Extreme oxidation state

A key property of metallocene is their ability to exist in the form of various oxidation states with the sandwich structure and a variable number of d electrons (NVE between 14 and 20), even if the stability decreases as the NVE is further away from 18. Each metallocene exists in various oxidation states, which is usually not the case for the other families of inorganic or organometallic complexes. These various oxidation states are characterized by cyclic voltammetry, each wave corresponding to a change of oxidation state. The shape of the wave shows that, for metallocenes, this oxidation or reduction is reversible and occurs without structural change. For instance, for ferrocene, the groups of Laviron (reduction, DMF) and Bard (oxidation, SO2) have reached extreme oxidation states (Fe and Fe ). The potentials below are given V5. the saturated calomel reference electrode (SCE) ... 

These elements show a kind of mini-periodicity [2] of characteristic extreme oxidation states, as seen in Table 3.1. A few of these known oxidation states, which represent exactly empty, half-full or full f subshells of electrons, are nevertheless not stable in water, as will be seen in the descriptions below. Table 3.2... 

Table 3.1. Periodicity in the characteristic extreme oxidation states of Ln, An and their neighbors, not aU in water...

Known halides are described in Table 8.3 and have been reviewed extensively [3, 4, 10-12], The only remarkable halides are those of extreme oxidation states. Dihalides of americium cannot be prepared by hydrogen reduction of trihalides, although hydrogen reduction is successful for the lanthanides Sm, Eu (the 4f analog of americiumX and Yb. Instead, americium metal must be reacted with a halide such as Hgl2 [329] or Hg02 [100]. It is likely that the reaction... 

Th e ability to perform m oleciilar orbital (MO ) calculation s on m et-als is extremely useliil because molecular mechanics methods are gen erally unable to treat m etals. This is becau se m etals h ave a wide range of valences, oxidation states, spin multiplicities, and have 1111 usual bonding situations (e.g.. d%-p% back bonding). In addition. the 11 on direction al n at are o ( m etallic hon din g is less am en a-ble to a ball and spring interpretation. 

The extensive hydrolysis of protactinium in its V oxidation state makes the chemical investigation of protactinium extremely difficult. Ions of protactinium(V) must be held in solution as complexes, eg, with fluoride ion, to prevent hydrolysis. 

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... 

Cu ( j -C5H5)2] is not. Likewise, Fe and Ni carborane derivatives are extremely stable. Conversely, metallocarboranes tend to stabilize lower oxidation states of early transition elements and complexes are well established for Ti", Zr , Hf , V , Cr and Mn" these do not react with H2, N2, CO or PPh3 as do cyclopentadienyl derivatives of these elements. 

In 1826 J. J. Berzelius found that acidification of solutions containing both molybdate and phosphate produced a yellow crystalline precipitate. This was the first example of a heteropolyanion and it actually contains the phos-phomolybdate ion, [PMoi204o] , which can be used in the quantitative estimation of phosphate. Since its discovery a host of other heteropolyanions have been prepared, mostly with molybdenum and tungsten but with more than 50 different heteroatoms, which include many non-metals and most transition metals — often in more than one oxidation state. Unless the heteroatom contributes to the colour, the heteropoly-molybdates and -tungstates are generally of varying shades of yellow. The free acids and the salts of small cations are extremely soluble in water but the salts of large cations such as Cs, Ba" and Pb" are usually insoluble. The solid salts are noticeably more stable thermally than are the salts of isopolyanions. Heteropoly compounds have been applied extensively as catalysts in the petrochemicals industry, as precipitants for numerous dyes with which they form lakes and, in the case of the Mo compounds, as flame retardants. 

The difference between the two extremes is essentially that, in the former, the Re retains its valence electrons in its d orbitals whereas in the latter it loses 6 of them to delocalized ligand orbitals. In either case paramagnetism is anticipated since rhenium has an odd number of valence electrons. The magnetic moment of 1.79 BM corresponding to 1 unpaired electron, and esr evidence showing that this electron is situated predominantly on the ligands, indicates that an intermediate oxidation state is involved... 

In moving across the transition series, iron is the first element which fails to attain its group oxidation state (-1-8). The highest oxidation state known (so far) is 4-6 in [Fe04] and even this is extremely easily reduced. On the... 

The most striking feature of the contrasts shown in Table 23-II is that the seventh-row elements display the multiplicity of oxidation states characteristic of transition elements rather than the drab chemistry of the +3 rare earth ions. Whereas Ce+3(aq) can be oxidized to Ce+4(aq) only with an extremely strong oxidizing agent, Th+Yaq) is the stable ion found in thorium salts and Th+3(aq) is unknown. In a similar... 

The mechanisms by which Pu(IV) is oxidized in aquatic environments is not entirely clear. At Oak Ridge, laboratory experiments have shown that oxidation occurs when small volumes of unhydrolyzed Pu(IV) species (i.e., Pu(IV) in strong acid solution as a citric acid complex or in 45 percent Na2Coj) are added to large volumes of neutral-to-alkaline solutions(23). In repeated experiments, the ratios of oxidized to reduced species were not reproducible after dilution/hydrolysis, nor did the ratios of the oxidation states come to any equilibrium concentrations after two months of observation. These results indicate that rapid oxidation probably occurs at some step in the hydrolysis of reduced plutonium, but that this oxidation was not experimentally controllable. The subsequent failure of the various experimental solutions to converge to similar high ratios of Pu(V+VI)/Pu(III+IV) demonstrated that the rate of oxidation is extremely slow after Pu(IV) hydrolysis reactions are complete. 

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