Other Common Transition Metal Oxidants

Mo(N2)2(dpe)2, and BnNMe3 OMoBr4. l l Many of these reagents have been shown to be selective for the oxidation of secondary alcohols, as in the oxidation of 1,10-undecanediol (122) to keto alcohol, 123.  

Cerium(IV) has been used extensively, and the two most common reagents are ceric ammonium sulfate [Ce(S04)2 2(NH4)2S04 2 H2O] and ceric ammonium nitrate [Ce(NH4)2(N03)6].l 3 Modified cerium reagents such as Ce(OH)302Hl 4 d [(N03)3Ce]3H2l06l - have been used by Firouzabadi et al. or the oxidation of primary alcohols, especially benzylic and allylic alcohols. 

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Table 20.2 lists the formulas of the oxides formed when the more common transition metals react with oxygen. With one exception (Cu20), the transition metal is present as a +2 and/or a +3 ion. In Co304 and in other oxides of general formula M304, there are two different types of cations +2 and +3. To be specific, there are twice as many +3 as +2 cations we might show the composition of Co304 as... 

This review has highlighted the key contributions of modern surface science to the understanding of the kinetics and mechanism of nitrogen oxide reduction catalysis. As discussed above, the conversion of NO has been taken as the standard to represent other NOx, and CO has typically been used as the reducing agent in these studies. The bulk of the work has been carried out on rhodium and palladium surfaces, the most common transition metals used in three-way catalytic converters. 

The reactions of MeOH with some transition metal oxide cluster anions [M O J, where M = Mn, Fe, Co, Ni, Cu n = 1,2 x = 2—4, have been studied (254). The [M03] anions were unreactive toward MeOH, unlike [Nb03]. The addition of the hydrogen molecule to the other cluster anions was the common reaction yielding the following transformations,... 

Of the steps listed in Table 1. some are encountered more frequently, while others are less common. Transition metal catalyzed processes usually begin with oxidative addition or coordination-addition as an Entry, which is commonly followed by transmetalation or insertion in the Attachment phase. The final Detachment step is either reductive elimination, or p-hydride elimination, depending on the nature of the intermediate. 

The loss of oxygen from transition metal oxides need not necessarily lead to vacancies accompanied by the redistribution of bonds. It can also lead to the compression of the structure along certain crystallographic planes by what may be called shear, where the polyhedra (octahedra in the well-documented cases) are condensed upon each other, so that the metal atoms close to these planes have more bridging oxygens in common. 

The chemistry of (87) resembles that of (83), allowing isolation of the analogs [ReCp2H2]+, [ReCp JK, ReCp Cl, and ReCp Me. C-H activation of the ring methyls, also common in other early transition metal MCp metallocenes, arises upon irradiation of ReCp Cl or oxidation of ReCp2, for example, (88). " ... 

Aluminum is the third most abundant element in the earth s crust (after oxygen and silicon), accounting for 8.2% of the total mass. It occurs most commonly in association with silicon in the aluminosilicates of feldspars and micas and in clays, the products of weathering of these rocks. The most important ore for aluminum production is bauxite, a hydrated aluminum oxide that contains 50% to 60% AI2O3 1% to 20% FeiOs 1% to 10% silica minor concentrations of titanium, zirconium, vanadium, and other transition-metal oxides and the balance (20% to 30%) water. Bauxite is purified via the Bayer process, which takes advantage of the fact that the amphoteric oxide alumina is soluble in strong bases but iron(III) oxide is not. Crude bauxite is dissolved in sodium hydroxide... 

Apart from availability (Section 1.2.1), there is another more chemical approach to commonality that we should dwell on, an aspect that we have touched upon already. This is a definition in terms of oxidation states. With the most common of all metals in the Earth s crust, the main group element aluminium, only one oxidation state is important - Al(III). However, for the most common transition metal (iron), both Fe(II) and Fe(III) are common, whereas other higher oxidation states such as Fe(IV) are known but very uncommon. With the rare element rhenium, the reverse trend holds true, as the high oxidation state Re(VI) is common but Re(III) and Re(II) are rare. What is apparent from these observations is that each metal can display one or more usual oxidation states and a range of others met much more rarely, whereas some are simply not accessible. 

Many heterogeneously catalyzed reactions are performed using metal particles dispersed on oxide supports, commonly on alumina or silica but also many others, including transition metal and rare earth metal oxides. Metal clusters have interesting reactions with or at the surfaces of such oxide materials. It has recently been shown that the oxide surface is actually a good medium for the synthesis of... 

More interesting is the commonly encountered situation where an ion diffuses in a majority electronic conductor. Thus, diffusion in metallic and semiconducting alloys or of inserted species in transition metal oxides and chalcogenides fall into this category. Many electrode reactions are of this type. Lithium diffusion in j5-LiAl and other alloys is of interest in negative electrode reactions for advanced hthium batteries hydrogen and lithium diffusion in oxides (e.g. VeOn) and sulfides (e.g. TiSa) are of importance as positive electrode reactions for batteries and elec-trochromic devices. 

Titanium and nickel also have only a single common nonzero oxidation state. All of the other 3r/-transition metals exhibit at least two oxidation states in their compounds. For example, cobalt can form Co and Co ions. 

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