Hydrogen usually forms a covalent bond with another element and, being the more electropositive of the two, donates its electron to form a stable outer shells for both atoms. In the case of hydrochloric acid, HCl, for example, the chlorine wants an extra electron to fill its outer shell, and the hydrogen atom is happy to donate the one it has, thereby achieving its own stable configuration of zero electrons. In aqueous solution, HCl ends up with an excess of H+ cations, and is therefore acidic.

In the case of certain metal or complex hydrides, however, the hydrogen is not the more electropositive element. In this case, the hydrogen sometimes receives an extra electron (thereby filling its S-orbital - also a stable configuration, the same as the noble gas helium next to it on the periodic chart), rather than donating one. In aqueous solution, if the molecule dissolves into its constituent ions, the result is an excess of H- ions (hydrogen with two electrons - a.k.a. hydrides) in solution. With an excess of H- anions, the solution is, therefore, basic.

That is correct for covalent hydrides... but for basic hydrides, it's a little more complicated than that.

are responsible for basicity, not ions.

When you put a metal hydride in water, there is a chemical reaction that occurs.



That's the 'easy' reaction equation.

Let's say that it didn't react at all, but merely dissolved...



And in water, you have an equilibrium:



As you can see, the H^+ and H^- have opposite charges, and therefore will attract each other and react!



And as H^+ ions are removed from the water, more OH^- ions are formed, making the solution more basic.

Very cool! Thanks for that, Jerry.