3.4 Synthesis and Reactions of Organometallic Compounds
8 min read•august 14, 2024
Organometallic compounds are key players in inorganic chemistry, featuring bonds between metals and carbon atoms. They come in various forms, from ionic to covalent, and their structure depends on the metal's coordination geometry and ligand properties.
Making organometallics involves methods like , , and . These compounds are super useful in organic synthesis, enabling new bond formation and serving as versatile intermediates in multi-step reactions.
Synthesis of organometallic compounds
Bonding and structure of organometallic compounds
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- Organometallic compounds contain at least one bond between a metal atom and a carbon atom of an organic molecule
- The metal-carbon bond can be ionic or covalent in character, depending on the electronegativity difference between the metal and carbon
- Ionic organometallic compounds (organolithium, ) feature a highly polarized metal-carbon bond, with a partially negative charge on the carbon atom
- Covalent organometallic compounds (ferrocene, Zeise's salt) have a more balanced electron distribution between the metal and carbon atoms
- The structure of organometallic compounds is influenced by the coordination geometry of the metal center and the steric and electronic properties of the ligands
- Common coordination geometries include linear, trigonal planar, tetrahedral, square planar, and octahedral
- The hapticity of a ligand (denoted as η) indicates the number of contiguous atoms in the ligand that are bonded to the metal center (η1, η2, η3, etc.)
Synthetic methods for preparing organometallic compounds
- Transmetallation is a reaction where an organometallic compound exchanges its organic group with another organometallic compound or a metal salt
- This method is used to prepare organometallic compounds with specific desired organic groups
- Example: the reaction of a Grignard reagent (RMgX) with a transition metal halide (MXn) to form a transition metal organometallic compound (RnM)
- Oxidative addition is a reaction where a metal inserts into a covalent bond of a substrate, increasing the oxidation state of the metal by two units
- This method can be used to prepare organometallic compounds from alkyl halides or hydrogen gas
- Example: the reaction of a low-valent metal complex (Mn) with an alkyl halide (R-X) to form an (Mn+2(R)(X))
- Direct metallation involves the reaction of a metal with an organic compound, typically involving C-H bond activation
- This method is used to prepare organometallic compounds directly from hydrocarbons
- Example: the reaction of a transition metal complex (MLn) with a hydrocarbon (R-H) to form a metal-alkyl complex (MLn-1(R))
- is the reverse of oxidative addition, where a high-valent organometallic compound eliminates two ligands to form a new covalent bond, reducing the oxidation state of the metal by two units
- This method can be used to prepare organometallic compounds with specific desired ligands
- Example: the elimination of an alkyl group and a halide from a high-valent metal complex (Mn+2(R)(X)) to form a metal complex (Mn) and an organic product (R-X)
- involve the substitution of one ligand for another on an organometallic compound
- This method is used to fine-tune the reactivity and selectivity of organometallic reagents
- Example: the replacement of a carbonyl ligand (CO) with a phosphine ligand (PR3) in a metal carbonyl complex (M(CO)n) to form a metal phosphine complex (M(CO)n-1(PR3))
Key reactions in organometallic chemistry
Ligand substitution reactions
- Ligand substitution reactions involve the exchange of one ligand for another on an organometallic compound
- The incoming ligand displaces the leaving ligand, typically following a dissociative or associative mechanism
- occurs when the leaving ligand departs first, forming an intermediate with a reduced coordination number, followed by the attachment of the incoming ligand
- occurs when the incoming ligand attaches first, forming an intermediate with an increased coordination number, followed by the departure of the leaving ligand
- The rate and mechanism of ligand substitution reactions depend on the metal center, its oxidation state, and the nature of the ligands
- First-order kinetics are observed for dissociative substitution, while second-order kinetics are observed for associative substitution
- Steric and electronic factors influence the relative rates of dissociative and associative substitution
- Example: the substitution of a carbonyl ligand (CO) with a phosphine ligand (PR3) in a metal carbonyl complex (M(CO)n) can proceed via a dissociative or associative mechanism, depending on the metal and the ligands
Oxidative addition and reductive elimination reactions
