Rules of Thumb for the Determinants of Structure

Rules of Thumb for the Determinants of Structure and Electronics Under High Pressure Roald Hoffmann (A) van der Waals space is most easily compressed. (B) Ionic and covalent structures, be they molecular or extended, respond to pressure by increasing coordination. (C) Electron deficient multi-center bonding is likely to be important as a mechanism for compactification in compounds that are octet or electron-deficient structures, i. e. involving group 13 and 14 atoms. (D) Increased coordination is achieved relatively easily for groups 14, 15, 16, 17 through donor-acceptor bonding shading over into electron-rich three-center bonding. (E) Orbital symmetry considerations will affect the chance that a high pressure product survives return to metastability in the ambient pressure world. (F) In ionic crystals, anions (Lewis bases) are more compressible than cations (Lewis acids); therefore the coordination number (especially that of cations) increases at high pressure. (G) Virtually all materials become metallic under sufficiently high pressure. (H) Thinking about Peierls distortions, their enhancement and suppression, is key to understanding the symmetrization (or its absence) of solids under high pressure. (I) Under extremes of really high pressure, electrons may move off atoms, and new bonding schemes need to be devised. (J) Close packing is the way, for a while. But keep an open mind – still denser packing may be achieved through electronic disproportionation. (K): Pressure may cause the occupation of orbitals that a chemist would not normally think are involved.