Chapter: Organometallic Chemistry

1. Introduction to Organometallic Chemistry

• Definition: Organometallic compounds are chemical compounds that contain at least one chemical bond between a carbon atom of an organic molecule and a metal atom. The metal can be a main group metal (e.g., Li, Mg, Al, Sn, Si, Pb) or a transition metal (e.g., Fe, Ni, Pd, Rh, Pt, Ti, Cu).
• Importance: Organometallic chemistry is a cornerstone of modern organic synthesis and catalysis. Organometallic reagents allow for the efficient formation of carbon-carbon and carbon-heteroatom bonds, which are crucial for constructing complex molecules. They are also vital in industrial processes, including polymerization, pharmaceutical production, and material science.
• Metal-Carbon Bond Character: The nature of the metal-carbon bond varies significantly depending on the electronegativity difference between the metal and carbon.
  • Highly Ionic: When the metal is very electropositive (e.g., alkali metals like Li, Na), the carbon atom carries a significant negative charge, making it highly nucleophilic and basic. (Mδ−Cδ).
  • Covalent with Significant Polarity: For less electropositive metals (e.g., Mg, Zn, Cd, Al), the bond is more covalent but still highly polarized towards carbon (Mδ+−Cδ−).
  • Covalent/Non-polar: With more electronegative or transition metals, the bond can be more purely covalent.

2. Organolithium Reagents (RLi)

• Preparation: Organolithium reagents are typically prepared by the reaction of an alkyl or aryl halide with lithium metal, often in a non-polar solvent like hexane or an ethereal solvent like diethyl ether or THF.
  R-X+2Liether or hexane​R-Li+LiX (Where R can be alkyl, aryl, vinyl; X can be Cl, Br, I)
• Properties:
  • Strong Bases: Due to the high partial negative charge on carbon, organolithium reagents are extremely strong bases, capable of deprotonating even weakly acidic protons (e.g., terminal alkynes, alcohols, water).
  • Strong Nucleophiles: The carbanionic character makes them potent nucleophiles, readily attacking electrophilic centers.
  • Highly Reactive: They are very sensitive to moisture and oxygen and must be handled under inert atmosphere.
• Reactions:
  • Alkylation of Carbonyl Compounds: Addition to aldehydes and ketones to form alcohols. Addition to esters and acid chlorides can lead to ketones (with careful control) or tertiary alcohols. Reaction with CO2​ forms carboxylic acids.
  • Epoxide Opening: Nucleophilic attack on epoxides to form alcohols.
  • Deprotonation: Used as strong bases (e.g., n-butyllithium) for metallation reactions.
  • Halogen-Metal Exchange: Can exchange a halogen on an aryl or vinyl halide for a lithium atom.

3. Grignard Reagents (RMgX)

• Preparation: Grignard reagents are formed by the reaction of an alkyl or aryl halide with magnesium metal, usually in an ethereal solvent (e.g., diethyl ether, THF). The solvent coordinates to the magnesium, stabilizing the reagent.
  R-X+Mgether or THF​R-MgX (Where R can be alkyl, aryl, vinyl; X can be Cl, Br, I; typically Br or I)
• Properties:
  • Strong Bases: Similar to organolithium reagents, Grignard reagents are very strong bases (stronger than hydroxide or alkoxide), reacting readily with protic sources.
  • Strong Nucleophiles: The carbon-magnesium bond is highly polarized, making the carbon a good nucleophile.
  • Sensitive: Must be prepared and handled under anhydrous conditions and an inert atmosphere (nitrogen or argon) to prevent reaction with water and oxygen.
• Reactions:
  • Alkylation of Carbonyl Compounds:
    • Aldehydes → secondary alcohols
    • Ketones → tertiary alcohols
    • Esters → tertiary alcohols (via a ketone intermediate that reacts further)
    • Formaldehyde → primary alcohols
    • CO2​→ carboxylic acids (after hydrolysis)
  • Epoxide Opening: Nucleophilic attack on epoxides to form alcohols, opening the less hindered side.
  • Reaction with Active Hydrogens: Reacts immediately with any compound containing an “acidic” or “active” hydrogen (e.g., O-H, N-H, S-H, C$\equiv$C-H). This is a limiting side reaction if such groups are present in the substrate.

