Organometallic Compounds Mock Test For CSIR-NET Chemical Science

organometallic compounds csir net mock test

Question 1: In the 18-electron rule, which of the following complexes violates the rule but is still isolable and stable?

A) V(CO)₆

B) Fe(CO)₅

C) Cr(CO)₆

D) Ni(CO)₄

Explanation: V(CO)₆ is a 17-electron complex but is stable due to delocalization and favorable metal–CO π-backbonding.

Question 2: Which of the following organometallic reactions involves the formal increase in oxidation state and coordination number of the metal center?

A) Oxidative addition

B) Reductive elimination

C) β-hydride elimination

D) Ligand substitution

Explanation: Oxidative addition increases both the oxidation state and coordination number of the metal by two units.

Question 3: Which of the following is NOT a prerequisite for a metal complex to undergo β-hydride elimination?

A) Hydride transfer to a trans ligand

B) Presence of a β-hydrogen on alkyl ligand

C) Vacant site cis to alkyl group

D) Proper orbital alignment for migration

Explanation: β-hydride elimination requires a vacant cis site; trans hydride transfer is not a typical feature of the mechanism.

Question 4: Which of the following ligands is expected to be the strongest π-acceptor in a metal complex?

A) CO

B) NO⁺

C) PPh₃

D) NH₃

Explanation: CO is a classic strong π-acceptor due to empty π* orbitals that engage in backbonding with metal d-orbitals.

Question 5: What is the product of the oxidative addition of H₂ to the square planar complex [Pt(PPh₃)₂Cl₂]?

A) cis-[Pt(H)₂(PPh₃)₂Cl₂]

B) trans-[Pt(H)₂(PPh₃)₂Cl₂]

C) [Pt(H)(Cl)(PPh₃)₂]

D) [Pt(PPh₃)₂]

Explanation: Oxidative addition adds two hydride ligands cis to each other on Pt(II), increasing coordination number to 6 and changing geometry to octahedral.

Question 6: In a metallocene such as ferrocene, what is the nature of bonding between Fe and the cyclopentadienyl rings?

A) Delocalized π-bonding involving η⁵ coordination

B) σ-bonding via a single C atom

C) Ionic bonding between Fe²⁺ and Cp⁻

D) Coordination via metal–hydride interactions

Explanation: Ferrocene has η⁵-Cp ligands bonded through delocalized π-interactions across the entire ring, leading to sandwich structure.

Question 7: What would be the expected ¹H NMR feature for a fluxional complex like [Fe(CO)₅] at room temperature?

A) A single CO signal due to rapid exchange

B) Two distinct CO signals (axial and equatorial)

C) Broad unresolved CO multiplet

D) No CO signal

Explanation: [Fe(CO)₅] undergoes rapid CO exchange, leading to averaging and a single peak in NMR at room temperature.

Question 8: Which of the following processes would convert a 16-electron complex to an 18-electron one?

A) Ligand association of CO

B) Ligand dissociation

C) β-hydride elimination

D) Reductive elimination

Explanation: Associating a 2-electron donor like CO with a 16-electron complex satisfies the 18-electron rule.

Question 9: The complex [Mn(CO)₅Br] is paramagnetic. Which of the following is the correct reason?

A) d⁷ configuration leaves one unpaired electron

B) Mn–CO backbonding causes electron pairing

C) Br⁻ is a strong-field ligand

D) CO acts as a 3-electron donor

Explanation: Mn in +1 oxidation state is d⁶, but with one extra electron due to odd coordination, it becomes d⁷, hence one unpaired electron.

Question 10: Which of the following organometallic complexes is used industrially in hydroformylation reactions?

A) HCo(CO)₄

B) Fe(CO)₅

C) Ni(CO)₄

D) Cr(CO)₆

Explanation: HCo(CO)₄ is the active species in hydroformylation (oxo process), where aldehydes are produced from alkenes and syngas.