- Oxidative addition is a key reaction where a metal center inserts into a covalent bond of a substrate, increasing the oxidation state and coordination number of the metal
- This reaction is favored by low-valent, electron-rich metal centers and substrates with relatively weak covalent bonds
- Concerted oxidative addition occurs when the metal center simultaneously breaks the covalent bond of the substrate and forms two new metal-ligand bonds in a single step
- SN2-type oxidative addition occurs when the metal center attacks the substrate in an SN2-like manner, breaking the covalent bond and forming a new metal-ligand bond in a single step
- Example: the oxidative addition of an alkyl halide (R-X) to a low-valent metal complex (Mn) to form an oxidative addition product (Mn+2(R)(X))
- Reductive elimination is the reverse of oxidative addition, where a high-valent organometallic compound eliminates two ligands to form a new covalent bond, reducing the oxidation state and coordination number of the metal
- This reaction is favored by high-valent, electron-poor metal centers and ligands that can form stable covalent bonds
- Example: the reductive elimination of an alkyl group and a halide from a high-valent metal complex (Mn+2(R)(X)) to form a metal complex (Mn) and an organic product (R-X)
- reactions involve the insertion of an unsaturated ligand (e.g., CO, alkene, alkyne) into a metal-ligand bond
- This reaction increases the oxidation state of the metal and forms a new metal-carbon bond
- Example: the insertion of carbon monoxide (CO) into a metal-alkyl bond (M-R) to form an acyl complex (M-C(O)R)
Reactivity of organometallic compounds
Factors influencing reactivity
- The reactivity of organometallic compounds depends on the metal center, its oxidation state, and the nature of the ligands
- Electron-rich, low-valent metal centers are typically more reactive than electron-poor, high-valent metal centers
- The d-electron count of the metal center influences its reactivity and stability (e.g., 16-electron complexes are more reactive than 18-electron complexes)
- The steric and electronic properties of ligands can significantly influence the reactivity of organometallic compounds
- Bulky ligands can hinder certain reactions due to steric crowding, while electron-donating or electron-withdrawing ligands can modulate the electron density at the metal center
- The is a measure of the steric bulk of a ligand, which can be used to predict the likelihood of ligand dissociation and the stability of different coordination geometries
- Example: the use of bulky, electron-rich phosphine ligands (PR3) can stabilize low-valent metal centers and promote oxidative addition reactions
Predictive tools for organometallic reactivity
- The can be used to predict the stability and reactivity of organometallic compounds
- Compounds with 18 valence electrons are generally stable and less reactive, while compounds with fewer than 18 valence electrons are more reactive and prone to oxidative addition or ligand coordination
- Example: a 16-electron square planar complex (ML4) is more likely to undergo oxidative addition than an 18-electron octahedral complex (ML6)
- The can be used to predict the preferred geometry of four-coordinate, d8 metal complexes based on the relative π-acceptor strengths of the ligands
- Strong π-acceptor ligands favor a square planar geometry, while weak π-acceptor ligands favor a tetrahedral geometry
- Example: a d8 metal complex with two strong π-acceptor ligands (e.g., CO) and two weak π-acceptor ligands (e.g., Cl) is likely to adopt a square planar geometry
- Organometallic reactions often follow a characteristic sequence of elementary steps, such as oxidative addition, migratory insertion, and reductive elimination
- By identifying the likely elementary steps, the overall reaction outcome can be predicted
- Example: a catalytic cycle for a cross-coupling reaction might involve oxidative addition of an aryl halide (Ar-X) to a low-valent metal complex (Mn), transmetallation with an organometallic reagent (R-M'), migratory insertion to form a new C-C bond, and reductive elimination to release the coupled product (Ar-R) and regenerate the active catalyst (Mn)
Multi-step syntheses with organometallics
Organometallic compounds as versatile intermediates
- Organometallic compounds are versatile intermediates in organic synthesis, enabling the formation of new carbon-carbon and carbon-heteroatom bonds under mild conditions with high selectivity
- The reactivity of organometallic intermediates can be tuned by the choice of metal, ligands, and reaction conditions
- Organometallic intermediates can be used in a wide range of synthetic transformations, including , nucleophilic substitution, and