4. Organocuprate Reagents (Gilman Reagents, R2​CuLi)

• Preparation: Organocuprates are prepared by reacting an organolithium reagent with cuprous iodide (CuI) in diethyl ether or THF.
  2RLi+CuIether or THF​R2​CuLi+LiI
• Properties:
  • “Soft” Nucleophiles: Unlike Grignard and organolithium reagents, organocuprates are considered “soft” nucleophiles. This means they are less reactive as bases and show a strong preference for conjugate addition (1,4-addition) over direct addition (1,2-addition) to α,β-unsaturated carbonyl compounds.
  • Less Basic, More Selective: Their softer nature makes them more selective nucleophiles, allowing for reactions that would be complicated by basicity or excessive reactivity with other organometallics.
• Reactions:
  • Conjugate Addition (1,4-Addition): To α,β-unsaturated ketones, aldehydes, and esters. This is their most characteristic reaction, adding the R group to the β-carbon and forming an enolate, which is then protonated.
  • Corey-House Synthesis (Coupling Reactions): Reaction with primary alkyl halides (or sometimes secondary) to form new C-C bonds, producing alkanes. This is a very useful method for synthesizing unsymmetrical alkanes.
  • Displacement of Halogens: Can displace halogens from vinyl and aryl halides to form new C-C bonds.

5. Other Important Main Group Organometallics

• Organozinc Reagents (RZnX or R2​Zn):
  • Less reactive than Grignards and organolithiums.
  • Reformatsky Reaction: Used to synthesize β-hydroxy esters from α-halo esters and aldehydes/ketones.
  • Simmons-Smith Reaction: Used for cyclopropanation of alkenes using diiodomethane (CH2​I2​) and zinc-copper couple (Zn(Cu)). Forms a carbene-like species (iodomethylzinc iodide, ICH2​ZnI) that adds to the double bond in a stereospecific syn fashion.
• Organoaluminum Reagents (R3​Al):
  • Strong reducing agents (e.g., DIBAL-H, diisobutylaluminum hydride).
  • Used in polymerization (Ziegler-Natta catalysis, with TiCl4​).
• Organotin Reagents (R4​Sn or R3​SnX):
  • Often used in Stille coupling reactions (transition metal catalyzed coupling).
• Organosilicon Reagents (R4​Si or R3​SiX):
  • Often used as protecting groups (e.g., for alcohols).
  • Used in cross-coupling reactions (Hiyama coupling).

