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

## TL;DR

This paper introduces a new type of excited-state aromaticity in metallabenzenes involving early transition metals, supported by various aromaticity indices and theoretical analysis.

## Contribution

The study provides the first direct evidence of Craig excited-state aromaticity in metallabenzenes and explains its origin through ab initio valence bond theory.

## Key findings

- Early transition metal-based metallabenzenes show Craig 6π aromaticity in their lowest singlet and triplet ππ* excited states.
- Ab initio valence bond theory reveals the d_yz orbital's role in cyclic electron delocalization and phase inversion in Craig aromaticity.
- Late transition metal metallabenzenes display Hückel or Baird (anti)aromaticity via the d_xz orbital.

## Abstract

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 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.