Abstract In this work, the cobalt(II) complexation with 1,10-phenanthroline (Phen) in the presence of the [trans-B₂₀H₁₈]^2– anion has been studied. At a Co : Phen = 1 : 2 ratio in acetonitrile, a binuclear cobalt(II) complex with bridging chlorine atoms and the boron cluster anion as a counterion [(Phen)₂Co(µ-Cl)₂Co(Phen)₂][trans-B₂₀H₁₈] has been isolated. However, during slow crystallization (within a month), spontaneous isomerization of [trans-B₂₀H₁₈]^2– into [iso-B₂₀H₁₈]^2– is observed. It has been established by X-ray diffraction analysis that in the crystal of the final compound [(Phen)₂Co(µ-Cl)₂Co(Phen)₂][trans-B₂₀H₁₈]_1/3[iso-B₂₀H₁₈]_2/3, co-crystallization of both isomeric forms of the octadecahydroeicosaborate anion is observed for the first time. The presence of the iso form of the boron cluster anion is also confirmed by IR spectroscopy data: the spectrum of the product shows a band of stretching vibrations of the BHB bridge groups at 1773 cm^–1, which is absent in the trans-form of the boron cluster.