6. Organotransition Metal Chemistry (Brief Overview)

• Ligands: Transition metals form bonds with a wide variety of ligands (molecules or ions that donate electrons to the metal). Common ligands include:
  • Carbon Monoxide (CO): Forms metal carbonyls (e.g., Ni(CO)4​).
  • Alkenes and Alkynes: π-donation from the C=C or C$\equiv$C bond to the metal.
  • Phosphines (PR3​): Excellent σ-donors and π-acceptors.
  • Cyclopentadienyl (Cp, C5​H5−​): A planar aromatic ligand that can bind to metals in a η5 (pentahapto) fashion (e.g., Ferrocene, FeCp2​).
• 18-Electron Rule: A useful guideline for predicting the stability of transition metal organometallic complexes. It states that stable diamagnetic complexes tend to have 18 valence electrons (counting the metal’s d-electrons plus electrons donated by ligands). This rule is analogous to the octet rule for main group elements.
• Fundamental Reaction Types (Catalytic Cycles): These are the elementary steps that frequently occur in transition metal catalyzed reactions.
  • Oxidative Addition: A substrate adds across a metal center, increasing the metal’s oxidation state by +2 and its coordination number by +2.
  • Reductive Elimination: The reverse of oxidative addition; two ligands on the metal couple together and depart, decreasing the metal’s oxidation state by -2 and coordination number by -2. Forms a new bond.
  • Migratory Insertion (or Carbonyl Insertion): An alkyl or aryl group migrates from the metal to a coordinated ligand (often CO or an alkene), forming a new M-C or C-C bond and decreasing the coordination number by 1.
  • β-Hydride Elimination: A hydrogen atom on a carbon β to the metal migrates to the metal, forming a metal-hydride bond and an alkene. This often occurs when a metal-alkyl bond is formed and can be a side reaction.
  • Ligand Exchange: One ligand is replaced by another.
• Key Catalytic Reactions:
  • Hydrogenation (e.g., Wilkinson’s Catalyst, RhCl(PPh3​)3​): Adds H2​ across C=C double bonds.
  • Heck Reaction: Palladium-catalyzed coupling of aryl/vinyl halides with alkenes. Forms new C-C bonds with stereocontrol.
  • Suzuki-Miyaura Reaction: Palladium-catalyzed coupling of aryl/vinyl halides with organoboron reagents. Versatile for C-C bond formation.
  • Sonogashira Coupling: Palladium/copper-catalyzed coupling of aryl/vinyl halides with terminal alkynes.
  • Stille Coupling: Palladium-catalyzed coupling of aryl/vinyl halides with organotin reagents.
  • Hydroformylation (Oxo Process): Adds H and CHO across an alkene using CO and H2​ with a cobalt or rhodium catalyst, producing aldehydes.
  • Polymerization (Ziegler-Natta): Formation of stereoregular polymers (e.g., polyethylene, polypropylene) using titanium and aluminum catalysts.

7. Applications in Organic Synthesis

• C-C Bond Formation: The most fundamental application, enabling the synthesis of complex carbon skeletons.
• Stereoselective Reactions: Many organometallic reactions can be made stereoselective or even stereospecific by using chiral ligands or specific reaction conditions, allowing chemists to control the configuration of new chiral centers.
• Functional Group Tolerance: Different organometallic reagents have varying tolerance for other functional groups, allowing for selective transformations.
• Industrial Applications: Large-scale production of polymers, pharmaceuticals, fine chemicals, and agrochemicals.

Multiple Choice Questions (MCQ) on Organometallic Chemistry

Instructions: Choose the best answer for each question.

1. What is the defining characteristic of an organometallic compound? a) It contains a bond between a metal and oxygen. b) It contains a bond between a carbon atom and a metal atom. c) It contains a metal complex with no carbon. d) It is an inorganic compound.

2. Which type of metal-carbon bond is typically described as having significant ionic character due to a large electronegativity difference? a) Carbon-silicon bond. b) Carbon-tin bond. c) Carbon-lithium bond. d) Carbon-platinum bond.

3. How are organolithium reagents typically prepared? a) By reacting an alkyl halide with magnesium metal. b) By reacting an alcohol with lithium metal. c) By reacting an alkyl or aryl halide with lithium metal. d) By reacting an alkene with lithium hydride.

4. Which of the following is a key property of organolithium reagents regarding their reactivity? a) They are weak bases and weak nucleophiles. b) They are strong acids and strong electrophiles. c) They are strong bases and strong nucleophiles. d) They are unreactive with protic solvents.

5. What is the product when an organolithium reagent reacts with an aldehyde? a) A carboxylic acid. b) A primary alcohol. c) A secondary alcohol. d) A tertiary alcohol.

6. What is the typical solvent used for the preparation and reactions of Grignard reagents? a) Water. b) Ethanol. c) Diethyl ether or THF. d) Hexane.

7. Why must Grignard reagents be handled under anhydrous conditions? a) They are sensitive to light. b) They react vigorously with water. c) They decompose in the presence of oxygen. d) They polymerize in humid air.

8. What is the main product when a Grignard reagent reacts with a ketone? a) A primary alcohol. b) A secondary alcohol. c) A tertiary alcohol. d) An aldehyde.

9. Reaction of a Grignard reagent with carbon dioxide (CO2​) followed by hydrolysis yields what functional group? a) Aldehyde. b) Ketone. c) Carboxylic acid. d) Ester.

10. How are Gilman reagents (organocuprates) typically prepared? a) By reacting a Grignard reagent with magnesium iodide. b) By reacting an organolithium reagent with cuprous iodide. c) By reacting an alkyl halide with copper metal. d) By reacting an alkene with copper salt.

