THE REDUCTION OF HIGHER OXIDATION STATES

The most important method for reduction of compounds to an oxidation state suitable for titration with one of the common oxidising titrants is based upon the use of metal amalgams, but there are various other methods which can be used, and these will be discussed in the following sections. 

138 REOUCTION WITH AMALGAMATED ZINC THE JONES REOUCTOR 

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The cyclopolyphosphines that were used to prepare the carbene-phosphinidene complexes described earlier were formed by the reduction of higher oxidation state phosphorus compounds, typically di-chlorophosphines, RPC12 (47-49). However, in some cases a separate reduction step is not necessary and it is possible to prepare the car-bene-phosphinidene complex (74) directly by reaction of a stable nucleophilic carbene with RPC12 (44). 

The zero and lower oxidation states are relatively unimportant in the classical coordination chemistry of iron. With increased electron density on the iron, d-d and d-n interactions are particularly important, and systems with n acceptor orbitals are the dominant ligand species. Amongst the most common ligands encountered are carbon monoxide, phosphines, phosphites and unsaturated hydrocarbons. There are, however, a few relatively well-characterized iron(0) complexes derived from the reduction of higher oxidation state complexes containing N donor ligands possessing delocalized ji systems. 

The toxic metals present in industrial effluent streams include heavy metals such as silver, lead, mercury, nickel, zinc, and chromium. These heavy metals accumulate in soil and are eventually transferred to the human food chain. In irradiation treatment the general strategy is the reduction of higher oxidation state ions to lower oxidation state ions in lower oxidation state the solubility is usually lower, so often the reduced ions can be separated by precipitation. The reduction is done by the hydrated electron and hydrogen atom (under oxygen-free conditions) and/or by other reducing-type radicals formed in hydroxyl radical + alcohol or in hydroxyl radical + acetic acid reaction (see for instance reaction (O 23.34) and (O 23.144)) (Haji-Saeid 2007 Chaychian et al. 1998 Belloni and Mostafavi 2004 Belloni and Remita 2008). 

There are two principal synthetic routes to dicarboxylate complexes. One of these uses an aqueous solution of the alkali metal dicarboxylate and the corresponding metal halide,93 while the other depends upon the dicarboxylic acid reduction of higher oxidation state metals. This reductive property of oxalic acid results in its ready dissolution of iron oxides and hence a cleaning utility in nuclear power plants.94 Mention must also be made of the successful ligand exchange synthesis of molybdenum dicarboxylates, Mo(dicarboxylate)2 H2 O, from the corresponding acetate complex. Unfortunately the polymeric, amorphous and insoluble nature of these complexes has restricted the study of these systems, which may well provide examples of multiple M—M bonding in dicarboxylate coordination chemistry.95... 

The action of the Z-R reagent is first to supply an adequate concentration of Mn(II), which reacts with local excesses of permanganate and ensures the reduction of intermediate oxidation states of manganese to Mn(III). The Mn(II) also depresses the potential of the reversible Mn(III)-Mn(II) couple. Phosphoric acid (and to a lesser extent, sulfuric acid) also lowers the Mn(III)-Mn(II) potential, so that Mn(III) is reduced by Fe(II) rather than by chloride. Schleicher S stressed the importance of the Mn(III)-Mn(II) couple and maintained that five Mn(II) ions should be present locally for each Mn(VII) ion, to ensure that no manganese oxidation state higher than Mn(III) can exist. For this purpose four Mn(II) ions should suflBce ... 

Under reducing conditions, the dissolution of some oxide and hydroxide minerals of transition metals is greatly accelerated. This includes the environmentally important oxide/hydroxide minerals of Mn(III/IV), Fe(III), Co(in), and Pb(IV) (Stone, 1986). The lower valence ions produced by reduction, e.g., Fe and Mn, are much more mobile in aqueous systems than the ions of higher oxidation states. 

At high Mo concentrations, two chronopotentiometric reduction waves are observed the more cathodic one, which is absent at low concentrations, is shorter than the first. The parameters of this second wave were not consistent with it being due to the reduction of another oxidation state of molybdenum. A higher oxidation state would have produced a new wave at more anodic potentials, while a lower oxidation state would have produced a wave having a longer transition time than the initial one and be apparent at all concentrations. However, this second wave is certainly due to molybdenum, since... 

AnXe, and some representative data for these are given in Table XII. The thermal stability of the halides toward reduction of higher oxidation state actinides decreases with increasing atomic number of the halogen. 

Oxidation-reduction potentials for complexes in solution are determined by the relative stabilities of the complexes of the metal ion in the lower and higher oxidation states. The thermodynamic cycle connecting redox potentials and stabifity constants is shown in Fig. 7. This cycle can be useful both in rationalizing aspects of aqueous solution chemistry of complexes and in predicting or estimating values for stabifity constants or redox potentials for systems which are difficult or impossible to access experimentally. Thus knowledge of stabifity... 

Vanadium(II) compounds are usually prepared by reduction of acidic solutions of higher oxidation states. The usual geometry is octahedral and the electronic spectra often show three d-d spin-allowed bands. These spectra, as well as those of tetrahedral and linear complexes, have been reviewed.58 For this cr configuration, a spin-only magnetic moment of 3.87 BM is expected. Slow substitution can be predicted for vanadium(II) complexes but such reactions are not as slow as those for the isoelectronic Crm ions. 

It is also possible to reduce metals of higher oxidation states to lower ones. These are called reduction reactions. In the case of iron, such a reduction is given by... 

The majority of reported Co complexes in oxidation states higher than 3-1- contain noninnocent ligands, and this hence renders the assignment of metal oxidation state difficult. This problem has existed since Werner s time indeed, he initially formulated the superoxo-bridged dimers [(NH3)4Co(02)(NH2)Co(NH3)4] + and [(en)2Co(02)(NH2)Co(en)2]" + as mixed-valence peroxo-bridged Co(III)/(IV) complexes. One-electron oxidation of the unusual square-planar Co(III) complex (31) using Ce(IV) gives a deep blue low-spin species. This is soluble and stable in benzene, and exhibits a reversible one-electron reduction... 

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