The gas-phase reaction of size-selected Ru-n(+) (n = 4-6) clusters with CO in an ion trap yields only one specific ruthenium carbonyl complex for each cluster size, Ru-4(CO)(14)(+), Ru-5(CO)(16)(+), and Ru-6(CO)(18)(+). First-principles density functional theory calculations reveal structures for these hitherto unknown carbonyl compounds that are in perfect agreement with the geometries predicted by Wade's electron counting rules. Furthermore, reactions with D-2 show that for Ra-4(+) and Ru-6(+), CO molecules can be partially replaced by D-2 to form hydrido carbonyl complexes while preserving the total ligand count corresponding to the Wade cluster sizes.