Methyl anion

The methyl anion is the simplest carbanion: a carbon with three C–H bonds and a lone pair, giving it a negative charge. Like carbocations and radicals, carbanions are usually short-lived, but they matter a great deal as electron-rich reagents.

Count the electron groups around carbon: three bonds and one lone pair makes four, the same as in ammonia. In fact the methyl anion has exactly the same number of electrons as ammonia. So the carbon is sp³-like and the shape is a trigonal pyramid, not flat. Look at it edge-on and compare with the flat methyl cation and methyl radical. Turn on Angles & lengths: the H–C–H angles are well below 120°.

The lone pair sits in an sp³-like orbital on the opposite side from the hydrogens, at the tip of the pyramid. Because it’s a full pair of electrons with nowhere to go, the methyl anion is a powerful base and nucleophile: it seeks out positively charged atoms or protons.

That completes the series: adding electrons one at a time to the same CH₃ group takes it from flat with an empty p orbital (cation), to flat with one electron (radical), to pyramidal with a lone pair (anion).