- Retrosynthetic analysis can be used to plan multi-step syntheses involving organometallic intermediates
- The target molecule is disconnected into simpler precursors, and organometallic reagents are chosen based on their reactivity and selectivity
- Example: the synthesis of a complex natural product might involve the disconnection of key C-C bonds, which can be formed using organometallic intermediates such as Grignard reagents or
Examples of organometallic reagents in organic synthesis
- Transition metal-catalyzed cross-coupling reactions, such as the Suzuki, Negishi, and Heck reactions, are powerful methods for forming new carbon-carbon bonds using organometallic intermediates
- These reactions typically involve a catalytic cycle of oxidative addition, transmetallation, and reductive elimination steps
- Example: the Suzuki coupling of an aryl halide (Ar-X) with an arylboronic acid (Ar-B(OH)2) using a palladium catalyst to form a biaryl product (Ar-Ar)
- Grignard reagents (organomagnesium compounds) are widely used in organic synthesis for the formation of new carbon-carbon bonds and the introduction of functional groups
- Grignard reagents can be used in nucleophilic addition reactions with carbonyl compounds, followed by hydrolysis to form alcohols
- Example: the reaction of a Grignard reagent (RMgX) with a ketone (R'C(O)R'') to form a tertiary alcohol (R'R''C(OH)R) after hydrolysis
- Organolithium reagents are highly reactive organometallic compounds that can be used in nucleophilic addition reactions, nucleophilic substitution reactions, and metal-halogen exchange reactions
- The high reactivity of organolithium reagents often requires low temperatures and strict moisture and air exclusion
- Example: the reaction of an organolithium compound (RLi) with a ketone (R'C(O)R'') to form a tertiary alcohol (R'R''C(OH)R) after hydrolysis
- Gilman reagents (organocuprates) are less reactive than organolithium reagents but offer greater selectivity in organic synthesis
- Gilman reagents can be used in conjugate addition reactions, nucleophilic substitution reactions, and cross-coupling reactions
- Example: the conjugate addition of a Gilman reagent (R2CuLi) to an α,β-unsaturated ketone (R'C(O)CH=CHR'') to form a 1,4-addition product (R'C(O)CH(R)CH2R'')
- Transition metal hydrides, such as borohydrides and aluminum hydrides, are valuable reducing agents in organic synthesis
- These organometallic reagents can selectively reduce functional groups, such as carbonyl compounds, esters, and nitriles, under mild conditions
- Example: the reduction of an ester (RC(O)OR') with lithium aluminum hydride (LiAlH4) to form a primary alcohol (RCH2OH)
Key Terms to Review (26)
18-electron rule: The 18-electron rule is a guideline used in organometallic chemistry that suggests stable transition metal complexes tend to have a total of 18 valence electrons. This rule is based on the idea that a full octet around the central metal, combined with contributions from its ligands, leads to enhanced stability in these compounds.
Aldehydes: Aldehydes are organic compounds characterized by the presence of a carbonyl group (C=O) attached to at least one hydrogen atom. This functional group is typically found at the end of a carbon chain, making aldehydes distinct from other carbonyl-containing compounds like ketones. Their reactivity is significant in organic chemistry, particularly in synthesis and reactions involving organometallic compounds.
Associative substitution: Associative substitution is a type of ligand substitution mechanism where a new ligand associates with the metal center before the original ligand departs, effectively forming a transition state in which both ligands are temporarily bound. This process is significant in the study of organometallic compounds, as it helps explain how metal-ligand interactions can influence reactivity and stability.
Bidentate Ligands: Bidentate ligands are molecules or ions that can form two bonds to a central metal atom or ion, thus bridging the metal with multiple coordination sites. This dual binding capability often enhances the stability of the resulting complex, and bidentate ligands can significantly influence properties such as isomerism, reaction mechanisms, and biological activity.
Caulton-Eisenstein Rules: The Caulton-Eisenstein rules are a set of guidelines that assist in predicting the reactivity and stability of organometallic compounds based on their electronic and steric properties. These rules help in understanding how different organometallic compounds will behave in chemical reactions, particularly in substitution reactions, and how they can be synthesized effectively. By considering factors like the nature of the metal center and the ligands attached, these rules streamline the process of designing and working with various organometallic compounds.
