Some "parent compounds" have not been or cannot be isolated, even though stable structural analogs with substituents have been discovered or synthesized (e.g. borole C4H4BH).
[1] Another stable compound, potassium trichromate, has been produced in a small scale and is known to be a very powerful oxidizing agent.
Despite the fact that rhenium heptahydride ReH7 has not been isolated, its salt potassium nonahydridorhenate(VII) is stable.
"[Using] the Born–Haber cycle to estimate ... the heat of formation ... can be used to determine whether a hypothetical compound is stable."
However, "a negative formation enthalpy does not automatically imply the existence of a hypothetical compound."