Resolving an Anomalous Ultrashort Cu-Cu Bond in the Ag16Cu18(C≡C-C6H11)24 Nanocluster: A Structural Reinterpretation
Farshad Shiri1, Herman H Y Sung1, Ian D Williams1
1Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 999077, China.
Abstract:
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.
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