# Why does ammonia causes unsymmetric cleavage of diborane, but carbon monoxide causes symmetric cleavage?

Diborane reacts with Lewis bases to form adducts.

I did some research on my own, and found that ammonia breaks it unsymmetrically but $\ce{CO}$ and $\ce{PF3}$ break it symmetrically. I cannot find the reason.

• – Mithoron Jan 26 '17 at 17:18
• Clearly not a duplicate, since "sterics" are no longer a good answer: $\ce{NH3}$ is intermediate in volume between $\ce{CO}$ and $\ce{PF3}$. – pentavalentcarbon Aug 13 '17 at 2:35