The number of structural isomers for ${\mathrm{C}}_6{\mathrm{H}}_{14}$ is :

The correct configuration assigned for compounds $\left(\mathrm{I}\right)$ and $\left(\mathrm{II}\right)$, respectively are:

"Hybridisation of central atom does not always change due to back bonding". This statement is valid for which of the following compounds?

($\mathrm{i}$) ${\mathrm{CCl}}_3^{-}$
($\mathrm{ii}$) ${\mathrm{CCl}}_2$
($\mathrm{iii}$) ${\mathrm{CF}}_2$
($\mathrm{iv}$) $\mathrm{N}{\left({\mathrm{SiH}}_3\right)}_3$