Classical and Nonclassical Methyleneboranes

Abstract Methyleneboranes are stable compounds when their reactive CB bonds are shielded by sterically demanding substituents and the electron deficiency at the dicoordinate boron atom is reduced through delocalization of adjacent electron pairs. In the classical amino(methylene)boranes of Nöth and Paetzold, the electron deficiency at the dicoordinate boron atom is relieved through π‐π delocalization of a neighboring nitrogen atom's formally nonbonding electron pair, that is, through formation of a classical two‐center, two‐electron (2c–2e) bond. In nonclassical methyleneboranes, which are the focus of this review, the electrondeficient center at the dicoordinate boron atom forms nonclassical three‐center, two‐electron (3c2e) bonds with neighboring σ bonds. These methyleneboranes exhibit structural and spectroscopic properties similar to carbenium ions having strong σ–π delocalization (hyperconjugation, bridging) and to transition metal complexes having strong agostic interactions.