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. 

Low-valence clusters are species in which the metal atoms have an oxidation state zero or negative. They are always associated with ligands able to behave as 7i-acceptors. Some examples of cluster belonging this class are shown in Fig. 2.2. Carbon monoxide is by far the most representative of these ligands but there are also many examples of compounds containing other typical soft ligands such as phosphine, olefines, acetylenes, cyclopentadiene, etc. 

Con A by the mannoside cluster at concentrations slightly above the nanomolar range, according to a mannose-dependent and partially reversible process. Finally, these results suggested that, rather than an increased avidity for Con A, the unusual potency of these molecular glycoasterisks was due to an efficient and kinetically controlled macromolecular assembly that strongly amplified the effect of these low-valency ligands. 

Some low-valency cations produced in early steps and at partial reduction are immediately neutralized by the counter anion if the solubility product is quite low, and they behave indeed as transient monomers able to coalesce into semiconductor clusters [19]. 

A stabilization of low valencies of metals and an induction time before cluster formation have been observed as well in the case of iridium [105], platinum [53], palladium [147], copper [94], or nickel [115]. 

In many cases, although and M are both readily reduced by radiolytic radicals, a further electron transfer from the more electronegative atoms (for example, M ) to the more noble ions ( °(M /M )electron transfer is also possible between the low valencies of both metals, so increasing the probability of segregation [174]. The intermetal electron transfer has been observe directly by pulse techniques for some systems [66,175,176], and the transient cluster (MM ) sometimes identified such as (AgTl) or (AgCo) [176]. The less noble metal ions act as an electron relay toward the precious metal ions, so long as all are not reduced. Thus, monometallic clusters M are formed first and M ions are reduced afterward in situ when adsorbed at the surface... 

As a group of typical metal elements, lanthanide elements can form chemical bonds with most nonmetal elements. Some low-valence lanthanide elements can form chemical bonds in organometallic or atom cluster compounds. Because lanthanide elements lack sufficient electrons and show a strong repulsive force towards a positive charge, chemical bonds between lanthanide metals have not yet been observed. Table 1.4 shows that 1391 structure-characterized lanthanide complexes were reported in publications between 1935 and 1995 and these are sorted by chemical bond type. 

The Michaelis-Menten character of the kinetics suggests that the formation of the reaction product is preceded by reversible coordination of the alkene, O2, and AcOH molecules by the cluster. The kinetic isotope effects give evidence that the mechanisms of oxidative acetoxylation (eq. (1)) catalyzed with Pd and low-valence Pd clusters are different. On the basis of kinetic data, including the H/D kinetic isotope effects [9], the reaction mechanism represented by Scheme 6 has been proposed for Pd-561-catalyzed reaction. 

Figure 5 shows a plot of average value of L, and the scatter of data points, against the number of valence electrons for all 3-dimensional low spin clusters. 

Jones and co-workers reported the fully characterized neutral mixed valence cluster [Pd3(Cl)(/r- Bu2P)3(CO)2], Pd3(4/3). Reaction of [PdCl(CO)] with Li- Bu2P in THF at —78 °C produces a mixture of compounds (Eq. 6) from which the trinu-clear phosphido-bridged complex was isolated in low yield. 

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. ... 

Substances similar to those just described in terms of their chemical composition and catalytic capabilities can also be obtained by a more convenient procedure, without isolation of clusters 3 and 4, by reduction of Pd(OAc)2 with CO in the presence of small quantities (more facile synthetic way to low-valence polynuclear Pd complexes is reduction of Pd(OAc)2 by H2 in the presence of L ligands. The colloids obtained in this way have been shown to be catalytically active in a number of redox reactions involving those of alkenes, alkylarenes, CO, alcohols, nitroarenes,... 

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