Complex sentence: Unstable gas-phase heteroelement-hydride ions can be extremely acidic, e.g. the helium hydride ion, but conventionally these are not counted since they exist only in extremely hot plasma, not in the liquid state.
Split into simpler sentences: Unstable gas-phase heteroelement-hydride ions can be extremely acidic, e.g. the helium hydride ion. However, they are not considered as such because they exist only in extremely hot plasma not in the liquid state.