Despite its importance in carbene chemistry, the amphoteric (both nucleophilic and electrophilic) behavior of the divalent carbon atom () in carbenes is not well understood. The electrostatic potential (EP) aroundis often incorrectly described by simple isotropic atomic charges, especially in singlet CF2. Instead, the multipole model should be used. This model can illustrate both negative and positive EPs, which favor the positively and negatively charged species often present around.

The amphotericity is much stronger in the singlet state due to its more conspicuous anisotropic charge distribution compared to the triplet state. This is validated by the complexation structures of carbenes interacting with Na+, Cl−, H2O, and Ag+. By studying diverse carbenes (including CH2, CLi2/CNa2, CBe2/CMg2, CF2/CCl2, C(BH2)2/C(AlH2)2, C(CH3)2/C(SiH3)2, C(NH2)2/C(PH2)2, cyclic systems of C(CH2)2/C(CH)2, C(BHCH)2, C(CH2CH)2/C(CHCH)2, and C(NHCH)2/C(NCH)2), we can elucidate important relationships.

These relationships involve electron configurations, electron accepting/donating strengths of atoms attached to, π conjugation, singlet-triplet energy gaps, anisotropic hard wall radii, anisotropic electrostatic potentials, and the amphotericities of carbenes. The (σ2, π2, or σπ) electronic configuration on theplane (where A is an adjacent atom) in singlet and triplet carbenes largely governs the amphoteric behaviors along thetip andface-on directions.

Thetip andface-on sites of σ2 singlet carbenes tend to show negative and positive EPs, respectively, favoring nucleophiles and electrophiles. In contrast, π2 singlet carbenes, such as highly π-conjugated 5-membered cyclic C(NCH)2, tend to show the opposite behavior. Open-shell σπ singlet carbenes, like highly π-conjugated 5-membered cyclic C(CHCH)2, and triplet carbenes exhibit less anisotropic and amphoteric behaviors.

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