Transition-metal silicon clusters

So far few reports have dealt with the reaction chemistry of transition-metal silicon clusters, but the diversity of structural types and the interesting ways in which silicon-based ligands can become incorporated into clusters suggest that new and useful types of transformations should be possible. 

The single-bond compounds between Si, Ge, and Sn and chalcogens (S, Se, Te) are reactive and used as synthetic intermediates in the synthesis of organic compounds, organochalcogen compounds, organo-silicon, -germanium and -tin compounds, and transition metal-chalcogen complexes and clusters. 

A few clusters of interest containing germanium and transition metals have been reported.136-138 Dimethylgermane was found to replace only the bridging carbonyls between cobalt in a mixed germanium/cobalt/iron cluster complex (Equation (107)), and replacement of the carbonyl bridging the iron metal centers was not observed.137 A similar reaction leads to replacement of a bridging carbonyl in a mixed cobalt/silicon cluster (Equation (108)).136... 

A related effect is that silicon-transition-metal compounds, once formed, undergo Si-M bond cleavage when dissolved in polar solvents (252,262,300,305,306,310). The products are often complex, but have been shown to include a cluster compound in one case involving cobalt (252). 

Following the many recent developments in the chemistry of metal carbonyl clusters and other metal-metal bonded species, interest has developed in silicon-transition-metal clusters. These will be defined for the present purpose as compounds containing (1) at least one metal-metal bond and (2) at least one silicon atom directly linked to two or more metal atoms. Thus compounds such as Me3SiOs(CO)4Os(CO)4-SiMe3 and R3SiOCCo3(CO)9 are excluded examples of compounds that meet these criteria are shown in Table XXIII. 

Silicon forms a variety of compounds containing clusters of transition metal atoms that exhibit a wide range of structures. DisUane reacts with Co2(CO)g to give the cobalt cluster (38) in which the silicon is coordinated to five Co atoms (equation 48). The Si-- Si distance of 2.817 A is too long for a bond (normally about 2.35 A) and the Si-Co distances average 2.336 A. ... 

As discussed earlier, it is now possible to make and study deposits of monosized, highly dispersed, transition metal clusters.(S) In this section we summarize results from the first measurements of the valence and core level photoemission spectra of mass selected, monodispersed platinum clusters. The samples are prepared by depositing single size clusters either on amorphous carbon or upon the natural silica layer of a silicon wafer. We allow the deposition to proceed until about 10 per cent of the surface in a 0.25 cm2 area is covered. For samples consisting of the platinum atom through the six atom duster, we have measured the evolution of the individual valence band electronic structure and the Pt 4f... 

In o-silaborane1 1 (1) the silicon atoms are in the unique situation of being incorporated in the skeleton of a borane cluster framework. We are interested in the reactivity patterns of the icosahedral cluster and especially in the silicon centres. The nucleophilic degradation was the first reaction of o-silaborane which has been enlightened.121 Removal of a silicon vertex from the icosahedral cluster results in almost quantitative isolation of the sila-tiido-undecaborate(l-) (2). This eleven vertex cluster became a versatile starting material for the synthesis of transition metal complexes comprising a silicon metal interaction.13,4 5,61... 

Under the attack of one equivalent OH in H2O, MeO in MeOH, or pure NH3 a silicon vertex was removed from the c/oso-cluster l.[21 A plausible mechanistic sequence for the degradation involves as the first step formation of an adduct consisting of the nucleophile and the c/oso-cluster. Afterwards this adduct should react with a protic solvent to give the isolated reaction product. In order to get indications for the expected adduct we studied the attack of nucleophiles in an aprotic solvent. Three types of nucleophiles were studied in this reaction. In the case of the dialkylamide nucleophile best results were achieved from the reaction of the close-cluster with transition metal amides of zirconium and tantalum,17,81 The anionic adduct cluster features a dialkylamide bridging both silicon centres of the silaborane cluster. 

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