High- and low-valence clusters

Synthetic Strategy for Interconverting High- and Low-Valence Clusters... 

I, etc.), satisfies the requirements of our synthesis stratagem for interconversions of high and low valence clusters. 

There are two large classes of molecular clusters in which the ligands are all of the same nature The halide cluster class and the carbonyl or, in general, organometallic cluster class. According to the formal oxidation numbers acquired by metal atoms in these two classes of clusters, they are often classified as high and low-valence clusters respectively. 

In this section the parameter L is used to quantify the distortions from sphericality, and define the relationship between cluster shape and the number of valence electrons. Figure 3 illustrated the splitting of the If shell under oblate and prolate distortions. The resultant orbital stabilisation energies in terms of p are shown in Table 8 for clusters with 4 to 8 valence electrons. Both high and low spin cases are considered. The most stable geometries predicted by this simple model, are summarised in Table 9. 

By this time, substantial experience in the synthesis of low-valence noble metal complexes in the form of non-crystalline, colloid-like samples has been accumulated. For instance, a series of amorphous, high-molecular weight palladium complexes, which had remarkable catalytic capability, had been obtained starting from low-nuclearity Pd(I) clusters and Pd(II) complexes. ... 

In this section, the redox chemistry and interconversion of several low-oxidation state carbonyl iron complexes has been detailed. These are mononuclear [-"Fe(CO)4] -, [H-"Fe(CO)4]-, Fe (CO)5, and dinuclear ( Fe2(CO)8] complexes, and mixed valence (Fe3(CO)n] and [Fe4(CO)i3] clusters. As for the oxidation of Fe(CO)s it has usually been reported to be irreversible, except at high scan rates in trifluoroacetic acid, a solvent of very low nucleophilicity and of moderate acidity (Guttman Donor Number is 0) [8]. 

However, we know numerous homogeneous catalytic reactions occurred with a high rate under the action of low-valence complexes of such metals as Ru, Rh, Co, Pt, Pd, Mn, etc. The typical ligands of such conqrlexes are phosphines, CO, Cr, CN , and crthers. Clusters of the Pd, Pt, Ni, and Os metals are u in recent years. The structure of one of these complexes obtained by the Hartree-Fock-Slate transition state method is presented in Fig. 17.6... 

Syntheses, crystallization, structural identification, and chemical characterization of high nuclearity clusters can be exceedingly difficult. Usually, several different clusters are formed in any given synthetic procedure, and each compound must be extracted and identified. The problem may be compounded by the instabiUty of a particular molecule. In 1962 the stmcture of the first high nuclearity carbide complex formulated as Fe (CO) C [11087-47-1] was characterized (40,41) see stmcture (12). This complex was originally prepared in an extremely low yield of 0.5%. This molecule was the first carbide complex isolated and became the foremnner of a whole family of carbide complexes of square pyramidal stmcture and a total of 74-valence electrons (see also Carbides, survey). 

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