Excited-state aromaticity expands the concept of aromaticity to describe additional molecular stability and reactivity upon photoexcitation. While both Hückel and Möbius excited-state aromatic species have been identified, Craig excited-state aromaticity involving 4n+2 electrons in planar metallacycles remains unrecognized. Herein, we report that early transition metal (M = Ti, Sc, Y, La, Ac)-based metallabenzenes exhibit Craig 6π aromaticity in their lowest singlet and triplet ππ* excited states, which is supported by a range of aromaticity indices based on electronic, geometric, energetic, and magnetic properties. Notably, ab initio valence bond theory reveals that the dyz orbital dominates the cyclic electron delocalization in the excited-state wave function, resulting in phase inversion between neighboring atomic orbitals of π-symmetry. In contrast, the dyz orbital is usually doubly occupied in well-identified metallabenzenes with late transition metals which thus display Hückel or Baird (anti)aromaticity via the dxz orbital. Our findings provide the first direct evidence and origin of Craig excited-state aromaticity and establish a unified framework for understanding the electronic structure of metallabenzenes, addressing a significant gap in the exploration of excited-state metalla-aromaticity.
Lin et al. (Wed,) studied this question.