In basic solution, six hydroxide ions are coordinated around the central Al3+ ion.

VSEPR theory predicts that six mutually repelling equal charges at fixed distances around a central ion will be most stable when positioned at the north and south poles and 90° apart around the equator, as shown at the right.

Since these are the six directions of the vertices of a regular octahedron, this is known as octahedral coordination.

Unless special features intervene, sixfold coordination of atoms, ions, or molecules around a central atom is always octahedral, as this represents the least clash or interference between neighboring groups.