For the two-atom inorganic backbone unit, $Z_{\rm Si}+Z_{\rm O}=14+8=22$ and $Z_{\rm P}+Z_{\rm N}=15+7=22$. Their valence-electron sums also agree: $4+6=5+5=10$. Since each unit bears the same two organic groups, \b[$\mathrm{R_2SiO}$ and $\mathrm{R_2PN}$ are <isoelectronic species>]. This electron-count relationship does not require identical bond localization, bond angles or chemical reactivity.

Solution

Past exam of the chemistry B course of the University of Cambridge / 2021 / Paper 2 / B9 / a / Solution

 Solution (source code)