8-Tetrahydrofuronium and 8-tetrahydropyronium derivatives of iron bis(dicarbollide)(-I) were synthesized. Their reactions of ring cleavage by MeOH, N3(-), amines and 1,2-bis(diphenylphosphino)ethane were investigated. 8-Tetrahydrofuronium iron bis(dicarbollide)(-I) was found to be more active in these reactions in comparison with 8-tetrahydropyronium and 8-dioxonium species, respectively. The first conjugates of iron bis(1,2-dicarbollide)(-I) with 2'-deoxyadenosine modified via the C-8 position of the purine base were synthesized. Reactions of these oxonium compounds with 1,2-bis(diphenylphosphino)ethane (dppe) led to zwitter-ionic monophosphonium salts. One of these compounds has given rise to a novel ferracarborane/cobaltacarborane hybrid complex.