2 - d io x et an ed ione ▁C 2 O 4 ; ▁and ▁the ▁stable ▁cro con ate ▁an ion ▁ ▁corresponds ▁to ▁the ▁neutral ▁cy c lop ent an ep ent one ▁C 5 O 5 , ▁which ▁has ▁been ▁detected ▁only ▁in ▁trace ▁amounts . ▁ ▁Red u ced ▁variants ▁Con vers ely , ▁some ▁ox oc ar bon ▁an ions ▁can ▁be ▁reduced ▁to ▁yield ▁other ▁an ions ▁with ▁the ▁same ▁struct ural ▁formula ▁but ▁greater ▁negative ▁charge . ▁ ▁Thus ▁rh od izon ate ▁ ▁can ▁be ▁reduced ▁to ▁the ▁ ▁t et rah ydro xy ben zo quin one ▁( TH B Q ) ▁an ion ▁ ▁and ▁then ▁to ▁ben z ene hex olate ▁. ▁ ▁Ac id ▁an h yd rides ▁An ▁ox oc ar bon ▁an ion ▁ ▁can ▁also ▁be ▁associated ▁with ▁the ▁an h yd ride ▁of ▁the ▁corresponding ▁acid . ▁ ▁The ▁latter ▁would ▁be ▁another ▁ox oc ar bon ▁with ▁formula ▁C x O y − ; ▁namely , ▁the ▁acid ▁minus ▁ ▁water ▁mole cules ▁H 2 O . ▁ ▁The ▁standard ▁example ▁is ▁the ▁connection ▁between ▁carbon ate ▁ ▁and ▁carbon ▁dio x ide ▁ ▁CO 2 . ▁The ▁correspond ence ▁is ▁not ▁always ▁well - defined ▁since ▁there ▁may ▁be ▁several ▁ways ▁of ▁performing ▁this ▁formal ▁de h yd r ation , ▁including ▁joining ▁two ▁or ▁more ▁an ions ▁to ▁make ▁an ▁o lig omer ▁or ▁polym er . ▁ ▁Un like ▁neutral ization , ▁this