11. Which of the following best describes the nucleophilic character of organocuprate reagents compared to Grignard or organolithium reagents? a) They are harder nucleophiles. b) They are stronger bases. c) They are softer nucleophiles. d) They are more electrophilic.

12. What is the characteristic reaction of organocuprates with α,β-unsaturated carbonyl compounds? a) Direct 1,2-addition. b) Elimination reaction. c) Conjugate 1,4-addition. d) Rearrangement.

13. The Corey-House synthesis, which forms new C-C bonds by coupling organocuprates with alkyl halides, is most effective with which type of alkyl halide? a) Tertiary alkyl halides. b) Secondary alkyl halides. c) Primary alkyl halides. d) Aryl halides.

14. The Reformatsky reaction uses an organozinc reagent to synthesize which type of product? a) Alkynes. b) β-hydroxy esters. c) Cyclopropanes. d) Aldehydes.

15. Which organometallic reaction is used for the stereospecific syn-addition to alkenes to form cyclopropanes? a) Reformatsky reaction. b) Grignard addition. c) Simmons-Smith reaction. d) Corey-House synthesis.

16. What is the typical oxidation state change of the metal in an oxidative addition reaction? a) Increases by +1. b) Decreases by -2. c) Increases by +2. d) Remains unchanged.

17. Which elementary step in transition metal catalysis involves the formation of a new C-C bond by an alkyl or aryl group migrating to a coordinated ligand (e.g., CO)? a) Reductive elimination. b) Oxidative addition. c) β-hydride elimination. d) Migratory insertion.

18. What is the general guideline for the number of valence electrons in stable diamagnetic transition metal complexes? a) 8-electron rule. b) 12-electron rule. c) 16-electron rule. d) 18-electron rule.

19. Which of the following is a common side reaction that can occur when an alkyl group is attached to a transition metal, leading to alkene formation? a) Oxidative addition. b) Reductive elimination. c) β-hydride elimination. d) Migratory insertion.

20. Wilkinson’s catalyst, RhCl(PPh3​)3​, is famous for its role in which reaction? a) Heck reaction. b) Hydrogenation of alkenes. c) Hydroformylation. d) Suzuki coupling.

21. Which type of cross-coupling reaction uses organoboron reagents with palladium catalysts? a) Heck reaction. b) Stille coupling. c) Suzuki-Miyaura reaction. d) Sonogashira coupling.

22. The Hydroformylation (Oxo) process is used to convert alkenes into which type of compound? a) Alcohols. b) Carboxylic acids. c) Aldehydes. d) Ketones.

23. Which industrial process for polymerization of alkenes utilizes titanium and aluminum organometallic catalysts? a) Friedel-Crafts. b) Ziegler-Natta. c) Fischer-Tropsch. d) Wacker process.

24. Organometallic reagents are most widely used in organic synthesis for the formation of what type of bonds? a) C-H bonds. b) O-H bonds. c) C-C bonds. d) N-H bonds.

25. Which of the following functional groups will not react with a Grignard reagent? a) Ketone. b) Alcohol. c) Aldehyde. d) Alkane.

26. If an alkyl halide is desired as a reactant in a reaction with a Grignard reagent, but the alkyl halide also contains an acidic proton (e.g., an -OH group), what might be a common strategy to prevent side reactions? a) Use a higher temperature. b) Use a different solvent. c) Protect the acidic proton first. d) Add excess Grignard reagent.

27. What is the main advantage of using organocuprates for 1,4-addition compared to Grignard or organolithium reagents? a) They are stronger bases. b) They are more reactive. c) They exhibit better regioselectivity for conjugate addition. d) They are less expensive.

28. Which organometallic compound is often used as a very strong, non-nucleophilic base for deprotonation reactions? a) Methylmagnesium bromide. b) Lithium dimethylcuprate. c) n-Butyllithium. d) Tetramethyltin.

29. What is the reverse reaction of oxidative addition in transition metal chemistry? a) Migratory insertion. b) Reductive elimination. c) β-hydride elimination. d) Ligand exchange.

