Question

The stability of adducts $H_3B.PF_3, H_3B.NMc_3, H_3B.CO, H_3B.OMe_2$ follows the order

• A. $H_3B.OMe_2 < H_3B.CO < H_3B-PF_3 < H_3B.NMc_3$
• B. $H_3B.PF_3,< H_3B.CO < H_3B.NMc_3 < H_3B.OMe_2$
• C. $H_3B.CO < H_3B.PF_3, < H_3B.NMc_3 < H_3B.OMe_2$
• D. $H_3B.PF_3, < H_3B.CO < H_3B.OMe_2 < H_3B.NMc_3 $

Answer 1

The Correct Option is D

The stability of adducts in question, where a Lewis acid and a Lewis base form a bonded pair, depends on several factors such as bond strength, steric hindrance, and electronic effects. In this case, we are analyzing boron-containing adducts with different ligands.

To determine the stability order of these adducts, let's consider each adduct individually with the general formula \( H_3B\cdot L \):

1. \(H_3B\cdot PF_3\): Phosphorus trifluoride is a weaker donor compared to other bases due to the electronegative fluorine atoms withdrawing electron density, making this adduct less stable.
2. \(H_3B\cdot CO\): Carbon monoxide (CO) is a good π-back-donor with boron, providing moderate stability to the adduct.
3. \(H_3B\cdot OMe_2\): Dimethyl ether (OMe₂) is an oxygen donor which is slightly better than carbon monoxide, leading to higher stability of this adduct due to better sigma donation capability.
4. \(H_3B\cdot NMe_3\): Trimethylamine (NMe₃) offers strong σ-donation from nitrogen, the best among these donors, causing the greatest stability in this adduct.

Based on these considerations, the order of stability is:

\(H_3B\cdot PF_3 < H_3B\cdot CO < H_3B\cdot OMe_2 < H_3B\cdot NMe_3\)

This order is primarily because of the increasing donor strength from PF₃ to NMe₃, with steric factors playing a minor role compared to electronic stabilizations.