The inset in Fig. illustrates the correlation between the concentration ratio [Fe 2+ ]/ [20-2] and the absorbance intensity ~567 nm. As the [Fe 2+ ]/ [20-2] ratio increased, the absorbance increased linearly until at [Fe 2+ ]/[20-2] = 0.18, where the absorbance plateaued with higher concentration ratios. This result suggests that [Fe 2+ ]/[20-2] = 0.18 represents the equivalence point of complex formation ([Fe 2+ ]/[20-2]eq). Typically, for the formation of [Fe(Tpy)2] from Fe 2+ and two Tpy molecules, the equivalence point is at [Fe 2+ ]/[Tpy]eq = 0.5. However, because the products obtained in this study mainly consist of 20 and 21, the equivalence point is lower. By introducing the composition distribution obtained from the ESI mass spectrometry data into the stoichiometric equation (Equation S10), a value of [Fe 2+ ]/[20-2]eq = 0.13 is obtained. This value is in relatively good agreement with the value of [Fe 2+ ]/[20-2]eq = 0.18 determined from the absorbance measurements, suggesting that 21, formed through ligand exchange, efficiently complexes with Fe 2+ . Fig. (curve b) shows the mass spectrum of the product obtained after adding an excess amount of Fe(BF4)2 beyond the equivalence point and removing excess Fe 2+ . The peak corresponding to 21 was absent and only the peak corresponding to unreacted 20 remained. When examining the mass spectrum over a broader range, peaks attributed to [(21)2Fe + (8-z)(BF4)] z+ (z = 3-7) were observed in the high mass-to-charge (m/z) ratio region (Fig. ). These peaks correspond to Au13 dimers formed by the complexation of Fe 2+ with two 21 units. For example, the charge state of +7 of the dimer molecule can be explained by a combination of two Au13 units with a charge of +3, one Fe ion with a charge of +2, and one counteranion [BF4] -. Because the product used in this study (Fig. ) has a low propensity of forming larger aggregates-such as 22, multimeric structures such as trimers (n = 3) or tetramers (n = 4), represented as ( )2( )n-2Fen-1 (n ≥ 3)-peaks corresponding to large aggregates were barely observed in the mass spectrum. These results indicate that BB clusters were successfully designed to selectively form dimers as the connected structure by simply adding Fe 2+ (Fig. ).