Iron-ruthenium-osmium mixed trimetallic carbonyl clusters: Theoretical studies and structural trends

The family of M-3(CO)(12) (M = Fe, Ru, Os) clusters, where Mg is any possible combination of Fe, Ru, and Os, was theoretically studied using DFT methods (ADF program). The optimized geometries were compared with the those of the clusters for which structures are available, and the agreement was quite good, especially when the metals were Fe and/or Ru. This allowed the prediction that the structure of the known, but not yet structurally characterized, Ru2Os(CO)(12) and RuOs2(CO)(12) clusters should be of the D-3 type, with only terminal carbonyls. On the other hand, the still unknown FeRuOs(CO)(12) cluster appears to prefer the C-2 form with two bridging carbonyls over the Fe-Ru edge, but the energy differs only by 2.7 kJ mol(-1) from that of the D-3 arrangement. The calculated enthalpy changes for the formation of mixed M-3(CO)(12) clusters, a process favored by entropy, are positive except for the formation of Ru2Os(CO)(12) and RuOs2(CO)(12) from the parent clusters, where the reaction is slightly exothermic.