Resolving an Anomalous Ultrashort Cu–Cu Bond in the Ag16Cu18(CC–C6H11)24 Nanocluster: A Structural Reinterpretation from DFT Analysis

The bimetallic nanocluster Ag16Cu18(CC–C6H11)24 was recently reported to display exceptionally short Cu–Cu separation, as short as 1.62 Å, within its central Cu6 ring, a distance far below typical Cu–Cu metallic bonds (∼2.64 Å) and even approaching that of a C–C single bond (∼1.54 Å). Through comprehensive density functional theory (DFT) investigations and detailed structural analysis, we demonstrate that this anomalously short bond length likely arises from a crystallographic misinterpretation. Crystal structural re-refinement revealed that the central Cu6 ring is actually a superposition of two oppositely oriented Ag3 triangular units, with each Ag site refined at 50% occupancy and the molecular formula being revised to Ag19Cu12(CC–C6H11)24. The re-refinement, in which the full Cu occupancy is replaced with 0.5 Ag occupancy, yields substantial improvement in R-factors (5.81% → 5.23%). Our results offer a chemically consistent framework that reconciles all experimental data with electronic-structure principles.