The concept of aromaticity after 200 years of benzene.

The concept of aromaticity has expanded over recent decades, from equilibrium structures to transition states, from two-dimensional to three-dimensional systems, from ground to excited states, from main-group elements to metals, and from classical π-aromaticity to σ-, δ- and even ϕ-aromaticity. Consequently, the number of species classified as aromatic has grown dramatically, with an estimated two-thirds of known compounds displaying full or partial aromatic character. However, the various forms of aromaticity are described in disparate and unconnected ways, preventing cross-disciplinary understanding. This Review summarizes the fundamental features of aromaticity and examines key developments of the past decade, with particular focus on excited-state aromaticity, three-dimensional aromaticity and metallaaromaticity.