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