Oxidation states, unstable

Lead, like tin, forms only one hydride, plumbane. This hydride is very unstable, dissociating into lead and hydrogen with great rapidity. It has not been possible to analyse it rigorously or determine any of its physical properties, but it is probably PbH4. Although this hydride is unstable, some of its derivatives are stable thus, for example, tetraethyllead, Pb(C2Hj)4, is one of the most stable compounds with lead in a formal oxidation state of + 4. It is used as an antiknock in petrol. 

Some metal thiosulfates are inherently unstable because of the reducing properties of the thiosulfate ion. Ions such as Fe " and Cu " tend to be reduced to lower oxidation states, whereas mercury or silver, which form sulfides of low solubiUty, tend to decompose to the sulfides. The stabiUty of other metal thiosulfates improves in the presence of excess thiosulfate by virtue of complex thiosulfate formation. 

Coordination Complexes. The coordination and organometaHic chemistry of thorium is dominated by the extremely stable tetravalent ion. Except in a few cases where large and stericaHy demanding ligands are used, lower thorium oxidation states are generally unstable. An example is the isolation of a molecular Th(III) complex [107040-62-0] Th[Tj-C H2(Si(CH2)3)2]3 (25). Reports (26) on the synthesis of soluble Th(II) complexes, such as... 

When the desired halide is hydrolytically unstable then dry methods must be used, often at elevated temperatures. Pre-eminent amongst these methods is the oxidative halogenation of metals (or non-metals) with X2 or HX when more than one oxidation state is available X2 sometimes gives the higher and HX the lower, e.g. ... 

Complexes of cations in unstable oxidation states in aqueous solutions as studied by pulse radiolysis. D. Meyerstein, Acc. Chem, Res., 1978,11,43-48 (95). 

The rate equation agrees with the experimental form, Eq. (6-14). However, it appears much less plausible than that in Eq. (6-14) in that it requires the intervention of two oxidation states of iron that are highly unusual and unstable under these reaction conditions. 

As a more complex example, we examine the stability of oxidation states of aqueous sulfur as a function of pH. This exercise will bring out the treatment of thermodynamically unstable species and the change of sulfur speciation with pH. 

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