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In a boroxine, each boron has three sigma bonds and is approximately trigonal planar, with an $s{p}^2$ description for its sigma framework and an available perpendicular $2p$ orbital. oxygen can be described approximately as $s{p}^2$: 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 $s{p}^3$ sigma framework. oxygen has two sigma bonds and two lone pairs, so an approximately $s{p}^3$ 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. 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.^[ref]

Ring cleavage of 1,3,5-Trioxane regenerates formaldehyde, whose strong carbon–oxygen pi bond stabilizes the monomer; in water much of it is further stabilized as its hydrate. acetal hydrolysis, particularly under acid catalysis, provides a route between the cyclic trimer and these monomeric forms.

The corresponding formal depolymerization of a siloxane would create ${\mathrm{R}}_2\mathrm{Si}=\mathrm{O}$ silanone units. Such ordinary small-substituent silanones have much less favourable pi bonding and are very reactive toward forming new Si–O–Si linkages. The connected siloxane is consequently preferred over a persistent aqueous solution of silanone monomers. This explains the intended contrast, but aqueous stability is conditional: acid or base can catalyse siloxane cleavage to silanols, and those silanols can condense again. Neither neutral-water inertness of every siloxane nor instantaneous neutral-water dissociation of every trioxane should be inferred.

First count the organic groups and then the unsatisfied oxygen valences. A is the fully condensed T8 silsesquioxane cage, $\mathrm{C}{\mathrm{y}}_8\mathrm{S}{\mathrm{i}}_8{\mathrm{O}}_{12}$, with silicon at the eight vertices of a cube and bridging oxygen on its twelve edges. Each silicon has one cyclohexyl group and three siloxane bridges. Its eight cyclohexyl groups supply exactly ${\mathrm{C}}_{48}{\mathrm{H}}_{88}$.

B is a fully condensed T10 silsesquioxane cage, $\mathrm{C}{\mathrm{y}}_{10}\mathrm{S}{\mathrm{i}}_{10}{\mathrm{O}}_{15}$, for which a pentagonal-prismatic silicon skeleton with an oxygen on every edge is a standard compatible structure. Its ten groups give ${\mathrm{C}}_{60}{\mathrm{H}}_{110}$. The formula establishes full condensation but does not, without more structural data, rule out every other possible ten-vertex cage connectivity.

C is a corner-deficient T7 trisilanol cage, $\mathrm{C}{\mathrm{y}}_7\mathrm{S}{\mathrm{i}}_7{\mathrm{O}}_9(\mathrm{OH}{)}_3$. Seven cyclohexyl groups account for ${\mathrm{C}}_{42}{\mathrm{H}}_{77}$, leaving three hydrogen atoms as Si–OH groups. Remove one silicon corner of the T8 cube and replace the three severed connections by terminal hydroxyl groups. Nine oxygen atoms remain as bridges and three become terminal silanol oxygen atoms, giving precisely twelve oxygen atoms. There are three silicon sites bearing OH, three further equivalent fully condensed sites, and one opposite fully condensed site, consistent with three silicon environments in a 3:3:1 site-count ratio. The less negative resonance is naturally associated with the three silanol-bearing sites; the remaining two resonances belong to the two fully condensed classes. Peak heights alone are not quantitative integrations.

The source figure appears to label the spectrum ${}^{27}\mathrm{Si}$. The chemically appropriate routine experiment here is silicon-29 NMR, using the spin-one-half isotope ${}^{29}\mathrm{Si}$; the figure label is treated as an apparent isotope-label typographical error, not as evidence for another cage.

A compound containing a silicon–oxygen double bond, ${\mathrm{R}}_2\mathrm{Si}=\mathrm{O}$. Ordinary small-substituent silanones are highly reactive; oligomerization to siloxane linkages is favoured over persistence as freely dissolved monomers.

An Si–O–Si linkage, or a compound containing such linkages. Cyclic and polymeric siloxanes have strongly polar bonds and flexible bond angles; their bonding should not be reduced to a dominant oxygen-2p/silicon-3d pi interaction.

A silicon–oxygen framework with ideal fully condensed composition $(\mathrm{RSi}{\mathrm{O}}_{3/2}{)}_{\mathrm{n}}$. Each silicon has one organic substituent and three siloxane bridges. Incompletely condensed cages retain silanol groups.