# How will Back-Bonding affect molecular geometry?

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.

• You might think of it as resonance instead of back-bonding... – jerepierre Mar 20 '17 at 23:20
• – Mithoron Mar 20 '17 at 23:56