We report a molecular dynamics study of chlorinated cobalt bis(dicarbollide) anions [(B₉C₂H₈Cl₃)₂Co]⁻ “CCD⁻” in nitrobenzene and at the nitrobenzene–water interface, with the main aim to understand the solution state of these hydrophobic species and why they act as strong synergists in assisted liquid–liquid extraction of metallic cations. Neat nitrobenzene is found to well solubilize CCD⁻, Cs⁺ salts in the form of diluted pairs or oligomers, without displaying aggregation. In biphasic nitrobenzene–water systems, CCD⁻ anions mainly partition to the organic phase, thus attracting Cs⁺ or even more hydrophilic counterions like Eu³⁺ into that phase. The remaining CCD⁻ anions adsorb at the interface, but are less surface active than at the chloroform interface. Finally, we compare the interfacial behavior of the Eu(BTP)₃³⁺ complex in the absence and in the presence of CCD⁻ anions and extractant molecules. It is found that in the absence of CCDs, the complex is trapped at the interface, while when the CCDs are concentrated enough, the complex is extracted to the nitrobenzene phase. These results are compared to those obtained with chloroform or octanol as organic phase and discussed in the context of synergistic effect of CCDs in liquid–liquid extraction, pointing to the importance of dual solvation properties of nitrobenzene or octanol to solubilize the CCD⁻ salts as well as the extracted complex.