The homoleptic phosphine with the bulky perchlorophenyl group, (C₆Cl₅)₃P (1), exhibits trigonal pyramidal structure (TPY-3), yet considerably flattened: Σ(C-P-C') = 321.0(1)º. Key steric and electronic properties of this simple organophosphorus species have been estimated by calculation. Attending to its characteristic features, 1 can be rated as a deactivated phosphine, where the less-basic P atom is sterically shielded by the bulky C₆Cl₅ groups. This marked inertness notwithstanding, it has been possible to obtain (under harsh conditions) derivatives with phosphorus in high oxidation state, namely the phosphine oxide (C₆Cl₅)₃PO (2) and the difluorophosphorane (C₆Cl₅)₃PF₂ (3). These four- and five-substituted derivatives respectively exhibit trigonal pyramidal (TPY-4) and trigonal bipyramidal (TBPY-5) structures. The Σ(C-P-C') value steadily increases along the series 1-3, according to the referred structural variation. The P-C bond length is, in turn, invariably maintained at about 185 pm regardless of the different oxidation state, the increasing number of substituents around the P atom and the overall geometry. The hypervalent difluorophosphorane (C₆Cl₅)₃PF₂ (3) dissociates one of the axial fluorides in the gas phase giving rise to the fluorophosphonium cation [(C₆Cl₅)₃PF]⁺, as detected by mass spectrometry. This cation is identified as a Lewis superacid.