Phosphide

In chemistry, a phosphide is a compound of phosphorus with a less electronegative element or elements. Binary compounds are formed with the majority of less electronegative elements with the exception of Hg, Pb, Sb, Bi, Te, Po. Typically there are a range of stoichiometries with each element; for example, potassium has nine phosphides (K₃P, K₄P₃, K₅P₄, KP, K₄P₆ K₃P₇, K₃P₁₁, KP_10.3, KP₁₅) and nickel has eight (Ni₃P, Ni₅P₂, Ni₁₂P₅, NiP₂, Ni₅P₄, NiP, NiP₂, NiP₃).

Classification of these compounds is difficult. On structural and reactivity grounds they can be broadly classified as:

• principally ionic with P−
  3 ions. Examples are the group 1 (e.g. Na₃P) and group 2 (e.g. Ca₃P₂) metal phosphides.
• polyphosphides with, for example, dumbbell P4−
  2 ions; cluster P3−
  11 ions; polymeric chain anions (e.g. the helical (P−)_n ion) and complex sheet or 3-D anions.
• compounds with individual P atoms in a metal lattice that can be semiconducting (e.g. GaP) through to metallic (e.g. TaP) in terms of electrical conductance.

Two polyphosphide ions, P40−
3 found in K₄P₃ and P5−
4 found in K₅P₄, are radical anions with an odd number of valence electrons making both compounds paramagnetic.