Zinc(I) compounds are rare and need bulky ligands to stabilize the low oxidation state. Most zinc(I) compounds contain formally the [Zn 2 ] 2+ core, which is analogous to the [Hg 2 ] 2+ dimeric cation present in mercury (I) compounds. The diamagnetic nature of the ion confirms its dimeric structure. The first zinc(I) compound containing the Zn–Zn bond, (η 5 -C 5 Me 5 ) 2 Zn 2 , is also the first dimetallocene . The [Zn 2 ] 2+ ion rapidly disproportionates into zinc metal and zinc(II), and has been obtained only a yellow glass only by cooling a solution of metallic zinc in molten ZnCl 2 .