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 possib le combination of Fe, Ru, and Os, was theoretically studied using DFT metho ds (ADF program). The optimized geometries were compared with the those of the clusters for which structures are available, and the agreement was quit e good, especially when the metals were Fe and/or Ru. This allowed the pred iction that the structure of the known, but not yet structurally characteri zed, Ru2Os(CO)(12) and RuOs2(CO)(12) clusters should be of the D-3 type, wi th 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 ove r the Fe-Ru edge, but the energy differs only by 2.7 kJ mol(-1) from that o f 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 exc ept for the formation of Ru2Os(CO)(12) and RuOs2(CO)(12) from the parent cl usters, where the reaction is slightly exothermic.