# Why BH3 its classified as a “Molecular Hydride” and AlH3 its a “Intermediary Hydride”

First, I understand that, on $\ce{BH3}$, they eletronegativity difference between $\ce{B}$ and $\ce{H}$ is very small. Which mean that the $\ce{B-H}$ bond is less polarized. But, I couldn't find a relation of this, to the fact that he is a Molecular hydride. And, secondly, I couldn't understand why $\ce{AlH3}$ its "intermediary", and not molecular, as well...

• (What do you mean by "relation"?) BH3 exists as the gaseous dimer molecule diborane. AlH3 is a solid that decomposes at 100°C. It's not molecular. – Karl Jun 28 '17 at 20:07
• I see. But, i still don't understand why this happen, you know? – Bruno Jun 28 '17 at 20:27
• "Intermediary" is meant in comparison to BH3 on one side and "real" metall hydrides on the other. The differences are explained in any advanced inorganic chemistry textbook. – Karl Jun 28 '17 at 20:42