Structure-Induced Reversing Basicity of α-Anionic Fischer Carbenes

Group VI metal carbenes and their α-anionic complexes play an important role in organic synthesis, yet structural characterizations and thermodynamic studies for reactive α-anions remain challenging. This work investigated the molecular structures and thermodynamics of α-anionic Fischer carbenes and their conjugated acids. The results clarify the negative charge localized on the M(CO)₅ moiety and the C═X (X = C, N, or O) double bond in all anionic structures. The conjugation between the π-donating group and the M(CO)₅ moiety significantly lowers the energy of neutral complexes, resulting in a reversing acidity between α-methyl, amino, and hydroxyl groups. This study deepens our understanding of the molecular structure, electronic structure, and thermodynamic property of Fischer carbenes.