NO-Induced Two-State Adaptive Aromaticity in Metallabenzynes: A Combined DFT and Machine Learning Study

Abstract Metallabenzynes are six-membered metallacyclic scaffolds bearing a formal metal–carbon triple bond (M≡C) and serve as metallic analogues of conventional benzyne. While the aromaticity of metallabenzenes has been extensively documented, the electronic-state-dependent aromatic behavior of metallabenzynes remains largely underexplored. Guided by experimentally validated structural frameworks and density functional theory calculations, we systematically investigate a series of simplified osmabenzyne derivatives with the general formula Os(≡CR)(L1)2(L2)2 (L1 = phosphine; L2 = halide). Computational results reveal that these osmabenzynes exhibit distinct state-dependent aromaticity: they are aromatic in the singlet ground state (S0) but lose aromatic character in the lowest triplet excited state (T1). Substituent modification on the metallacyclic skeleton effectively tunes this aromatic behavior. Among all examined substituents (R = NO, NO2, CN, Me, SiH3, SiMe3, OH), only the nitroso (NO) group enables persistent aromaticity in both S0 and T1 states, endowing the metallabenzyne system with adaptive aromaticity. In the T1 state, the two singly occupied molecular orbitals (SOMOs) of NO-functionalized osmabenzyne are predominantly localized on the N═O moiety, with approximately 80% of the total spin population concentrated on the NO substituent. Thus, nitroso substitution represents a reliable and synthetically feasible strategy as a spin acceptor to realize two-state adaptive aromaticity in metallabenzyne systems.