Organometallic compounds feature unique metal-carbon bonds. These can be σ or π bonds, with varying strengths and properties. Understanding these bonds is crucial for grasping how organometallics behave and react.

Hapticity, back-bonding, and ligand effects all play key roles in organometallic bonding. These concepts help explain the stability, reactivity, and properties of these compounds. Molecular orbital theory provides a deeper understanding of these interactions.

Bonding in Organometallic Compounds

Types of Metal-Carbon Bonds

• Organometallic compounds contain at least one metal-carbon bond, which can be either a σ bond or a π bond
• σ bonds are formed by the overlap of a metal orbital with a carbon orbital, resulting in a single bond with electron density concentrated between the two atoms
• π bonds are formed by the sideways overlap of a metal d orbital with a carbon p orbital, resulting in a bond with electron density above and below the bond axis
• Metal-carbon multiple bonds can exist, such as metal carbenes (M=C) and metal carbynes (M≡C), which involve a combination of σ and π bonding

Hapticity and Bonding Modes

• The hapticity of a ligand refers to the number of contiguous atoms in the ligand that are bonded to the metal, with the η (eta) notation used to describe the bonding mode (η2, η3, η4, η5, η6)
• Examples of hapticity include η2-alkene, η3-allyl, η4-butadiene, η5-cyclopentadienyl, and η6-benzene ligands
• The bonding mode can influence the reactivity and stability of the organometallic complex
• Higher hapticity ligands generally provide greater stability to the metal center due to increased electron donation and steric protection

Back-bonding in Organometallic Complexes

Concept and Significance of Back-bonding

• Back-bonding, also known as π back-bonding, occurs when electron density is donated from a filled metal d orbital to an empty π orbital of a ligand
• Back-bonding is most significant when the metal is in a low oxidation state and has a high electron density, and the ligand has low-lying empty π orbitals (CO, CN-, alkenes, alkynes)
• The strength of back-bonding depends on the metal (more electron-rich metals exhibit stronger back-bonding), the ligand (ligands with lower-energy π orbitals accept more back-bonding), and the ancillary ligands (electron-donating ligands enhance back-bonding)
• Back-bonding stabilizes organometallic complexes by reducing the electron density on the metal, strengthening the metal-ligand bond, and increasing the ligand's ability to accept electron density

Effects of Back-bonding on Properties

• The concept of back-bonding helps explain trends in bond lengths, vibrational frequencies, and NMR chemical shifts of ligands in organometallic complexes
• Stronger back-bonding leads to shorter metal-ligand bond lengths, lower vibrational frequencies (CO stretching), and increased shielding of the ligand nuclei (upfield NMR shifts)
• Examples of back-bonding include the decreased CO stretching frequency in Ni(CO)4 compared to free CO, and the upfield 13C NMR shift of the carbene carbon in Schrock carbene complexes

Ligand Influence on Organometallics

Ligand Classification and Properties

• Ligands can be classified as L-type (two-electron donors), X-type (one-electron donors), or Z-type (two-electron acceptors) based on their bonding and electron-counting contributions
• The electronic and steric properties of ligands significantly influence the bonding and reactivity of organometallic compounds
• Electron-donating ligands (alkyl, aryl, amine, phosphine) increase electron density on the metal, favoring low oxidation states and promoting back-bonding to π-acidic ligands
• Electron-withdrawing ligands (halides, CN-, CO) decrease electron density on the metal, favoring high oxidation states and reducing back-bonding

Steric Effects and the Trans Effect

• The steric bulk of ligands can affect the stability and reactivity of organometallic compounds by influencing the coordination geometry, protecting the metal center, and modulating the accessibility of reactive sites
• Bulky ligands (tert-butyl, adamantyl) can stabilize low-coordinate complexes, while smaller ligands (methyl, ethyl) allow for higher coordination numbers
• The trans effect, where ligands trans to each other influence each other's bonding and reactivity, is an important consideration in the design and synthesis of organometallic complexes
• Strong σ-donor ligands (H-, alkyl, carbenes) and strong π-acceptor ligands (CO, CN-) exhibit a strong trans effect, labilizing the ligand trans to them

Molecular Orbital Theory for Organometallics

Bonding and Antibonding Orbitals

• Molecular orbital (MO) theory describes the bonding in organometallic complexes in terms of the formation of bonding and antibonding orbitals through the combination of metal and ligand orbitals
• The symmetry and energy of the metal and ligand orbitals determine the type and strength of the interactions, leading to the formation of σ, π, and δ bonding and antibonding orbitals
• The relative energies of the metal and ligand orbitals influence the degree of orbital mixing and the resulting MO diagram, which can be used to predict the electronic configuration, magnetic properties, and spectroscopic features of the complex

18-Electron Rule and Electronic Spectra

• The 18-electron rule, which states that stable organometallic complexes often have 18 valence electrons, can be rationalized using MO theory by considering the occupation of the available bonding and nonbonding orbitals
• Complexes with 18 valence electrons have a closed-shell configuration, with all bonding and nonbonding orbitals filled, leading to increased stability
• MO theory can be used to explain the electronic spectra of organometallic complexes, including the presence of metal-to-ligand charge transfer (MLCT) and ligand-to-metal charge transfer (LMCT) transitions
• MLCT transitions involve the excitation of an electron from a metal-based orbital to a ligand-based orbital, while LMCT transitions involve the excitation of an electron from a ligand-based orbital to a metal-based orbital

Computational Methods

• Computational methods, such as density functional theory (DFT), can be employed to generate MO diagrams and gain insights into the electronic structure and bonding of organometallic complexes
• DFT calculations can provide information on the relative energies and compositions of the molecular orbitals, as well as the charge distribution and bond orders within the complex
• Examples of computational studies include the analysis of the bonding in ferrocene (Fe(C5H5)2) and the investigation of the mechanism of olefin metathesis catalyzed by Grubbs catalysts