High pressure chemistry tend to return compounds with very interesting and unusual bonding structures, such as $\ce{NaCl7}$, metallic hydrogen and $\ce{Na2He}$

However, for most of these, the resulting product is only at best metastable (e.g. diamond) when returning to ambient conditions, and does not survive when pressure returns to ambient conditions (r.t.p.).

For the following requirements, a compound that has unusual bonding similar to those 3 reported above, that is things like n centre m electron bonds is desired

Are there exceptions to this rule, where the high pressure is mainly needed to overcome the energy barrier and the resulting product is more stable than the reactants such that the product stays stable (instead of metastable) even when it is decompressed (That is, a spontaneous but kinetically slow reaction where pressure is the major factor to activate the reaction)?

If there are no known exceptions, what are possible theoretical reasons that products formed from high pressure are usually metastable and cannot survive at low pressures. Put it simply, why are most reactions reversible under the change of pressure?

  • $\begingroup$ "Usually" is a wrong generalisation. Completely wrong. Heard of Le Chatelier? Phase diagrams? Enthalpy, entropy? Activation energy? The full answer to your question is several months of chemistry lectures. $\endgroup$ – Karl Jan 30 '17 at 9:31
  • $\begingroup$ For substances with phase diagrams, a given phase can only exists at the given pressure and temperature (e.g. liquid CO2). While my undergrad physical chemistry course have went through all the concepts you mentioned above, there are few examples of pressure activated reactions (except when gases equlibria are involved). Most reactions we went through are heat, light activated. Graphite to diamond would have been the answer had diamond being not metastable near ambient conditions $\endgroup$ – Secret Jan 30 '17 at 9:42
  • $\begingroup$ To overcome an activation energy only by increasing pressure is nonsense. At high pressure, the activation energy can however be smaller, the kinetics can be different, etc. But why do complicated, dangerous pressure experiments, when very often a rather small temperature increase does the trick? $\endgroup$ – Karl Jan 30 '17 at 10:15
  • $\begingroup$ "But why do complicated, dangerous pressure experiments, when very often a rather small temperature increase does the trick?" The edited questions shows how compounds with unusual bonding tend to form under pressure driven reactions. In particular, metallic hydrogen requires cooling in addition to high pressure to form, which probably due to more complicated reasons then energy barrier arguments $\endgroup$ – Secret Feb 8 '17 at 8:12
  • $\begingroup$ What do you want, a STP stable product or some arcane high pressure phase? Metallic hydrogen still does not exist, that recent article was humbug. Any speculation what is "complicated" about it just that, speculation. Unless you are some highly talented theoretical chemist. :-/ $\endgroup$ – Karl Feb 11 '17 at 9:52