While going through a textbook, I came across the following line:
$\ce{R3C-}$ is pyramidal while $\ce{(CN)3C-}$ is planar due to lack of back bonding from the lone pair of carbon atom into the $\pi^*$ orbital of the $\ce{CN}$ group.
I am having trouble understanding how exactly back-bonding occurs in the first species and how it causes the structure to be pyramidal while the latter is planar.