Please do not adjust margins The obtained BB cluster was dissolved in acetonitrile, and a solution of iron(II) tetrafluoroborate (Fe(BF4)2) in acetonitrile (12.3 μM) was added. Complex formation was monitored using ultraviolet-visible (UV-vis) absorption spectroscopy. The chemical composition of the products after complexation was evaluated by mass spectrometry. For products obtained by adding Fe(BF4)2 in excess of the equivalence point, mass spectrometry was conducted after removing excess Fe 2+ .