity and designing new complexes

Examples and Exceptions to Organometallic Rules

• Ferrocene follows the 18-electron rule (18 valence electrons)
• Titanocene dichloride is stable with only 16 electrons
• Square planar complexes often have 16 electrons (Vaska's complex)
• Some reactive intermediates have fewer than 18 electrons (14-electron Pt(II) complexes)
• Hypervalent compounds can exceed 18 electrons (19-electron cobaltocene)