Metal vapour technique

The dropwise addition of sodium naphthalenide to THF containing vanadium trichloride and l,2-bis(dimethylphosphino)ethane (dmpe) causes changes of colours suggesting a stepwise reduction from +2 to 0. Brown [V(dmpe)3] was isolated41 (fieff = 2.10BM) and IR data suggest octahedral coordination. The same complex was synthesized by a metal vapour technique.42 The ESR was that expected and the unit cell is cubic with a = 11.041(3) A. 

As we shall see, there are now other rare-earth elements that are stable enough in the divalent state in solution, allowing for the successful isolation of molecular complexes that can be fully characterised, including structurally, and their reactivity studied. Recent reviews and highlights have partially addressed this topic (Bochkarev, 2004 Cassani et al., 2002 Evans, 2000, 2002a,b, 2007 Izod, 2002 Meyer, 2008). Eurthermore, there are a few reports on low-valent scandium complexes, and also a number of extremely reactive zero-valent rare-earth molecular compounds that could only be made via metal vapour techniques, which have been reviewed some time ago (Cloke, 1993). 

Bisarene Complexes The new triple-decker-sandwich complex (46) has been proposed as a product of a metal vapour synthesis of chromium with mesitylene.The metal vapour technique has also led to the isolation of (C5H5AS)2Cr. 

Very little work has been carried out using metal vapour techniques. There are therefore significant gaps between possible preparative routes to supported metal clusters and what has actually been investigated. 

The relative advantages and disadvantages ofvoltammetric and atomic absorption methodologies are listed below. It is concluded that for laboratories concerned with aquatic chemistry of metals (which includes seawater analysis), instrumentation for both AAS (including potentialities for graphite furnace AAS as well as hydride and cold vapour techniques) and voltammetry should be available. This offers a much better basis for a problem-orientated application of both methods, and provides the important potentiality to compare the data obtained by one method with that obtained in an independent manner by the other, an approach that is now common for the establishment of accuracy in high-quality trace analysis. 

In analogy to sample introduction by hydride generation, mercury trace analysis is possible by reducing Hg compounds to the metal using the cold vapour technique or the determination of iodine at the ultratrace level (after oxidation with 70 % perchloric acid of iodide to iodine) via the gas phase. 

This is a very restricted oxidation state of copper but may be considered to occur in the polynuclear copper species Cu2, Cu3 and Cus, which have been characterized by matrix isolation techniques. Copper(O) also occurs in species formed by the reaction of copper metal vapour and carbon monoxide gas. Matrix isolation techniques have characterized a monomeric [Cu(CO)3] trigonal planar species and a dimeric [(CO)3CuCu(CO)3] species.32... 

Krause et a/.123-125 have recently reported a series of measurements of the spin-orbit relaxation of the alkali metals in their first excited states (2P). The technique, for example for atomic caesium with AE = 554 cm-1, consists of irradiating the metal vapour with light from a monochromator to excite only one of the 2P states. The vapour pressure of the metal is controlled at 10-6 torr to avoid imprisonment of the resonance radiation. The components of the fluorescence light are measured with a photomultiplier by isolating the 2P - 2S lines with interference filters. In the presence of added gases which cause the transitions... 

An example of the application of the MPE technique is given in Table 4.6 for AI2O3, which was studied using Sn and Co as the auxiliary liquids. The dihedral angles xp, xp° and 0 in these systems were measured by Nikolopoulos (1985). For both systems, at each temperature, it was found that the difference between xp and xp° is of the order of the experimental error, meaning that metallic vapours on A1203 surfaces was negligible. This result is to be expected for a non-reactive metal /oxide system in... 

As aheady mentioned, the simplest way of generating metal nanoparticles in the gas phase is to produce atoms that are subsequently allowed to coalesce under controlled conditions. This so-called metal-vapour synthesis requires more or less expensive equipment. Numerous modifications of this well established technique have been successfully applied. However, a detailed description of all the devices would exceed the scope of this article. For a summarizing overview, see Reference 7. Some more recent and relevant results shall be mentioned here. 

The sensitivity to air of alkali-metal-doped fullerenes (A C ) limits the choice of sample preparation and characterization techniques. To avoid sample degradation, we carried out reactions with the alkali metal vapour and Ceo in sealed tubes either in high vacuum or under a partial pressure of helium. The Cso was purified by chromatography of fullerite and was heated at 160 °C under vacuum to remove solvents. 

Deposition. - Deposition, as used in preparing supported catalysts, is the laying down or placing of the active components on the exterior surface of a support. One means by which this may be achieved is the preparation of catalysts by sputtering, which involves condensing the metal vapour onto an agitated finely dispersed support. However, as this process is performed under a high vacuum, the technique is probably only useful for the preparation of model catalysts. Alternatively, the process may be performed in the liquid phase by the deposition of a metal sol onto a suspended support. 

Metal vapour s)mthesis in rare-earth chemistry has been used as early as 1977 to prepare rare-earth butadiene and alk)me complexes by interaction of butadienes and alk)mes (respectively) with vaporised metals (Evans, 1987 Evans et al., 1977), and we have seen in Section 3 that low-valent scandium complexes can also be made by this technique. Additionally, condensation of rare-earth vapours with a 7i-acceptor such as 1,4-di-(t-Butyl)diazadiene (DAD) has produced compounds of general formula [R(DAD)3], except with scandium where the composition is [Sc(DAD)2]. Extensive metal-ligand electron transfer is effective in these molecules so the real oxidation state of the rare earth (at least at room temperature) is in fact + 3. Note that the first structurally characterised organosamarium(II)... 

Matrix Modification Technique Up to this point, consideration of the mechanisms and kinetics of sublimation and evaporation have been limited to crystalline substances (representing an individual phase). However, the thermodynamic approach used above is not restricted to these processes only. There are some other heterogeneous processes where it can be of use. Among these are, in particular, the processes of heterogeneous catalysis and of the interactions of solid substances with gases and metal vapours. The application of this approach to a mechanism of sample atomization in electrothermal AAS (ET AAS) will be considered further. 

Cold vapour technique. An analytical technique for the determination of mercury. Mercury is first reduced to the metallic element, vaporized, and introduced into the light path of the instrument. 

The determination of mercury is possible by the cold vapour technique, since it is based on the unique properties of this element. Mercury has an appreciable vapour pressure at ambient temperatures (0.16 Pa at 293 K) and the vapour is stable and monoatomic. Mercury can easily be reduced to metal from its compounds. By selective reductions of inorganic mercury(ii) compounds and organomercury compounds, it is possible to determine successively the inorganic and organic mercury fraction in the same sample. 

---