3H-1,2,3,4-Triazaphosphole derivatives can be selectively alkylated with Meerwein's reagent at the most nucleophilic nitrogen atom. According to the principle of valence isoelectronicity, the corresponding phosphorus heterocycle represents the first formal phosphorus analogue of the well-known 1,2,3-triazolylidenes (mesoionic carbenes). Theoretical calculations revealed that the cation in triazaphospholenium tetrafluoroborate is an aromatic system with a high degree of π-conjugation. First investigations showed that the cationic phosphorus heterocycle can stabilize a [Cu2Br4]2− dianion by formation of a neutral coordination compound with an unusual bonding situation between phosphorus and copper(I).