The maximum number of carbonyl groups around an Ru6C polyhedral cluster: hexanuclear rutheniumcarbonyl carbides

Abstract

Octahedral, trigonal prismatic, and capped square pyramidal structures have been optimized for the Ru₆C(CO)_n clusters (15 ≤ n ≤ 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. A trigonal 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 required energy of only 4.9 kcal mol⁻¹ for disproportionation into Ru₆C(CO)₂₀ + Ru₆C(CO)₁₈. The predicted instability of Ru₆C(CO)_n (n ≥ 18) derivatives suggests a maximum of 17 external carbonyl groups around a stable polyhedral Ru₆C structure.