Oxidation state III

Until comparatively recently only vanadium had a significant coordination chemistry and even so the majority of its compounds are easily oxidized and must be prepared with air rigorously excluded. The usual methods are to use VCI3 as the starting material, or to reduce solutions of vanadium(V) or (IV) electrolytically. However, the reduction of pentahalides of Nb and Ta by Na amalgam or Mg, has facilitated the expansion of Nb and Ta chemistry particularly with S-and P-donor ligands. 

A few trinuclear oxo-centred carboxylates [V30(RC00)gL3]+ of a type more common for later transition metals (see Fig. 23.9, p. 1030) have been obtained, as well as [Nb302(MeC00)6(thf)3]+ whose structure differs essentially only in that there are two bridging O atoms above and below the Nb3 plane. 

tetranuclear, orange coloured derivatives Li4[Nb4S2(SPh),2] and [Nb4S2(SPh) -(PMe2R)4] (R = Me, have been shown to 

In spite of the preponderance of 6-coordinate complexes, other coordination numbers are known the ions [VCU] and [VBr4] are tetrahedral and are notable in that 4-coordination 

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The optical absorption spectra of Pu ions in aqueous solution show sharp bands in the wavelength region 400—1100 nm (Fig. 4). The maxima of some of these bands can be used to determine the concentration of Pu ions in each oxidation state (III—VI), thus quantitative deterrninations of oxidation—reduction equiUbria and kinetics are possible. A comprehensive summary of kinetic data of oxidation—reduction reactions is available (101) as are the reduction kinetics of Pu + (aq) (84). 

A third access to isocorroles was found7 when a tetrapyrrole 11 having an acrylaldehyde side chain was cyclized in presence of copper(II) or cobalt(II) salts. In this case isocorrole-9-carb-aldehydes 12 are formed with copper and cobalt in the oxidation state + III. The copper compound can easily be demetaled by hydrochloric acid to yield the metal-free isocorrole. In contrast, the cyclization of the tetrapyrrole in the presence of palladium(II) gives the isopor-phycene (see Section 1.7.1.). 

Iron in the oxidation state + III in dimeric p-oxo compounds 5 can be reduced to iron + II, leading to monomeric phthalocyanines 6.351... 

All the oxidation states III, IV, V and VI can exist in aqueous solutions in the Eh-pH-range of environmental interest (37). Strong complexes are formed between plutonium and oxygen containing ligands (02-, OH-, CO32-, etc.) as well as F but not with S2-. 

In the first series, with high oxidation states > + III) they are strong oxidants (for instance, Cr C ). This is not the case for the high oxidation states of the elements in the second and third series. 

The aqueous solution chemistry of Ir in its higher oxidation states III, IV, and V has been explored by Sykes et al.41,48 Chemical and electrochemical oxidation of Ir(H20)6]3+ gives a brown-green Irv product, which undergoes chemical and electrochemical reduction to a blue and a purple IrIV complex. 170 NMR studies are consistent with double- and single-bridged dimeric structures, with likely formulas [(H20)4Ir(/i-0H)2Ir(H20)4]6+ for the blue complex and [(H20)5Ir(/r-0)Ir(H20)5]6+ for the purple one. 

The diazaphosphane or aminoiminophosphane ligands with a NPN framework are another subclass of cyclophosphazenes. These compounds with both phosphorus in oxidation state (III) [104-110] and (V) [111-112] have been employed in the synthesis of four membered heterocycles and coordination chemistry with group 13 derivatives. Several complexes of trivalent phosphorus derivatives with both aluminum halide and alkyls are known as illustrated for 48 in Scheme 21 [113-119]. The structure determination of 48 confirms the formation of a four membered metallacycle [116, 117]. 

Tunicates (ascidians or sea-squirts) are invertebrate marine organisms, which can accumulate vanadium at concentrations approaching 350 mM (the concentration of vanadium in seawater is 10 8 M). This vanadium is taken up as V(V) from seawater (Figure 17.16), reduced to oxidation state III or IV and stored in a soluble form in the blood cells within acidic vacuoles at concentrations a million fold higher than in their external surroundings. 

The actinides. The actinides metals are electropositive and very reactive they are pyrophoric in finely divided form. They tarnish rapidly in air forming an oxide protective coating in the case of Th, but more slowly for the other actinides. The metals react with most non-metals. With steam or boiling water, oxide is formed on the surface of the metal and H2 evolves in this way hydrides are produced that react rapidly with water and facilitate further attack on the metals. The oxidation states observed in the chemistry of lanthanides and actinides are shown in Fig. 5.9. Notice the predominant oxidation state III for the lanthanides... 

It is interesting to note that these complexes are mixed-valent MnmMnIV complexes. Based on the relative structural data [the bond distances of the MnA atom are shorter than those of MnB], it has been concluded that in [Mn202(bipy)4]3+ one of the manganese ions is in the oxidation state IV [Mn(B)] and the other in the oxidation state III [Mn(A)]. Hence, the complex would have to be classified as a mixed-valent derivative with localized charge (Robin-Day Class I). Conversely, the two manganese sites are identical in [Mn202(phen)4]" +, from which one can infer that the charge is delocalized over the two centres (Robin-Day Class III). 

The Osl-Nl-N2-Os2 axis is not perfectly linear. The inequality of the bond distances (which would assign the oxidation states III to the Osl atom and II to the Os2 atom, respectively) should classify the monocation as a charge localized (Class I) mixed-valent species. Nevertheless, this result is in contrast with the electrochemical separation of the two... 

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