Negative oxidation state
Aluminum does assume negative oxidation states, see the list of elemental oxidation states in Wikipedia. Aluminum assumes a negative oxidation state when forming compounds with more electropositive elements like magnesium.

The magnesium compounds referred to above include $\ce{Mg_{17}Al_{12}}$.  In this compound there is enough magnesium to half-fill the valence $p$ orbital on aluminum and thus attain a $-2$ oxidation state for the aluminum, which is the minimum in Wikipedia's list. (Part of the magnesium would then be below $+2$).  However, this is only a formal designation. Magnesium aluminides are far from ionic in character.