Lewis Acid-Base Pairs: The Bonding Rule of Closed-Shell M···M' Interactions (M = HgII/PdII; M' = AgI/AuI)

Metallophilic interactions are the widespread interactions in multimetal clusters to orientate closed-shell metal self-assembly form linear, facet, or block clusters. The closed-shell metal cation does not have empty valence orbitals, but is able to attract each other. It is still a conundrum to understand the resource in balancing the strong Coulomb repulsion between two cations. Most traditional descriptions attribute the counterintuitive attractions to London dispersion, Pauli repulsions, and ambiguous orbital interactions. However, neither the dispersion nor the unsourced donor-acceptor interaction can be applied to explain the saturability and directionality in multimetal clusters, where the M···M' structure is the basic molecular unit. Here, we clarify the origination of the covalency in closed-shell metallophilic interactions based on the study of heterobimetallic compounds composed of d¹⁰-d⁸ species (Ag^I/Au^I-Pd^II) and d¹⁰-d¹⁰ species (Ag^I/Au^I-Hg^II) obtained from experiments. The inner d electrons not only participate in the metallophilic interactions but also show different Lewis acidity and basicity in the formation of M···M' structures. The present work not only provides us a novel covalent perspective to visualize the closed-shell M···M' interactions but also unveils the truth of metallophilic interactions.