The existence of a dihydrogen bond (S-(HH)-H-...-B) and its combination with a C-(HS)-S-... hydrogen bond in an unusual cooperative effect are demonstrated from a combination of experimental and theoretical methods. This cooperative effect seems to be responsible for self-assembly of mercaptane-metallacarborane complexes such as closo-[3-Ru(eta(6)-C6H6)-8-HS-1,2-C2B9H10] (1) and closo-[3-Co(eta(5)-C5H5)-8-HS-1,2-C2B9H2O] (3), which present identical supramolecular two-dimensional polymeric networks. The findings, besides documenting structurally the first S-H...(H-B)(2) dihydrogen bond and the unconventional cooperative ability of a boron-attached SH group, prove that substituted carboranes have the potential to serve as building blocks for assembling complex structures.