Cluster compounds, organometallic stability

One major reason for the great interest in the processes of thin metal-containing films is that reactions on the surface of small metal clusters can be studied. Indeed, prior to the development of thin-film chemistry, reactions of similar particles were studied only in the gas phase at rather high temperatures. Under these conditions, most of the primary products are unstable and decompose in the course of further reaction, which is non-selective. As a result, the information obtained on the routes and mechanisms of reactions of disperse metals appears to be scarce, while the use of such reactions in synthesis is inexpedient. Conversely, low-temperature reactions in the films of co-condensates are very promising from the standpoint of determining the detailed reaction mechanism, as well as for synthesis of previously unknown complexes and organometallic compounds. It is important that atoms of only a few metals react with organic compounds immediately at the instant of their contact on the cooled substrate. Rather often, atoms and/or small (molecular) clusters are first stabilized in the film, and then their transformations are observed. 

As with any rule, there are exceptions. For example, cluster compounds don t always satisfy the 18-electron rule. And some organometallic compounds find stability with only 16 electrons instead (more on this in the next section). But the 18-electron rule is a good place to start. 

Many of the different pillaring agents reported in the literature (organic compounds, metal trischelates, organometallic complexes, metal cluster cations, metal oxide sols, polyoxocations, etc.) have drawbacks such as low reactivity or lack of thermal stability, and polyoxocations are by far the most widely employed. Many different polyoxocations (Al, Ni, Zr, Fe, Cr, Mg, Si, Bi, Be, B, Nb, Ta, Mo, Ti, and, more recently, Cu, Ga, and Ce) have been reported in the open and patent literature and clays with multielement or doped pillars also have been claimed [2,8,9,19,20,68-70,73-75]. However, the chemical composition, structure and charge are at present well defined only for the Al-polyoxocation, in which the Keggin ion [Al,304(0H)24(H20)i2] is identified. 

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