Isolobal fragments of tetrahedral and octahedral molecules can be related. The analogy applies to other shapes besides tetrahedral and octahedral geometries. The adducts are usually either octahedral or tetrahedral. The cation and the anion are tetrahedral and octahedral, respectively. These ratios refer to the proportion of tetrahedral sheets to octahedral sheets. Other common coordination geometries are tetrahedral and square planar. White phosphorus forms molecular crystals composed of tetrahedral P molecules. All of these consist of regular tetrahedral and icosahedral cells. Alkyl amines characteristically feature tetrahedral nitrogen centers. As in simpler alkanes, carbon in the CFCs bonds with tetrahedral symmetry. The smaller anions, and feature tetrahedral centres. The tetrahalides are volatile solids existing as monomeric tetrahedral molecules in the vapor phase. The most prevalent compounds of phosphorus are derivatives of phosphate (PO), a tetrahedral anion. In most organosilicon compounds, Si is tetravalent with tetrahedral molecular geometry. NaBH is a salt, consisting of the tetrahedral BH anion.