ELECTRONIC AND GEOMETRIC STRUCTURE OF BIMETALLIC CLUSTERS - DENSITY-FUNCTIONAL CALCULATIONS ON [M(4)(FE(CO)(4))(4)](4-) (M=CU, AG, AU) AND [AG-13(FE(CO)(4))(8)](N-) (N=0-5)

The results of all-electron density functional calculations on the bim etallic cluster compounds [M(4){Fe(CO)(4)}(4)](4-) (M = Cu, Ag, Au) an d on the corresponding naked species M(4)Fe(4) are reported. The trend s within the triad have been investigated. The bare metal clusters exh ibit a strong magnetization which is quenched on addition of CO ligand s. The bonding in the bare clusters is different for the silver deriva tive compared to that of copper and gold, resulting in comparatively w eaker Ag-Fe and Ag-Ag bends. This can be rationalized in terms of the different d-sp mixing, which for Cu and Au is larger than for Ag. Rela tivistic effects act to increase the 4d-5s mixing in Ag and to strengt hen the intermetallic bond with Fe. In the carbonylated clusters a cha rge transfer from the metal M (M = Cu, Ag, or Au) to the Fe(CO)(4) gro ups occurs so that the atoms M can be considered in a formal +I oxidat ion state, rationalizing the nearly square-planar geometry of the meta l frame. In fact, the local coordination of the M atoms is almost line ar, as expected for complexes of M(I). The addition of extra electrons results in a stabilization of the clusters, indicating the electron-d eficient nature of these compounds. Similar features have been found f or the largest cluster synthesized so far for this class of compounds, [Ag-13{Fe(CO)(4)}(8)](n-), (n = 0-5). The nature and localization of the unpaired electron in the tetraanion is also discussed.