Question 11: Which statement best explains why Ni(CO)₄ is volatile and toxic despite being a neutral 18-electron complex?

A) Strong metal–CO bonding and absence of polarity make it volatile

B) The compound has ionic character and decomposes easily

C) It contains Ni(II) which is unstable

D) The 18-electron rule makes it reactive

Explanation: Ni(CO)₄ is neutral, nonpolar, and obeys the 18-electron rule, making it volatile. The Ni–CO bond also stabilizes the molecule, but its volatility leads to high inhalation toxicity.

Question 12: In the Tolman electronic parameter (TEP) system, which ligand would show the highest ν(CO) stretching frequency when coordinated to Ni(CO)₃L?

A) PF₃

B) PPh₃

C) PMe₃

D) PH₃

Explanation: PF₃ is a strong π-acceptor, reducing back-donation to CO ligands and resulting in higher ν(CO) frequencies in IR spectroscopy.

Question 13: What is the correct geometry and electron count of the complex [IrCl(CO)(PPh₃)₂]?

A) Square planar, 16 electrons

B) Octahedral, 18 electrons

C) Tetrahedral, 16 electrons

D) Square planar, 18 electrons

Explanation: Ir(I) complexes like [IrCl(CO)(PPh₃)₂] adopt square planar geometry with 16 electrons—common for catalytically active species.

Question 14: Which mechanism best explains migratory insertion in organometallic chemistry?

A) Alkyl group inserts into a coordinated ligand like CO, forming an acyl

B) Reductive elimination of two ligands

C) Oxidative addition followed by ligand dissociation

D) Alkyl hydride elimination followed by recombination

Explanation: Migratory insertion involves the movement of an alkyl or hydride ligand into a coordinated π-ligand like CO to form a new bond.

Question 15: Which metal center is most likely to form stable agostic interactions in organometallic complexes?

A) Early transition metals with low d-electron count

B) Late transition metals with full d-orbitals

C) Alkali metals

D) Lanthanides

Explanation: Early transition metals (like Ti, Zr) often form agostic interactions due to their electron deficiency and vacant d-orbitals.

Question 16: What happens during reductive elimination in an organometallic complex?

A) Two cis ligands combine and leave, lowering the oxidation state

B) A ligand is substituted by a nucleophile

C) The metal is oxidized by losing electrons

D) A π-bonded ligand inserts into an alkyl group

Explanation: Reductive elimination is the coupling of two ligands (often cis) on the metal center, reducing the metal's oxidation state by 2.

Question 17: Which of the following is a likely intermediate in the Heck reaction involving Pd(0) catalysts?

A) A Pd(II)-aryl complex

B) A Pd(IV)-hydride complex

C) A Pd(0)-chloride dimer

D) Pd metal nanoparticles

Explanation: The Heck reaction proceeds via oxidative addition of aryl halide to Pd(0), forming a Pd(II)-aryl complex as a key intermediate.

Question 18: Which factor favors the formation of η³-allyl complexes over η¹-allyl in organometallic chemistry?

A) Delocalization of π-electrons in η³ bonding stabilizes the complex

B) η¹-allyl has better overlap with metal d-orbitals

C) Steric hindrance of η³ leads to dissociation

D) η³-allyl forms unstable free radicals

Explanation: η³-allyl complexes benefit from delocalized π-bonding and are more stabilized compared to η¹ coordination.

Question 19: The species [Fe(Cp)(CO)₂]₂ dimerizes through which interaction?

A) Metal-metal bond between Fe centers

B) Hydrogen bonding between CO groups

C) π–π stacking of Cp rings

D) Formation of Fe–CO–Fe bridges

Explanation: The dimer of [Fe(Cp)(CO)₂] involves an Fe–Fe bond, making it a good example of metal-metal bonded organometallics.

Question 20: In catalytic cycles involving Wilkinson's catalyst, which step is typically rate-determining in hydrogenation reactions?

