However tellurium hydride and polonium hydride are both volatile and highly labile. Some of the carbonyl ligands are highly labile. It is also the first ionization method to ionize nonvolatile and thermally labile compounds. Many related complexes with cis labile sites are active catalysts. This occurs because of preferential decomposition of labile molecules over refractile molecules. The Oppenauer oxidation is still used for the oxidation of acid labile substrates. The more labile compounds decompose quickly, leaving an increasing proportion of recalcitrant material. NHS reacts to form a less labile activated acid. In general terms, any solvent that contains a labile H is called a protic solvent. This rapid increase in available labile substrates causes short-term enhanced soil respiration. The fourth coordination position is taken up by a labile water molecule. Non-directed alternations are further subdivided into labile, equipollent, and suppletive alternations. Intermittent or nonreciprocal responses lead to labile vacillation between despair and ecstacy. In medicine, the term "labile" means susceptible to alteration or destruction. It, along with other labile viruses, can also persist longer in serum and tissue debris.