Reaction of HB(C₆F₅)₂ with (^tBu₃C₆H₂)P(vinyl)₂ does not yield the expected ethylene-bridged frustrated Lewis pair (FLP) but its cyclic methylene phosphonium isomer which is formed in a unique way by subsequent internal 1,2-P/B addition to the vinylphosphane unit. The product is a rare example of a stable phosphorus analogue of the ubiquitous iminium salts. It features a short PC bond (1.637(3) Å). Subsequent reactions with pyridine and with dihydrogen probably proceed by equilibration with the “invisible” ethylene-bridged FLP isomer. The reaction of (^tBu₃C₆H₂)P(vinyl)₂ with B(C₆F₅)₃ gave the zwitterionic acyclic methylene phosphonium product that was crystallized at low temperature and isolated. The X-ray crystal structure analysis also revealed a short PC bond (1.635(5) Å).