We report the first evidence that the antiviral active (inhibitor of HIV protease) boron cluster [3-cobalt bis(1,2-dicarbollide)](-) anion. CoD-, interacts and forms a stable complex with one of the most widely used polymeric components of drug delivery systems: poly(ethylene oxide), PEO. The metallacarborane/polymer complex is insoluble in aqueous solutions. The amount of the precipitate depends oil concentration of alkaline or earth-alkaline cations. The formation of insoluble complex is the result of a combination of several Factors. One of the decisive contributions is the formation of dihydrogen bonds between negatively charged hydrogen atoms attached to boron atoms and slightly positively charged H atoms in repealing -CH2-CH2-O- units. It is also important that alkaline cations interact with oxygen atoms of PEO. The formation of the insoluble NaCoD/PEO complex can be exploited ill design of water-soluble [3-cobalt bis(1,2-dicarbollide)]-containing nanoparticles which could offer applications ill medicine. We studied the boron cluster interaction with well-defined double hydrophilic block copolymer: poly(ethylene oxide)-block-poly(methacrylic acid), PEO-PMA. The interaction leads to it spontaneous formation of core-shell nanoparticles. The insoluble core contains the PEO/CoD-complex, while the polyanionic PMA blocks, which do not interact with the cobaltacarborane, form the pH-responsive micellar shell and stabilize the particles in aqueous media. The nanoparticles were studied by light and X-ray scattering, NMR spectroscopy, electrophoresis, and microscopy techniques like AFM and cryo-TEM. It was found that the cores are not completely frozen in aqueous media. Their composition depends oil salt concentration, and the metallacarborane call diffuse from/into the nanoparticle after the salinity change.