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boron is sufficiently small for effective oxygen-$2p$/boron-$2p$ overlap, and three-coordinate planar boron benefits from the pi donation just described. A six-membered boroxine ring provides this overlap without severe angle strain. The larger aluminium centre has poorer oxygen-$2p$/aluminium-$3p$ matching and remains strongly Lewis acidic. It can lower its electron deficiency by accepting additional oxygen coordination, making a three-dimensional aluminoxane cage favourable. Four-coordinate aluminium does not require a boroxine-like planar pi system.

An appropriate hexamer model is an alternating Al/O hexagonal prism, with six aluminium and six oxygen vertices. Colour the twelve-vertex prism alternately Al and O: each aluminium then has three oxygen neighbours and an outward organic substituent, making it four-coordinate, while each oxygen bridges three aluminium centres. The graph has 18 Al–O edges, hence six Al centres with three Al–O bonds each. Different polyhedral connectivities and sizes can meet similar local coordination requirements, so the hexamer is a proposal rather than a claim that the empirical repeat unit fixes a unique cage.