Properties and applications of [ n ]cumulenes

[ n ]Cumulenes are highly versatile sp-carbon scaffolds whose properties and potential applications are strongly dictated by their parity. This review provides an overview of the geometric and electronic structure, optical activity, conductance, and physicochemical behavior of [ n ]cumulenes containing three or more cumulated C=C double bonds, with particular emphasis on the contrast between even- and odd-membered systems. We also discuss their coordination chemistry and the related metalla[ n ]cumulenes, which exhibit notable similarities. Key features such as reverse bond-length alternation (BLA), the HOMO–LUMO gap, and electrohelicity are examined in the context of anti-ohmic behavior, chiroptical activity, and current-density patterns, and how these properties translate into design principles for molecular wires, spin filters, and single-molecule magnets (SMMs).