30. Which ligand commonly forms stable complexes with transition metals by donating electrons from its C$\equiv$O triple bond? a) Alkene. b) Phosphine. c) Carbonyl (CO). d) Water.

31. Ferrocene, a famous organometallic compound, features a central iron atom bonded to which type of ligand? a) Alkyl groups. b) Carbonyl groups. c) Cyclopentadienyl rings. d) Halides.

32. The relative reactivity of organometallic reagents often correlates with the electronegativity difference between the metal and carbon. Which order correctly lists reagents from most ionic (most reactive nucleophile/base) to least ionic? a) RMgX>RLi>R2​CuLi b) RLi>RMgX>R2​CuLi c) R2​CuLi>RLi>RMgX d) RLi>R2​CuLi>RMgX

33. Which of the following organometallic reagents is NOT considered a “soft” nucleophile? a) Lithium dialkylcuprate. b) Organozinc reagent. c) Grignard reagent. d) Organoaluminum reagent.

34. The reaction of an alkyl halide with lithium metal to form an organolithium reagent is an example of what type of reaction? a) Reduction. b) Oxidation. c) Metal-halogen exchange. d) Insertion.

35. If an ester is reacted with two equivalents of a Grignard reagent, what is the final product after aqueous workup? a) A ketone. b) An aldehyde. c) A primary alcohol. d) A tertiary alcohol.

36. What is the role of the ethereal solvent (e.g., diethyl ether or THF) in the formation and stability of Grignard reagents? a) It acts as a catalyst. b) It donates electrons to the magnesium atom, stabilizing it. c) It removes impurities from the magnesium. d) It prevents the reaction from becoming too exothermic.

37. Which of the following is an example of a π-bonded organometallic complex? a) Methylmagnesium bromide. b) Dimethylcuprate. c) Ferrocene. d) Ethyllithium.

38. The Suzuki-Miyaura reaction is a type of cross-coupling that forms a new C-C bond between an aryl/vinyl halide and an organoboron reagent. What role does the palladium catalyst play? a) It acts as a strong base to deprotonate the organoboron reagent. b) It facilitates the formation of a radical intermediate. c) It undergoes a catalytic cycle involving oxidative addition, transmetalation, and reductive elimination. d) It simply provides a surface for the reaction to occur.

39. What type of bond is formed when H2​ adds across a metal center in an oxidative addition reaction? a) Metal-metal bond. b) Metal-hydride bonds. c) Metal-oxygen bond. d) Carbon-carbon bond.

40. Which of these is a common strategy to achieve stereoselectivity in organometallic reactions, particularly with transition metal catalysts? a) Using high temperatures. b) Employing very dilute solutions. c) Incorporating chiral ligands. d) Running the reaction in the dark.

