The maximum number of carbonyl groups around an Ru6C polyhedral cluster: hexanuclear ruthenium carbonyl carbidesElectronic supplementary information (ESI) available: Table S1, Table S2 and Fig. S1. See DOI: 10.1039/c0dt00670j

Octahedral, trigonal prismatic, and capped square pyramidal structures have been optimized for the Ru 6 C(CO) n clusters (15 ≤ n ≤ 20) using density functional theory. The experimentally known very stable Ru 6 C(CO) 17 is predicted to have an octahedral structure in accord with experiment as well as the Wade-Mingos rules. The stability of Ru 6 C(CO) 17 is indicated by its high carbonyl dissociation energy of ∼37 kcal mol −1 and the high energy of ∼33 kcal mol −1 required for disproportionation into Ru 6 C(CO) 18 + Ru 6 C(CO) 16 . Theoretical calculations predict a doubly carbonyl bridged octahedral Ru 6 C(CO) 17 structure to be ∼0.7 kcal mol −1 more stable than the experimentally observed singly bridged structure. A trigonal prismatic Ru 6 C(CO) 19 cluster isoelectronic with the known Co 6 C(CO) 15 2− dianion does not appear to be viable as indicated by a low carbonyl dissociation energy of 8.8 kcal mol −1 and a required energy of only 4.9 kcal mol −1 for disproportionation into Ru 6 C(CO) 20 + Ru 6 C(CO) 18 . The predicted instability of Ru 6 C(CO) n ( n ≥ 18) derivatives suggests a maximum of 17 external carbonyl groups around a stable polyhedral Ru 6 C structure. The very stable Ru 6 C(CO) 17 is predicted to have an octahedral structure in accord with experiment as well as the Wade-Mingos rules. The maximum number of external CO groups around an Ru 6 octahedron in a stable compound appears to be 17.