Almost Mixed-Metal Clusters

Because of the requirements set at the beginning of this chapter concerning the scope of this review, we have had to exclude several interesting compounds, but, as these may be of interest to some readers, they are briefly mentioned here although this section is not meant to be comprehensive. 

A number of linear mixed-metal carbonyls are known. Sheline and coworkers prepared a series of M—Fe—M (M = Tc, Re) trimers by photolysis of Fe(CO)5 and the corresponding M2(CO)10, Eq. (101) (66, 109). 

Schmid and co-workers (136) prepared several derivatives that possess the [Co3CO10]- unit as an oxygen donor ligand to another metal. Reactions (102) (138), (103) (152), (104) (152), and (105) (139) are illustrative. 

HMnRe2(CO)i4 41 was shown to have a bent disposition of metals with the hydride lying between the Re atoms (44, 69). 

Dahl and co-workers prepared the unusual compound Mo2Re2(Cp)2(CO)u(S)2 (42) containing both tri- and tetracoordinated sulfur ligands, Eq. (107) (158). 

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Anionic clusters are good nucleophiles (see Section III,A) and are often easy to make. On the other hand, the electrophilic nature of most monometallic complexes is obvious from ligand substitutions. The combination of these properties makes a strategy for cluster expansion. This strategy was used for the first time by Hieber (130) in making Fe4(CO)fc from Fe3(CO),7 and Fe(CO)s. It is probably active in many syntheses of large metal carbonyl clusters because the Re, Os, Rh, Ir, Ni, and Pt clusters involved are almost always anionic. However, simple stoichiometries can rarely be written for such reactions (122). This route makes mixed metal clusters accessible, e.g.,... 

Some of the best known mixed-metal clusters are group Vlll triangular clusters [M3(CO)i2] (M = Fe, Ru, Os) [111-113], where almost aU possible metal combinations have been prepared [114, 115], and their bonding properties, electrochemistry and photochemistry have been studied in much detail [111-117]. 

Banerjee and Harbola [69] have worked out a variation perturbation method within the hydrodynamic approach to the time-dependent density functional theory (TDDFT) in order to evaluate the linear and nonlinear responses of alkali metal clusters. They employed the spherical jellium background model to determine the static and degenerate four-wave mixing (DFWM) y and showed that y evolves almost linearly with the number of atoms in the cluster. 

The interaction of a Pd4 (C4v) cluster with the oxide surface was analyzed in more detail with the help of electron density difference plots and other theoretical tools, such as population analysis, core level shifts as well as induced and dynamic dipole moments [175]. Three interaction mechanisms were found to contribute to different extent metal polarization with the subsequent electrostatic attraction, Pauli repulsion, and covalent orbital interactions. Electrostatic interactions make up a sizeable fraction of the adhesion energy the polarization of the metal adsorbate by the surface electric field provides an important bonding mechanism. For the adsorption of Pd on-top or in the vicinity of the surface Mg " cations this electrostatic interaction accounts for almost the entire adsorption energy, albeit counteracted by Pauli repulsion. For adsorption on-top 0 , on the other hand, mixing of adsorbate and substrate orbitals becomes noticeable. This hybridization or covalent bonding at the interface with the oxide anions is complemented by electrostatic polarization. Further work is required to establish in a more quantitative way the relative importance of electrostatic and chemical bonding contributions. However, in line with our other studies of... 

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