Bonding in CuP 2 has some interesting features that are worth a brief analysis. Bulk CuP 2 crystallizes in the monoclinic structure shown in Fig. , with space group P2 1 /c. The structure consists of alternating sheets of CuP 4 tetrahedra and of homoelement-bonded P atoms in planes parallel to (100) (Fig. ). Each CuP 4 tetrahedron shares an edge and three corners with other analogous tetrahedra. The existence of anion-anion bonding is a key qualitative difference between P-rich compounds like CuP 2 and most optoelectronic compounds such as III-V and II-VI semiconductors. The generalized 8 -N rule can then be used to interpret bonding. In this framework, one may assign the -1 oxidation state to one half of the P atoms, since they are bonded to two other P atoms and three Cu atoms. The remaining P atoms have three P-P bonds and one Cu-P bond and are formally neutral, as the three homoelement bonds complete their octet. To achieve charge neutrality, Cu should then be in the +1 oxidation state. While explicit calculations indicate that only about 30% of this charge is actually transferred to P due to significant covalency, they also confirm that the charge is only accepted by the P atoms that are in the -1 oxidation state. Thus, CuP 2 can be considered a relatively rare example of a compound with mixed anion valence.