Structural evolution of small ruthenium cluster anions

The structures of ruthenium cluster anions have been investigated using a combination of trapped ion electron diffraction and density functional theory computations in the size range from eight to twenty atoms. In this size range, three different structural motifs are found: Ru₈⁻–Ru₁₂⁻ have simple cubic structures, Ru₁₃⁻–Ru₁₆⁻ form double layered hexagonal structures, and larger clusters form close packed motifs. For Ru₁₇⁻, we find hexagonal close packed stacking, whereas octahedral structures occur for Ru₁₈⁻–Ru₂₀⁻. Our calculations also predict simple cubic structures for the smaller clusters Ru₄⁻–Ru₇⁻, which were not accessible to electron diffraction measurements.