A) Oxidative addition of H₂ to Rh(I)

B) Reductive elimination

C) Ligand substitution of PPh₃

D) Dissociation of alkene

Explanation: In many hydrogenation reactions, the oxidative addition of H₂ to Rh(I) is the slowest and hence the rate-determining step.

Question 21: Which of the following ligands, when coordinated to a metal, is most likely to promote oxidative addition reactions?

A) Electron-rich phosphine ligands

B) π-acceptor ligands like CO

C) Bulky N-heterocyclic carbene ligands

D) Halide-only coordination environment

Explanation: Electron-rich phosphines increase electron density at the metal, making oxidative addition more favorable by stabilizing higher oxidation states.

Question 22: Which feature of olefins enhances their coordination strength to low-valent transition metals in organometallic complexes?

A) Substitution by electron-withdrawing groups

B) Presence of bulky alkyl groups

C) Olefins with sp-hybridized carbons

D) Planarity of the olefin

Explanation: Electron-withdrawing groups lower the π* energy level of the olefin, enhancing π-backbonding with metal d-orbitals.

Question 23: Which is the correct 18-electron count for [Mo(CO)₆]?

A) Mo (Group 6) contributes 6 e⁻; 6 CO ligands contribute 12 e⁻

B) Mo contributes 4 e⁻; CO contributes 10 e⁻

C) Mo contributes 6 e⁻; each CO contributes only 1 e⁻

D) Mo contributes 10 e⁻; CO contributes 8 e⁻

Explanation: Mo⁰ provides 6 valence electrons; each CO contributes 2 e⁻ → total = 6 + (6×2) = 18 electrons.

Question 24: Which of the following complexes is most likely to undergo homolytic cleavage under photochemical conditions?

A) Mn₂(CO)₁₀

B) Cr(CO)₆

C) Mo(CO)₆

D) Fe(CO)₅

Explanation: Mn₂(CO)₁₀ has a metal-metal bond that can undergo homolytic cleavage under light to give Mn(CO)₅ radicals.

Question 25: What is the formal oxidation state of Ru in [RuH₂(CO)(PPh₃)₃]?

A) +2

B) 0

C) +4

D) –2

Explanation: H = –1 each, CO and PPh₃ are neutral. Total ligand charge = –2 → Ru must be +2 to neutralize the charge.

Question 26: Which type of insertion is more common in alkene polymerization catalyzed by Ziegler-Natta catalysts?

A) 1,2-insertion

B) 2,1-insertion

C) 1,3-insertion

D) β-insertion

Explanation: In olefin polymerization, 1,2-insertion is predominant due to favorable stereoelectronic and steric factors.

Question 27: What is the key intermediate in hydrocyanation of alkenes using Ni(0) catalysts?

A) Ni(II)-alkyl cyano complex

B) Ni(IV)-alkyl intermediate

C) Ni(CN)₂

D) Ni-hydride

Explanation: The Ni(0) species undergoes oxidative addition of H–CN and then alkene insertion to form a Ni(II)-alkyl cyano intermediate.

Question 28: Which of the following processes will regenerate the active catalyst in a catalytic cycle?

A) Reductive elimination

B) Migratory insertion

C) Oxidative addition

D) β-hydride elimination

Explanation: Reductive elimination often leads to product release and regeneration of the catalytically active low-valent species.

Question 29: Why is backbonding stronger in low oxidation state metals?

A) More d-electron density available for π-backdonation

B) Ligand field splitting increases

C) CO bond becomes stronger

D) σ-donation is enhanced

Explanation: Low oxidation state metals have more d-electron density to donate into π* orbitals of ligands like CO or olefins, increasing backbonding strength.

Question 30: Which of the following is an example of a fluxional organometallic compound?

A) Fe(CO)₅

B) Ni(CO)₄

C) Cr(CO)₆

D) FeCp₂

Explanation: Fe(CO)₅ exhibits Berry pseudorotation, making axial and equatorial CO ligands exchange rapidly—characteristic of fluxional behavior.