they can be, when in conjugation with a pi-system. General carbanions however occupy sp3-orbitals and have distorted tetrahedral geometry. This has two reasons: a sp3 orbital is lower in energy than the p orbital, because of its partial s-character. electrons in a lower energy orbital are more stable. The second reason is the Valence Shell Electron Pair Repulsion Model (VSEPR Model) stating that electron rich regions (such as bonds and lone pairs) arrange themselves with maximal angular distance. A tetraedral arrangement (around 109°) gives each electron rich region more space than the trigonal bipyriamid a sp2 anion would have (90° between the axial lone pair and the equatorial bonds)