Comproportionation route

The reduction of rare earth metal trihalides, RX3, is in principle possible with all kinds of reducing agents as long as they have standard electrode potentials E° that can overcome that of the respective potentials of E° (R + R +). This is discussed below in more detail. Therefore, the classical reducing agents, nonmetals such as hydrogen or carbon, or like metals (comproportionation route) and unlike metals (metallothermic reduction) are all possible but (may) lead to different products. Cathodic reduction of appropriate melts is also an option. 

In the following sections, only the comproportionation route and the metallothermic reduction route are discussed as the two most coimnonly used synthetic methods. 

The comproportionation route is widely used and is very efficient when pure phases are desired, especially when the phase relationships are known. It led to a great variety of reduced rare earth haUdes, binary, ternary, and higher, simple and complex salts and such that incorporate metal clusters interstitially stabiUzed by a nonmetal atom or by a (transition) metal atom for example,... 

The Comproportionation Route 120 4.7 Layers of Edge- and Face-Connected Clusters 162... 

The comproportionation route is straightforward whenever the respective phase diagram is known (Corbett, 1973). One can also make use of a melt of some kind. If properly carried out, pure products will be obtained. This method can also be used to prepare ternary and quaternary compounds with the proper choice of starting materials. The problem here is that the phase diagrams are in most cases unknown. This approach is, therefore, exploratory and rather serendipitous. 

It gave birth to a new approach to reduced rare-earth metal halides that were previously synthesized only by the comproportionation route or by reduction with hydrogen, viz.,... 

The bis-MeCsELi or bis-Cp framework is a useful template for organoimido and organophosphinidene complexes. Comproportionation of uranium(III) and ura-nium(V) metallocenes provides a route to dinuclear ura-nium(IV) imido complexes (equation 11). The molecular structure of [(MeC5H4)2U(NPh)]2 is depicted in (5). 

A frequently used indirect method involves cyclizable (cf. (7)) or other mechanistic probes which should provide evidence for free radical intermediates and thus for SET [19,37,59]. However, Newcomb and Curran have pointed out the pitfalls of such an approach especially if iodide precursors are used [17]. The supposedly radical-indicative reaction may come about albeit slower by a different, nonradical mechanism or the radical formation may occur via a secondary process which is not directly related to the first reaction step. A similar side-route can be made responsible for the appearance of stable radical compounds which may arise via a comproportionation reaction between non-reduced starting material and the doubly reduced species which can be formed from a hydro form (the normal product, Eq. (5)) and the usually strongly basic organometallic or hydridic reagents (Eq. (9)) [58]. The ability of strong bases to produce reduced radical species via complicated electron/proton transfer processes has been known for some time in the chemistry of quinones and quaternary salts [60,61]. 

A conceptually distinct route to some intercalation compounds is through chemical reaction of preformed intercalation compounds. Thus, the comproportionation reaction, Eq. (g), is used to synthesize first-stage graphite-FeClj from first-stage graphite-FeCl3, as shown in Eq. (h) °. 

Mono-Cp complexes are generally prepared by the same routes as are the bis-Cp versions (see under preparation), but in addition they may be synthesized via comproportionation (CP 2MX2 -I- MX4 = 2 Cp MXa X = halide) (67) or by reaction of Cp salts with titanium compounds having only one substituent that is readily exchanged (68), such as ClTi(OR)3. 

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