Answer Key with Explanations

1. b) It contains a bond between a carbon atom and a metal atom.
   • Explanation: The defining feature of organometallic compounds is the direct covalent or highly polarized bond between carbon and a metal atom.
2. c) Carbon-lithium bond.
   • Explanation: Lithium is highly electropositive, leading to a large electronegativity difference with carbon. This results in significant ionic character in the C-Li bond, making the carbon very nucleophilic/basic.
3. c) By reacting an alkyl or aryl halide with lithium metal.
   • Explanation: This is the standard method for preparing organolithium reagents. The lithium metal inserts into the C-X bond.
4. c) They are strong bases and strong nucleophiles.
   • Explanation: The highly polarized C-Li bond places a substantial negative charge on carbon, making organolithium reagents behave as both very strong bases and powerful nucleophiles.
5. c) A secondary alcohol.
   • Explanation: Organolithium reagents (like Grignard reagents) add to the carbonyl carbon of aldehydes. After aqueous workup, this results in a secondary alcohol. Formaldehyde yields a primary alcohol.
6. c) Diethyl ether or THF.
   • Explanation: Ethereal solvents are crucial for Grignard reagent formation and stability because they coordinate to the magnesium atom, helping to solvate and stabilize the reagent.
7. b) They react vigorously with water.
   • Explanation: Grignard reagents are strong bases and nucleophiles; they react immediately with water (a protic source) to form an alkane and magnesium hydroxide, effectively destroying the reagent.
8. c) A tertiary alcohol.
   • Explanation: Grignard reagents add to the carbonyl carbon of ketones. After aqueous workup, this yields a tertiary alcohol.
9. c) Carboxylic acid.
   • Explanation: Grignard reagents react with carbon dioxide (CO2​) via nucleophilic addition to form a carboxylate salt, which upon acidic workup, protonates to yield a carboxylic acid.
10. b) By reacting an organolithium reagent with cuprous iodide.
    • Explanation: Organocuprates (Gilman reagents) are synthesized by combining two equivalents of an organolithium reagent with one equivalent of cuprous iodide (CuI).
11. c) They are softer nucleophiles.
    • Explanation: The inclusion of copper changes the character of the nucleophile, making organocuprates “softer” compared to the “harder” Grignard and organolithium reagents. This affects their reactivity and selectivity.
12. c) Conjugate 1,4-addition.
    • Explanation: This is a hallmark reaction of organocuprates. Their softer nucleophilic character leads them to preferentially attack the β-carbon of α,β-unsaturated carbonyl compounds (conjugate addition) rather than the carbonyl carbon directly.
13. c) Primary alkyl halides.
    • Explanation: The Corey-House synthesis (using Gilman reagents) is most effective for forming new C-C bonds with primary alkyl halides, as it avoids issues like elimination or rearrangement that can occur with secondary or tertiary halides.
14. b) β-hydroxy esters.
    • Explanation: The Reformatsky reaction uses an organozinc reagent (derived from an α-halo ester and zinc) to react with aldehydes or ketones, producing β-hydroxy esters.
15. c) Simmons-Smith reaction.
    • Explanation: The Simmons-Smith reaction uses a zinc-copper couple and diiodomethane to generate an iodomethylzinc iodide species, which adds to alkenes in a stereospecific syn fashion to form cyclopropanes.
16. c) Increases by +2.
    • Explanation: In an oxidative addition, the metal’s oxidation state increases by two units, and its coordination number also increases by two.
17. d) Migratory insertion.
    • Explanation: Migratory insertion is a key step where an alkyl or aryl group attached to the metal moves to a coordinated ligand (often CO or an alkene), creating a new C-C bond and forming an acyl or new alkyl group.
18. d) 18-electron rule.
    • Explanation: The 18-electron rule is a heuristic that helps predict the stability of transition metal complexes. Complexes that achieve an 18-electron valence shell (like noble gas electron configuration) are often stable.
19. c) β-hydride elimination.
    • Explanation: β-hydride elimination is a common side reaction in organotransition metal chemistry. A hydrogen atom on a carbon beta to the metal transfers to the metal, simultaneously forming an alkene and a metal hydride.
20. b) Hydrogenation of alkenes.
    • Explanation: Wilkinson’s catalyst (RhCl(PPh3​)3​) is a widely used homogeneous catalyst for the hydrogenation of alkenes, adding H2​ across the double bond.
21. c) Suzuki-Miyaura reaction.
    • Explanation: The Suzuki-Miyaura reaction is a palladium-catalyzed cross-coupling reaction involving organoboron compounds (organoboranes or boronic acids) and organic halides.
22. c) Aldehydes.
