Craig Excited-State Aromaticity in Metallabenzenes: How, When, and Why?

High Resolution Image Download MS PowerPoint Slide 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 [4 n +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 d yz 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 d yz orbital is usually doubly occupied in well-identified metallabenzenes with late transition metals which thus display Hückel or Baird (anti)aromaticity via the d xz 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.