Phenyl- and 1,2-diphenyl-1,2-dicarba-closo-dodecaborane have been synthesized and deboronated to yield [7-C6H5-7,8-C2B9H11]– and [7,8-(C6H5)2-7,8-C2B9H10]–. Reaction of the monophenyl with KOBut and CoCl2 in 1,2-dimethoxyethane (dme) led to [3,3′-Co(1-C6H5-1,2-C2B9H10)2]– 1–. This has been fully characterized by X-ray analysis. A similar reaction with [7,8-(C6H5)2-7,8-C2B9H10]– did not yield the expected sandwich compound. On the contrary, reaction of [7,9-(C6H5)2-7,8-C2B9H10]– with KOBut and CoCl2 in a mixture of dme and diethylene glycol dimethyl ether led to [3,3′-Co{1,7-(C6H5)2-1,7-C2B9H9}2] 2–. The two sandwich complexes were tested for liquid–liquid extraction and transport through supported liquid membranes of 137Cs, 90Sr and 152Eu. Extractions for 137Cs were very good at pH 3 but decreased considerably at pH 1 for H[1], while H[2] showed good efficiency even at this pH. The extraction of 90Sr was performed with the incorporation of a synergistic linear polyether and was good at pH 3 and 1. For the extraction of 152Eu, compound H[2] displayed good efficiency. Transport experiments of Cs using supported liquid membranes were performed with H[1] and H[2] and o-nitrophenyl hexyl ether as the membrane solvent. The permeability of 137Cs for compound H[2] was 30.9 cm h–1 while for H[1] it was 7.95 cm h–1. These permeabilities are much higher than those of usual carriers under comparable conditions.