Tiara-like metal nanoclusters (TNCs) composed of group 10 transition metals and thiolates can easily change their number of polymerization and include various molecules or metal ions as guests within their ring structures. Therefore, they are expected to be applied in sensing, storage, and catalyst materials based on their selective inclusion characteristics. However, there are very few reports regarding the principles of selective inclusion for guest molecules/ions in TNCs. In this study, we focused on the nickel (Ni) hexanuclear TNC, Ni6(PET)12 (PET = 2-phenylethanethiolate), to clarify the kinds of metal ions that can be included in TNCs. As a result, we found that the gold (Au) ion can be selectively included in Ni6(PET)12 among various metal ions to form a stable structure. From various experiments and density functional theory calculations, we concluded that the main reasons for this are that [Ni6(PET)12]0 has (1) a pore large enough to include Au ions and (2) Ni and Au favour the formation of bonding orbitals based on charge transfer interactions. These insights are expected to lead to a better understanding of host-guest interactions in TNCs and provide clear design guidelines for forming various inclusion structures in the future.