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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.

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