Do ionic or metallic structures have higher melting points?Watch
Do ions not undergo disproportionation? Why? e.g. HBr(aq) + Cl-(aq)
But it's hard to make that comparison because you need to consider factors like charge and ionic radius. I.e the charge density.
Its harder to compare the groups holistically and is more accurate and reliable to compare on an individual scale.
Mercury = 234 K
MgO = 3125 K (I am sure there will be higher, but I can't be bothered to find them)
HgI = 532 K (there are ionic liquids at RTP, you can look them up, if you like)
My point: there is a range of values for both. The question, "which one is higher" makes no sense.
Lots of disproportionation reactions involve ions: 2Sn^2+ → Sn^4+ + Sn, 2S2O4^2− + H2O → S2O3^2− + 2HSO3^− etc.