Here's why:

* Ammonium ion (NH₄⁺): This is a positively charged ion formed by the protonation of ammonia (NH₃).

* Hydride ion (H⁻): This is a negatively charged ion consisting of a single hydrogen atom.

While both ions exist, they are highly reactive and would not stably combine to form ammonium hydride (NH₄H).

Instead, the reaction between ammonium ions and hydride ions would likely result in the formation of ammonia (NH₃) and hydrogen gas (H₂):

NH₄⁺ + H⁻ → NH₃ + H₂