Electronic and Geometric Structure of Bimetallic Clusters: Density Functional Calculations on [M₄{Fe(CO)₄}₄]^4- (M = Cu, Ag, Au) and [Ag₁₃{Fe(CO)₄}₈]^n- (n = 0-5)

The results of all-electron density functional calculations on the bimetallic cluster compounds [M₄{Fe(CO)₄}₄]^4- (M = Cu, Ag, Au) and on the corresponding naked species M₄Fe₄ are reported. The trends within the triad have been investigated. The bare metal clusters exhibit a strong magnetization which is quenched on addition of CO ligands. The bonding in the bare clusters is different for the silver derivative compared to that of copper and gold, resulting in comparatively weaker Ag-Fe and Ag-Ag bonds. This can be rationalized in terms of the different d-sp mixing, which for Cu and Au is larger than for Ag. Relativistic effects act to increase the 4d-5s mixing in Ag and to strengthen the intermetallic bond with Fe. In the carbonylated clusters a charge transfer from the metal M (M = Cu, Ag, or Au) to the Fe(CO)₄ groups occurs so that the atoms M can be considered in a formal +I oxidation state, rationalizing the nearly square-planar geometry of the metal frame. In fact, the local coordination of the M atoms is almost linear, as expected for complexes of M(I). The addition of extra electrons results in a stabilization of the clusters, indicating the electron-deficient nature of these compounds. Similar features have been found for the largest cluster synthesized so far for this class of compounds, [Ag₁₃{Fe(CO)₄}₅]^n-, (n = 0-5). The nature and localization of the unpaired electron in the tetraanion is also discussed.