First-principles Studies On Ru₆C(CO)_n(n=15-20) Metal Carbonyl Carbides

The chemistry of metal carbonyl carbide clusters dates back to the 1962 isolation of the square pyramidal iron carbonyl carbide Fe5C(CO)15 Subsequently the ruthenium carbonyl carbide Ru6C(CO)17 was isolated from the pyrolysis of Ru3(CO)12 in hydrocarbon solvents and shown by X-ray crystallography to have a structure consisting of a Ru6 octahedron with a carbon atom in the center.Octahedral, trigonal prismatic, and capped square pyramidal structures have been optimized for the Ru6C(CO)n clusters (n=15～20) using density functional theory. The experimentally known very stable Ru6C(CO)17 is predicted to have an octahedral structure in accord with experiment as well as the Wade-Mingos rules. The stability of Ru6C(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 Ru6C(CO)18+Ru6C(CO)16.Theoretical calculations predict a doubly carbonyl bridged octahedral Ru6C(CO)17 structure to be～0.7 kcal mol-1 more stable than the experimentally observed singly bridged structure.Atrigonal prismatic Ru6C(CO) 19 cluster isoelectronic with the known Co6C(CO)152-dianion does not appear to be viable as indicated by a low carbonyl dissociation energy of 8.8 kcal/mol-1 and a requiredenergy of only 4.9 kcal/mol-1 for disproportionation into Ru6C(CO)20 +Ru6C(CO)18.The predictedinstability of Ru6C(CO)n (n≥18) derivatives suggests a maximum of 17 external carbonyl groupsaround a stable polyhedral Ru6C structure.