    • Explanation: Hydroformylation, also known as the Oxo process, uses transition metal catalysts (typically Co or Rh) to add a hydrogen atom and a formyl group (CHO) across a C=C double bond, producing aldehydes.
23. b) Ziegler-Natta.
    • Explanation: Ziegler-Natta catalysts, typically composed of titanium compounds (TiCl4​) and organoaluminum compounds, are used industrially for the stereoregular polymerization of alkenes (e.g., forming isotactic polypropylene).
24. c) C-C bonds.
    • Explanation: The most significant application of organometallic reagents in organic synthesis is their ability to form new carbon-carbon bonds, enabling the construction of complex molecular frameworks.
25. d) Alkane.
    • Explanation: Grignard reagents are strong nucleophiles and bases. They will react with ketones, alcohols (acidic proton), and aldehydes. Alkanes are unreactive with Grignard reagents under normal conditions as they lack electrophilic centers or acidic protons.
26. c) Protect the acidic proton first.
    • Explanation: To prevent the Grignard reagent from reacting with an acidic proton (like an -OH group) elsewhere in the molecule, that group must first be protected (e.g., as a silyl ether), then deprotected after the Grignard reaction.
27. c) They exhibit better regioselectivity for conjugate addition.
    • Explanation: Organocuprates are “soft” nucleophiles, making them more selective for 1,4-conjugate addition to α,β-unsaturated carbonyls, while “harder” reagents like Grignards prefer direct 1,2-addition.
28. c) n-Butyllithium.
    • Explanation: n-Butyllithium (n-BuLi) is a common organolithium reagent often employed as a very strong, non-nucleophilic base due to the high basicity of its carbanion.
29. b) Reductive elimination.
    • Explanation: Reductive elimination is the microscopic reverse of oxidative addition. Two ligands on the metal couple, forming a new bond and reducing the metal’s oxidation state and coordination number.
30. c) Carbonyl (CO).
    • Explanation: Carbon monoxide (CO) is a common ligand in organotransition metal chemistry, forming stable metal carbonyl complexes. It acts as both a σ-donor and a π-acceptor.
31. c) Cyclopentadienyl rings.
    • Explanation: Ferrocene (Fe(C5​H5​)2​) is a classic example of a “sandwich” compound where an iron atom is bonded symmetrically between two planar cyclopentadienyl rings.
32. b) RLi>RMgX>R2​CuLi
    • Explanation: Reactivity generally correlates with the ionic character of the M-C bond and the “hardness” of the carbanion. Organolithium reagents are most ionic/hard, followed by Grignard reagents, and then the “softer” organocuprates.
33. c) Grignard reagent.
    • Explanation: Grignard reagents are considered “hard” nucleophiles/strong bases, readily undergoing 1,2-addition to carbonyls and reacting with protic sources. Organocuprates and organozinc reagents are considered “softer.”
34. a) Reduction.
    • Explanation: In this reaction, the carbon atom of the alkyl halide gains electrons (effectively a hydride equivalent), and the lithium metal is oxidized. This is a net reduction of the organic moiety.
35. d) A tertiary alcohol.
    • Explanation: Esters react with two equivalents of a Grignard reagent. The first equivalent adds to the carbonyl to form a ketone intermediate, and the second equivalent then adds to the ketone, ultimately forming a tertiary alcohol after aqueous workup.
36. b) It donates electrons to the magnesium atom, stabilizing it.
    • Explanation: The lone pairs on the oxygen atoms of ethereal solvents (like diethyl ether or THF) act as Lewis bases, coordinating to the electron-deficient magnesium atom and stabilizing the Grignard reagent.
37. c) Ferrocene.
    • Explanation: Ferrocene is a classic example of a π-bonded organometallic complex where the cyclopentadienyl rings donate π-electrons to the central iron atom. Methylmagnesium bromide and ethyllithium are σ-bonded.
38. c) It undergoes a catalytic cycle involving oxidative addition, transmetalation, and reductive elimination.
    • Explanation: Palladium catalysts in cross-coupling reactions operate through a catalytic cycle typically involving three key steps: oxidative addition of the halide to the metal, transmetalation where the organic group from the boron reagent transfers to the metal, and reductive elimination to form the new C-C bond and regenerate the catalyst.
39. b) Metal-hydride bonds.
    • Explanation: When H2​ undergoes oxidative addition to a metal center, the H-H bond breaks, and two new metal-hydride (M-H) bonds are formed, increasing the metal’s coordination number and oxidation state.
40. c) Incorporating chiral ligands.
    • Explanation: A powerful strategy for achieving stereoselectivity (e.g., enantioselectivity or diastereoselectivity) in organometallic catalysis, especially with transition metals, is to use chiral ligands that interact specifically with the reactants, guiding the reaction to form one stereoisomer preferentially.