The metal complex formation and the electrical properties of amorphous solid polymer electrolytes, based on poly(butadiene–acrylonitrile) copolymer (PBAN) and CoCl2, have been studied over the homogeneity region of the system limited by the CoCl2 concentration of 1.89molkg−1. It has been found that ionic conductivity is carried out by the unipolar anion transfer at lower CoCl2 concentrations (up to 0.10molkg−1). As the CoCl2 concentration increases, electronic conductivity appears in addition to ionic conductivity, and the former becomes dominant, starting from 0.38molkg−1. It has been shown that the nature of charge carriers is determined by the composition of metal complexes formed by CoCl2 and the macromolecular solvent PBAN. At lower concentrations, the [Co2L2Cl4]0 dimers are the predominant species (L being macromolecule side groups CN), and their dissociation is followed by the formation of mobile Cl− anions and immobile binuclear [Co2Cl3]+ complexes. As CoCl2 concentration increases, polynuclear [Co n L2Cl2n ]0 (n >2) complexes appear (L being CN and CC groups of PBAN). Specific features of chemical bonds in π-complexes of transition metals result in the appearance of electronic charge carriers. The abrupt increase in conductivity observed at the highest CoCl2 concentration is connected with the formation of a percolation network of polynuclear [Co n L2Cl2n ]0 complexes.