Phosphide is an ion. Its chemical formula is sometimes P3-. It does not always have that formula. It is a reducing agent. Phosphides (chemical compounds having the phosphide ion in them) react with acids to make phosphine gas. Sodium phosphide is an example. Some phosphides are semiconductors and are not ionic with P3- ions. They are less reactive. Some metals form alloys with phosphorus to make phosphides. If it is ionic, then the phosphorus is in the -3 oxidation state.