Precursor NC 1 was synthesized following a reported method (Scheme S1). Subsequently, 2 was prepared by completely exchanging all 18 PET ligands of 1 with TBBT at 60 °C using a modified version of the method for synthesizing 2, which was previously reported by the authors 26 (Scheme S2). During this high-temperature ligand exchange reaction, greenish-brown product was generated and isolated by preparative thin-layer chromatography (PTLC). The purity and composition of the isolated product were confirmed by comparing the UV-vis spectroscopy and electrospray ionization-mass spectrometry (ESI-MS) data of the product with the data reported for [Au24Pt(TBBT)18] 0 (Figure ). Interestingly, 3 was serendipitously obtained while attempting to synthesize 2 using 97% pure TBBTH (TCI Chemicals) (Schemes 1 and S3). A blackish-green crude reaction mixture was obtained following the exchange reaction, and its UV-vis absorption spectrum was notably different from that expected for 2 (Figure ). Major product 3 was separated by PTLC (Figure ), and its purity was verified by reverse-phase high-performance liquid chromatography (RP-HPLC) (Figure ). The separated product generated only one RP-HPLC peak, confirming that it was pure. However, the retention time (30.9 min) of this peak was different from that of 2 (31.7 min), further indicating that a new NC was obtained. Efforts to determine the chemical composition of the newly formed NC using ESI-MS (Figure ) proved unsuccessful because the major peak (single peak) could not be assigned to any feasible chemical composition based on various combinations of the initial (PET) and exchanging (TBBT) ligands or their fragments, such as S 2-, 32 with Au and Pt from precursor 1 (Table ). To determine whether newly formed 3 contained any of the initial thiolate, namely PET from 1 ([Au24Pt(PET)18] 0 ), a different starting NC, [Au24Pt(C6)18] 0 , was used to synthesize 3 (Scheme S4). Surprisingly, the newly formed NC exhibited the same ESI-MS and UV-vis spectra as the material generated using 1 (Figure ), confirming that the same NCs were formed in both reactions. Hence, the new NCs did not contain the starting thiolate (PET or C6) but instead incorporated either a new ligand generated in situ or impurities present in TBBTH.