= Solution
In a <boroxine>, each <boron> has three <sigma bonds> and is approximately trigonal planar, with an $sp^2$ description for its sigma framework and an available perpendicular $2p$ orbital. <oxygen> can be described approximately as $sp^2$: two in-plane hybrids form B–O <sigma bonds>, one contains an in-plane <lone pair>, and a perpendicular $2p$ <lone pair> overlaps with neighbouring <boron> $2p$ orbitals. These three <oxygen> donor pairs supply a six-electron pi system around the alternating ring. The combination of good same-shell overlap and a low-strain six-membered ring favours planarity and partial B–O multiple-bond character. The bonds remain polar; a six-electron diagram alone does not establish benzene-like aromatic stabilization quantitatively.
In a <siloxane>, each <silicon> instead has four <sigma bonds>, two to <oxygen> and two to its organic groups, giving an approximately tetrahedral $sp^3$ sigma framework. <oxygen> has two <sigma bonds> and two <lone pairs>, so an approximately $sp^3$ localized description is a useful starting point, although actual Si–O–Si angles and lone-pair hybrids vary considerably. There is no comparable ring of vacant <silicon> $2p$ orbitals. A description based on strong oxygen-$2p$ to silicon-$3d$ <pi bonds> is also misleading: those orbitals have poor energy and radial matching. \b[Strongly polar Si–O sigma bonding, supplemented by <negative hyperconjugation> from <oxygen> <lone pairs> into appropriate Si–R <antibonding orbitals>, gives a better account]. The ring can therefore pucker rather than requiring continuous planar pi overlap. This follows the broader conclusion that strong main-group bonds and apparently expanded valence do not imply dominant valence $d$-orbital participation.<Chemical bonding in hypervalent molecules. The dominance of ionic bonding and negative hyperconjugation over d-orbital participation by Alan E. Reed and Paul von Ragué Schleyer>[JACS 112, 1434–1445: ionic contributions and negative hyperconjugation versus d-orbital models]{ref}
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