Chelating Agents: Chelating agents are molecules that can form multiple bonds to a single metal ion, effectively 'grabbing' onto the metal and forming a stable complex. This ability to bind metals in a stable manner is crucial for various applications, including the design of coordination compounds, the development of medicinal treatments, and the synthesis of organometallic compounds. Chelating agents play a vital role in controlling metal reactivity and solubility, influencing isomerism, therapeutic functions, and catalytic processes.
Cross-coupling reactions: Cross-coupling reactions are a type of chemical reaction that allows for the formation of new carbon-carbon or carbon-heteroatom bonds by coupling two distinct fragments, typically facilitated by transition metal catalysts. These reactions are significant in synthesizing complex organic molecules and play a crucial role in the field of organometallic chemistry, where the interaction between organometallic compounds and organic substrates leads to the creation of valuable products.
Direct metallation: Direct metallation is a chemical process where a metal species is directly introduced into an organic molecule by replacing a hydrogen atom. This method allows for the selective introduction of metal centers into organic substrates, making it an essential technique in the synthesis of organometallic compounds. By enabling precise modifications of organic frameworks, direct metallation plays a crucial role in creating functionalized materials and enhancing reactivity in subsequent chemical transformations.
Dissociative Substitution: Dissociative substitution is a mechanism of ligand exchange where a leaving group departs from a coordination complex before a new ligand binds to the metal center. This process involves the formation of a transient intermediate that contains an empty coordination site, allowing for the incoming ligand to attach subsequently. Understanding this mechanism is crucial for studying how organometallic compounds behave and react during synthesis and transformations.
Electrophilicity: Electrophilicity refers to the ability of a species to accept electrons during a chemical reaction. This property is crucial in understanding the behavior of electrophiles, which are typically positively charged or neutral molecules with vacant orbitals that can engage in reactions with nucleophiles, which donate electrons. The degree of electrophilicity influences the reactivity and stability of organometallic compounds, impacting their synthesis and reactions.
Grignard Reagents: Grignard reagents are a class of organometallic compounds formed by the reaction of an alkyl or aryl halide with magnesium in an ether solvent, resulting in compounds that contain a carbon-magnesium bond. These highly reactive reagents play a crucial role in organic synthesis by acting as nucleophiles, enabling the formation of new carbon-carbon bonds and allowing for the construction of complex organic molecules.
Ketones: Ketones are a class of organic compounds characterized by a carbonyl group (C=O) bonded to two other carbon atoms. They play a significant role in organic chemistry due to their reactivity and the unique properties that arise from the presence of this functional group. Ketones are often involved in various synthesis reactions and can be produced through the oxidation of secondary alcohols or through the reactions of organometallic compounds.
Kinetic Stability: Kinetic stability refers to the tendency of a chemical species, such as a coordination compound or an organometallic compound, to resist change or decomposition over time due to the energy barrier associated with reactions. It indicates how long a compound can exist without undergoing a reaction, which is influenced by factors such as bond strength and the nature of ligands. Understanding kinetic stability helps explain the reactivity and behavior of coordination and organometallic compounds in various chemical contexts.
Ligand exchange reactions: Ligand exchange reactions are processes where one ligand in a coordination complex is replaced by another ligand. These reactions are essential for understanding the behavior of organometallic compounds, as they influence the reactivity and stability of metal centers. Ligand exchange can significantly affect the electronic properties of the metal, altering its catalytic activity, and is a key mechanism in many synthetic and biochemical pathways involving metal complexes.
Migratory insertion: Migratory insertion is a chemical process in which a ligand, typically an alkyl or aryl group, moves from a metal center to an adjacent bond, usually involving a metal-ligand complex. This process is crucial in various organometallic reactions, facilitating the formation of new bonds and altering the structure of the complex. It plays a significant role in understanding how organometallic compounds can be synthesized and utilized in various applications.
Nucleophilic addition: Nucleophilic addition is a fundamental reaction mechanism in which a nucleophile attacks an electrophilic carbon atom, resulting in the formation of a new covalent bond. This process is crucial in organic and organometallic chemistry, as it enables the synthesis of various compounds by converting carbonyl groups and other electrophiles into alcohols, amines, or other functional groups. It plays a key role in the reactivity of organometallic compounds, which often act as strong nucleophiles due to their metal-carbon bonds.
