4-2 oxidation state discovery

Plutonium was the first element to be synthesized in weighable amounts (6,7). Technetium, discovered in 1937, was not isolated until 1946 and not named until 1947 (8). Since the discovery of plutonium in 1940, production has increased from submicrogram to metric ton quantities. Because of its great importance, more is known about plutonium and its chemistry than is known about many of the more common elements. The metallurgy and chemistry are complex. MetaUic plutonium exhibits seven aUotropic modifications. Five different oxidation states are known to exist in compounds and in solution. 

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 successful operation of the reactor and plutonium extraction plant at Oak Ridge, Tennessee led to the availability of first milligram, and then gram, amounts of plutonium starting early in 1944, The availability of milligram amounts of plutonium led to the immediate discovery of the III oxidation state. 

The serendipitous discovery of this compound has proven to be extremely important to our visions of possibilities for new metal-metal bonded structures in reduced oxide phases. In retrospect it is amazing that oxide phases containing molybdenum in oxidation states less than 4+ were essentially unknown and certainly structurally uncharacterized. The existence of the previously mentioned series M2 Mo30q and LiM Mo303 should have been a tip-off to an extensive chemistry for metal-metal bonded molybdenum oxide systems. Indeed, subsequent work has revealed a plethora of new compounds all of which (where structure has been determined) feature strong metal-metal bonding in either discrete cluster units or extended chain arrays. 

E.O. Fischer s discovery of (CO)sW[C(Ph)(OMe)D in 1964 marks the beginning of the development of the chemistry of metal-carbon double bonds (1). At about this same time the olefin metathesis reaction was discovered (2), but It was not until about five years later that Chauvln proposed (3) that the catalyst contained an alkylidene ligand and that the mechanism consisted of the random reversible formation of all possible metallacyclobutane rings. Yet low oxidation state Fischer-type carbene complexes were found not to be catalysts for the metathesis of simple olefins. It is now... 

Several organo-titanium compounds with the oxidation states IV, III, II, 0,-1 have been prepared. A starting point was the discovery by Ziegler et al. (1955), Ziegler (1963) and Natta et al. (1955) and Natta (1963) of the catalytic properties of TiClj-Al-alkyl mixtures in hydrocarbons in reactions such as the ethylene and propylene polymerization. 

In the years since the discovery of nickel and iron in the catalytic centres, numerous different descriptions of the catalytic cycle of hydrogenase have been proposed. These were based on different oxidation states of the metal centres, and different sequences of transfer of electrons and hydrous. Although the reaction cycle has not been definitively resolved, the spectroscopic evidence places constraints on possible models that should be considered. 

The lower oxidation states are stabilized by soft ligands e.g. CO (Prob. 3). The aquated vanadium ions represent an interesting series of oxidation states. They are all stable with respect to disproportionation and labile towards substitution. They undergo a number of redox reactions with one another, all of which have been studied kinetically. Many of the reactions are [H ]-dependent. There has been recent interest in the biological aspects of vanadium since the discovery that vanadate can mimic phosphate and act as a potent inhibitor (Prob. 4). 

Electrochemical studies on V(II) complexes have been relatively few compared to the quantity of work on higher oxidation states. The recent discovery of a general synthesis has improved the availability of data on derivatives of vanadocene [15, 16]. For example, Ef values of L2V decrease in the order L = indenyl (—2.38 V versus Cp2Fe/TFlF)> Cp (—2.72 V versus... 

McMillan had been sure that another element was present in his neptunium fractions. In December, 1940, Seaborg, A. C. Wahl, and J. W. Kennedy separated from neptunium a fraction which had alpha activity and which showed at least two oxidation states. It required stronger oxidizing agents to oxidize this substance than were needed for neptunium. The new element was identified as 94. The notes reporting this discovery were submitted to the journals early in 1941, but were not published until 1946 (67, 68). 

Immediately after the discovery of YBa2Cus07 x, numerous groups employed iodometric titration procedures to measure the effective oxidation state of the material, and therefore the value of x. The procedure described below involves two different titrations (10X17X18) and is more accurate than a procedure in which the first titration is omitted (19). Experiment A measures the total copper content of the superconductor and Experiment B measures the total charge of the copper. The two experiments, together, give the average oxidation state of copper. 

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