Polyphosphide

Polyphosphide refers to a class of compounds that contain polymeric phosphorus skeletons, typically as negatively charged anions or neutral clusters. The defining feature is P–P bonded phosphorus atoms arranged in chains, rings, or clusters, rather than discrete P4 molecules alone. In many salts, these polyphosphide units are balanced by metal cations such as alkali metals or alkaline earths.

Structural motifs include linear chains (P_n)^{m−}, cyclic cyclo-P_n rings, and larger phosphorus clusters. The negative charge is delocalized over the framework and stabilized by coordination to metal centers. Polyphosphides can be isolated as salts M_x[Pn], where n≥3, and are often characterized by X-ray crystallography and vibrational spectroscopy.

Preparation commonly involves the reduction of white phosphorus (P4) with metals in coordinating solvents, such as liquid ammonia or ethylenediamine, producing polyphosphide salts. Other routes include reaction of P-containing precursors with low-valent metal complexes, or high-temperature condensation of phosphide sources. They are reactive toward air and moisture, requiring inert atmosphere.

In coordination and organometallic chemistry, polyphosphide ligands can bridge metals and support unusual bonding modes. Polyphosphide chemistry intersects with materials science, offering routes to materials with interesting electronic or magnetic properties and providing insight into phosphorus–phosphorus bonding.