(i) N(SiMe 3 ) 3 is trigonal planar because in it silicon uses its vacant d-orbital for p π -d π back bonding with lone pair of electrons of central N-atom and the p π -d π bonding is delocalised as given in the structure. So, N(SiMe 3 ) 3 with steric number three is trigonal planar.

(ii) In N(Me 3 ) , there is no such p π -d π delocalisation of lone pair of electrons on N atom as carbon does not have vacant d-orbital. So N(Me) 3 with steric number four is trigonal pyramidal with a lone pair at the apex.

(i) (ii)

Hence both are not isostructural.