The structure of neodymium hexaboride (NdB₆) is an example of this latter complex three-dimensional structural type, in which B₆ octahedra are arranged in a body-centered cubic lattice with the octahedra linked to the apices of other octahedra in all six directions, giving a rigid yet relatively open structure. The strong multicenter, covalent bonding of these boron polyhedra is believed to impart the observed high stability, hardness, and high melting points to most of the boride materials. While it is not possible to account for the boride structures in simple bonding terms, it is generally believed that the metal center donates electrons to the boron units in the boron-rich compounds, such as NdB₆. In the case of NdB₆ , the closo-boron octahedra require 14 valence electrons (2n+2), of which 12 are provided by the boron atoms. If the neodymium then provides two electrons to the cage, one ‘free’ valence electron should remain per metal center, making the material an excellent conductor. This analysis is consistent with Hall-effect, solid-state 11 B NMR and conductivity measurements on these materials.