Nucleophilicity: Nucleophilicity refers to the ability of a species to donate an electron pair to form a chemical bond, typically with an electrophile. This property is essential in many chemical reactions, particularly in the context of organometallic compounds, where nucleophiles can interact with metals or other electrophilic centers. Understanding nucleophilicity helps in predicting the outcomes of reactions and the stability of intermediates formed during these processes.
Organolithium compounds: Organolithium compounds are a class of organometallic chemicals that contain a carbon-lithium bond, where lithium is bonded to a carbon atom of an organic group. These compounds are highly reactive and play a crucial role in various synthetic reactions, particularly in the formation of new carbon-carbon bonds. Their reactivity and unique properties make them valuable in organic synthesis, allowing chemists to create complex molecules.
Oxidative Addition: Oxidative addition is a fundamental reaction mechanism in organometallic chemistry where a metal center increases its oxidation state by forming new bonds with a substrate, typically involving the addition of two substituents to the metal. This process is crucial for various transformations and plays a significant role in bonding interactions, as well as enabling further chemical reactivity. By allowing the metal to expand its coordination sphere, oxidative addition facilitates the formation of important intermediates that are essential for many synthetic pathways.
Oxidative Addition Product: An oxidative addition product is a complex formed when a transition metal catalyst reacts with a substrate, resulting in the addition of two ligands to the metal center. This process typically involves the oxidation of the metal, increasing its oxidation state, and it plays a crucial role in various synthetic pathways, particularly in the context of organometallic chemistry.
Reductive elimination: Reductive elimination is a fundamental reaction mechanism in organometallic chemistry where a metal complex undergoes a transformation, resulting in the removal of ligands and the formation of a product with lower oxidation states. This process is crucial in the synthesis and reactivity of organometallic compounds, allowing for the generation of new carbon-carbon or carbon-heteroatom bonds.
Thermodynamic stability: Thermodynamic stability refers to the stability of a system in terms of its energy state, where a lower energy state indicates greater stability. In the context of chemical compounds, particularly coordination compounds and organometallics, thermodynamic stability is essential as it influences reaction pathways, ligand binding, and overall reactivity. Compounds that are thermodynamically stable tend to resist change and do not easily undergo transformations under standard conditions.
Tolman Cone Angle: The Tolman cone angle is a measure of the steric bulk of ligands in coordination complexes, representing the angle formed by the lines connecting the metal center to the edges of a hypothetical cone that extends from the metal atom. This concept is crucial in assessing how the spatial arrangement of ligands around a metal center can influence reactivity, stability, and the overall geometry of organometallic compounds. Understanding this angle helps chemists predict how changes in ligand size or shape affect the properties and behavior of these compounds.
Transmetallation: Transmetallation is a chemical reaction where a metal atom or ion is transferred from one compound to another, often involving organometallic complexes. This process plays a crucial role in the reactivity and synthesis of various organometallic compounds, allowing for the exchange of metals and leading to the formation of new compounds with different metal centers. It is essential for understanding how different metal species can be interconverted during chemical reactions.
Wilkinson's Catalyst: Wilkinson's Catalyst is a coordination complex of rhodium, specifically RhCl(PPh₃)₃, that is widely used as a catalyst in the hydrogenation of alkenes and alkynes. This catalyst is known for its ability to facilitate reactions under mild conditions, making it an important tool in organometallic chemistry and synthetic organic chemistry. Its selectivity and efficiency in hydrogenation reactions have made it a standard in the field, helping to produce a variety of valuable organic compounds.
Ziegler-Natta catalyst: A Ziegler-Natta catalyst is a type of coordination catalyst used primarily in the polymerization of alkenes, such as ethylene and propylene, to produce high-density polyethylene and polypropylene. This catalyst typically consists of a transition metal compound, usually titanium, combined with an organoaluminum compound, allowing for the control of polymer structure and properties during synthesis. Its invention revolutionized the production of polymers and led to significant advancements in industrial applications.