Calcium monophosphide explained

Calcium monophosphide is the inorganic compound with the formula CaP. It is sometimes also known as "calcium phosphide", which also describes a different compound with composition Ca₃P₂. Calcium monophosphide is a black solid.

Structure and properties

The structures of CaP and sodium peroxide (Na₂O₂) are very similar.^[1] The solid is described as a salt: (Ca²⁺)₂P₂⁴⁻, or Ca₂P₂. Since the bonding is ionic, the diphosphide centers carry negative charge and are easily protonated. Upon hydrolysis this material releases diphosphine (P₂H₄):^[2]

Ca₂P₂ + 4 H₂O → 2 Ca(OH)₂ + P₂H₄The hydrolyses of CaP and calcium carbide (CaC₂) are similar, except that diphosphine spontaneously ignites in air. Thus, CaP must be protected from air.

CaP decomposes to Ca₃P₂ at about 600 °C.

3 CaP → Ca₃P₂ + 1/4 P₄

References

1. Iandelli, A. and Franceschi, E., "On the crystal structure of the compounds CaP, SrP, CaAs, SrAs and EuAs", Journal of the Less Common Metals, 1973, volume 30, pp. 211-216.
2. Marianne Baudler, Klaus Glinka . Monocyclic and polycyclic phosphines . . 1993 . 93 . 1623–1667 . 10.1021/cr00020a010 . 4.