The synthesis of a new bridged [8,8′-μ-CH2O(CH3)-(1,2-C2B9H10)2-3,3′-Co]0 derivative (2), arising from the acid-catalyzed reaction of cobalt bis(1,2-dicarbollide)(1−) ion with formaldehyde, is reported. The proposed reaction path is supported by the isolation of side products including two zwitterionic compounds, the known bridged [8,8′-μ-(CH3O)-(1,2-C2B9H10)2-3,3′-Co]0 derivative (3), the new zwitterion [(8-(CH3)2O-1,2-C2B9H10)-(1′,2′-C2B9H11)-3,3′-Co]0 (4), and two anionic compounds—the known [(8,8′-Cl2-1,2-C2B9H10)2-3,3′-Co]− and the newly characterized dimethoxy derivative [(8,8′-(CH3O)2-1,2-C2B9H10)2-3,3′-Co]− of the cobalt bis(dicarbollide) ion. Compound 2 serves as a versatile building block for the construction of zwitterionic derivatives, as exemplified by the synthesis of a series of compounds of general formulation [(8-X-CH2-1,2-C2B9H10)(8′-CH3O-1′,2′-C2B9H10)-3,3′-Co]0 (6). Compounds of type 6 bear organic end groups (X = NC5H5 (6a), C6H13NH2 (6b), 2-HOC2H4NH2 (6c), (C6H5)3P (6d)) adjacent to the cluster via a methylene spacer. The reactions with alcoholates or phenolates, demonstrated by the isolation of a derivative with X = 1-O(4-t-Bu-C6H4) (7–) in low yield, seem less advantageous due to the competing demethylation of the oxonium bridge in 2, which results in the preferential formation of the anion [8,8′-μ-CH3O-(1,2-C2B9H10)2-3,3′-Co]− (8–). A similar side reaction was found to occur in the synthesis of a tetrasubstituted tert-butyl-calix[4]arene (9–), where one calixarene OH site was methylated producing the metallacarborane ion 8– instead of the bridge opening. The molecular structures of 6a,c–e, 8–, and 9– were determined by single-crystal X-ray diffraction analyses, and all compounds were characterized by high-resolution NMR (1H, 13C, and 11B) and mass spectrometry.