A different number of bridging carbonyls is found in bi- or trinuclear clusters having the title formulas. Comparative calculations at the SCF, MP2, and DFT levels of theory show that only the latter is able to describe properly the energetics of various isomers of the whole triad. For the first-row transition metal, DFT gives excellent agreement with the experimental structures, whereas the MP2 approach fails completely. Conversely for the second- and third-row metals, the best agreement with the experiment is obtained by the MP2 optimizations. The quantitative computational results, associated with a qualitative MO analysis, allow one to conclude that the structural preferences are determined by a critical balance of metal-bridge bonding, metal-metal bonding, and intermetallic repulsion. Although the M-M bond order is expected to be 1 in all cases, the bridge-supported bond is experimentally and computationally shorter than the unsupported one. By contrast, the trend for the overlap population (OP) is reversed, with even negative values for the shorter bridge bonds. For the latter, only a weak attractive interaction stems from the almost pure t(2g) orbitals, taken as metal lone pairs or eventually responsible for back-donation (formation of metal-bridge sigma bonds). Thus, the negative OP values are consistent with a prevailing repulsion between the latter levels. In the iron systems, with more contracted metal orbitals, the direct metal-metal repulsion is relatively weak while the metal-bridge bonds are sufficiently strong. This is not equally true for the more diffuse ruthenium and osmium orbitals, so the alternative nonbridged structure is preferred.

DFT calculations were performed on Os 3 (CO) 10 (α-diimine) clusters, for α-diimine = DAB (1,4-diaza-1,3-butadiene), PYCA (σ-N,σ-NЈ-pyridine-2-carbaldehyde-imine), and BIPY (2,2Ј-bipyridine). Geometry optimizations were performed for several models of the possible isomers under different conditions. Axial isomers were always found to be the most stable, in agreement with the available X-ray determined structures. A full optimization of the geometry was required in order to improve the quality of the results, but the dis-

The family of M3(CO)12 (M = Fe, Ru, Os) clusters, where M3 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 M3(CO)12 clusters, a process favored by entropy, are positive except for the formation of Ru2Os(CO)12and RuOs2(CO)12 from the parent clusters, where the reaction is slightly exothermic.

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