Salts (and hydroxides)
[R4P+]X− containing tetracoordinate phosphonium ion and the associated anion.
See: onium compounds
Source:
PAC, 1995, 67, 1307. 'Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)' on page 1358 (https://doi.org/10.1351/pac199567081307)