unstable

https://doi.org/10.1351/goldbook.U06569
The opposite of @S05900@, i.e. the @CT01038@ concerned has a higher molar Gibbs energy than some assumed standard. The term should not be used in place of @R05180@ or @T06451@, although more reactive or transient species are frequently also more unstable. (Very unstable chemical species tend to undergo exothermic @M03989@ decompositions. Variations in the structure of the related chemical species of this kind generally affect the energy of the @T06468@ for these decompositions less than they affect the stability of the decomposing chemical species. Low stability may therefore parallel a relatively high rate of @M03989@ @D01547@.)
Source:
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1175 [Terms] [Paper]