Polyanionic species have been obtained in high yield by a new route in the ring-opening reaction of cyclic oxonium [3,3'-Co(8-C4H8O2-1,2-C2B9H10)(1',2'-C2B9H11)] (2) by using carboxylic acids, Grignard reagents, and thiocarboranes as nucleophiles. The crystal structures of Na-3(H2O)(C2H5OH)[1 '',3 '',5 ''-[3,3'-Co(8-O(CH2CH2O)(2)-1,2-C2B9H10)(1',2'C2B9H11)}(3)-C6H3] and Na(H2O)[3,3'-Co(8-O(CH2CH2O)(2)C(O)CH3-1,2-C2B9H10)(1',2'-C2B9H10)] show that the chain contributes three or two oxygen atoms for coordination to Na+, and interestingly, the [3,3'-Co(1,2-C2B9H11)(2)](-) moiety provides extra B-H coordination sites. These B-H center dot center dot center dot Na interactions in the solid state have also been confirmed by dynamic NMR studies in solution. These new polyanionic compounds that contain multiple carborane or metallacarborane clusters at their periphery may prove useful as new classes of boron neutron capture therapy compounds with enhanced water solubility and as a core to make a new class of dendrimers.