12.
Between metal carbonyl complexes A = M(CO)4(PEt3) and B = M(CO)4(PPh3), the
ability to act as a catalyst for nucleophilic addition to olefin is
a. similar for both
b. higher for A
c. higher for B
d. indeterminate based on the given information alone

PEt3 outperforms PPh3 in metal carbonyl catalysis for olefin activation.

Question Analysis

The query compares metal carbonyl complexes A = M(CO)₄(PEt₃) and B = M(CO)₄(PPh₃) for catalytic activity in nucleophilic addition to olefins. Catalysis requires labilization of a CO ligand to create a vacant site for olefin coordination, followed by nucleophilic attack. PEt₃ (triethylphosphine) is a smaller, more electron-donating ligand than PPh₃ (triphenylphosphine), enhancing CO dissociation rates.

Ligand Properties

PEt₃ has a Tolman cone angle of ~132° and strong σ-donation, while PPh₃ has a larger cone angle (~145°) with weaker donation due to phenyl groups. Smaller steric bulk in PEt₃ reduces crowding, increasing lability of trans-CO ligands. Electron donation from PEt₃ raises metal electron density, weakening M-CO π-backbonding and promoting dissociation.

Option Evaluation

• a. Similar for both: Incorrect, as PEt₃’s lower sterics and higher donation make A more labile than B.

• b. Higher for A: Correct, since easier CO loss in A facilitates olefin binding and nucleophilic attack.

• c. Higher for B: Incorrect; PPh₃’s bulk hinders site vacancy despite common use in stable catalysts.

• d. Indeterminate: Incorrect; steric/electronic differences predict A’s superiority.

Metal carbonyl complexes play a key role in metal carbonyl catalysis olefin addition PEt3 PPh3 reactions, especially for nucleophilic addition to olefins in CSIR NET exam contexts. Between M(CO)₄(PEt₃) (A) and M(CO)₄(PPh₃) (B), complex A exhibits higher catalytic ability due to ligand effects.

Phosphine Ligand Effects

Triethylphosphine (PEt₃) offers less steric hindrance (cone angle ~132°) than triphenylphosphine (PPh₃, ~145°), accelerating CO dissociation for olefin coordination. PEt₃’s stronger σ-donation increases metal electron density, labilizing CO via reduced backbonding.

Catalytic Mechanism

Olefin catalysis involves CO loss to form M(olefin)(CO)₃L, enabling nucleophilic attack. PEt₃ facilitates faster labilization than bulky PPh₃, enhancing turnover.

Answer: b. higher for A.