Symproportionation reactions

The most obvious and frequently used route to sub-valent species in molten halide solution is the reduction of a normal-valent halide with its parent metal, i.e. sym-proportionation reactions such as (2). This methodology is natural, since it disfavors disproportionation of the sub-valent compound to the zero-valent state according to (1). In addition, the methodology minimizes the number of components in the system. Post-transition metal-molten salt systems from which solid, sub-valent compounds have been isolated through symproportionation reactions are summarized in Table 1. 

Many molten halides are insufficiently Lewis-acidic to allow for the formation of appreciable amounts of sub-halides through symproportionation reactions with the parent metal. As described above, the addition of an excess of a more acidic halide is the obvious way to increase the yield. The most commonly used acids of this type are GaCls and AICI3, which are both very potent halide acceptors (Table 2). 

The use of inert halides as solvents for symproportionation reactions seems not to have been investigated extensively. However, such solvents may provide effective, alternative routes to sub-valent compounds. This is illustrated by the improved synthesis of BieCl from Bi and BiCl3 in liquid antimony(III) chloride. ... 

A major drawback connected with the symproportionation reaction is that the number of redox reactions that can take place is limited. A more recent development, which shows obvious similarities to the traditional molten salt route, but whieh avoids the limitations of symproportionations, is the solid-state technique developed by Beck in which a volatile, high-valent transition-metal chloride acts both as halide acceptor and as oxidizing agent. The synthesis of Tes " by oxidation of tellurium with WCl6 according to (4) is representative. ... 

Finally, a symproportionation reaction of the Kocheskov type may be carried out between (C6H5)3SnMn(CO)5 and Cl3SnMn(CO)5 to produce the mixed derivative (C6H5)2ClSnMn(CO)5... 

A special case of symproportionation in a solid-state reaction is shown in equation (7) ... 

Table 1. Metal-molten salt systems in which solid, sub-valent compounds with metal-metal bonds have been synthesized by symproportionation or similar reactions. The many sub-halides of gallium and indium have been omitted. These compounds contain either sub-valent monoatomic cations e.Lj. (Ga )(GaCl4 )l > and (In )(Cl )] " and/or ligand-stabilized metal-metal bonded anions (Ga+)2(Ga2X6- ), X = Br, For a review of the sub-valent chemistry of the group...

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