The addition of Fe(BF4)2 acetonitrile solution to the BB cluster containing 20-2 resulted in an increase in absorbance at ~567 nm (Fig. ). The peak displayed a comparable shape and absorption wavelength to the metal-to-ligand charge transfer (MLCT) absorption peak of [Fe(Tpy)2] 2+ , which is formed by Fe 2+ and two Tpy molecules. This result suggests that the addition of Fe 2+ led to the formation of a complex between Fe 2+ and the Tpy sites of 21.