Heterometal Substitution

Thus far, dimensional reduction has not been applied in this way to any of the other parent solids listed in Table I. Of particular interest are systems that straddle the divide between the edge-bridged and face-centered octahedral geometries. Will reduced niobium-molybdenum halide mixtures simply disproportionate or will they lead to new mixed-metal clusters If the latter, then where does the crossover in cluster geometries occur An important factor in applying dimensional reduction is recognizing the variability in the stable electron counts within a particular cluster system. An extreme case occurs 

Substitution at one or more metal sites will generally break the symmetry of the cluster core, and can greatly influence its electronic properties and reactivity. Consider, for example, the possible substitutions of a metal M into an octahedral core of composition MfiX v (x = 8, 12). The first substitution will afford an MsM X core, for which the symmetry has been lowered to C4v. A second substitution generates an M4M 2Xx core with two possible isomers One in which the M atoms are positioned at trans vertices (D4/,) and another where they are positioned at cis vertices (C2v). With a third substitution to give an M3M 3Xx core, fac and mer isomers become possible, while further substitutions simply repeat the pattern with M and M interchanged. Here again, the substitutions can be anticipated to alter the basic electronic properties of the cluster. Moreover, the outer-ligand substitution chemistry could potentially be quite different 

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The other way to introduce heterometals is their isomorphous substitution for Si in the framework, in a similar manner to the isomorphous substitution of Al. The heteroatoms should be tetrahedral (T) atoms. In hydrothermal synthesis, the type and amount of T atom, other than Si, that may be incorporated into the zeolite framework are restricted due to solubility and specific chemical behavior of the T-atom precursors in the synthesis mixture. Breck has reviewed the early literature where Ga, P and Ge ions were potentially incorporated into a few zeolite structures via a primary synthesis route [9]. However, until the late 1970s, exchangeable cations and other extraframework species had been the primary focus of researchers. 

Similar magnetochemical arguments have been used in mixed-metal derivatives of dodecanuclear Ni 2 and Co coordination complexes in which one of the 12 metal centers is replaced, resulting in Ni Co11 and Co j Ni11 stoichiometries in which the heterometal position can assume one of two symmetry-equivalent sites. In this case, simulations of the susceptibility data for the two possible scenarios of each MuM derivatives were able to identify the preferred substitution site.42... 

In contrast, those cluster compounds with an 18-electron environment at the central heterometal do not undergo nucleophilic addition reactions. Ligand substitution reactions at both the central heterometal and the peripheral gold atoms are observed to occur for both 16- and 18-electron cluster compounds, as in the reactions shown below ... 

From 1985 to the present which is now witnessing an unprecedented spurt in the chemistry of heterometal alkoxides and oxometal alkoxides (42) in general, along with alkoxide derivatives of special (substituted, chelating, or unsaturated) alcohols, as well as the rapidly developing applications of metal alkoxides for ceramic and catalytic materials coupled with an increasing emphasis on their actual structural elucidation. 

Bases such as amines, phosphines, or phosphites were found to react with the heterometal catalysts in three different ways Coordination to the heterometal without CO displacement, substitution of CO, and disproportionation, respectively. The latter reaction occurred only with the mercury compound, which reacted with amines and phosphines... 

Ligand reactions of amino substituted heterometal clusters... 

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