Transition metal oxidation numbers

Group lA and group 2A metals have only one oxidation number. Transition metals and metals on the right side of the periodic table often have more than one oxidation number. To distinguish between multiple oxidation numbers of the same element, the name of the chemical formula must indicate the oxidation number of the cation. The oxidation number is written as a Roman numeral in parentheses after the name of the cation. For example, the compound formed from Fe + and has the formula FeO and is named iron(II) oxide. The compound formed from Fe + and has the formula Fe203 and is named iron(III) oxide. 

E3.43 AgjS and CuBr (low-oxidation-number metal chalcogenide and halide) would be a p-type, and VO2 (high-oxidation-number transition metal oxide) would be an n-type. 

Vigorous oxidation leads to the fonnation of a carboxylic acid, but a number of methods pemnit us to stop the oxidation at the intemnediate aldehyde stage. The reagents most commonly used for oxidizing alcohols are based on high-oxidation-state transition metals, particularly chromium(VI). 

The performance of a number of single oxides of transition metals was studied by Skorbilina et al. [295] using a differential reactor. As usual, o-tolualdehyde, phthalic anhydride and carbon oxides are the main reaction products. The initial selectivity with respect to partial oxidation products decreases in the order Co > Ti > V > Mo > Ni > Mn > Fe > Cu from 71% to 33%. The relatively high initial selectivities demonstrated by the deep oxidation catalysts (e.g. Co, Ni, Mn) indicates that the primary activation is probably the same for all these catalysts, while the differences that actually determine the character of the catalyst are connected with the stability of intermediates and products. 

Oxides of transition metals exhibit more complex add-base properties that may depend on the oxidation number of the metal. 

In contrast to the processes described above, the electrooxidation of metals and alloys still cannot be considered as an accepted electrosynthetic method as yet only its principal possibilities have been demonstrated. At the same time, the anodic oxidation of transition metals, which forms the basis for a number of semiconductor technologies, is extremely effective and convenient for varying and controlling the thickness, morphology, and stoichiometry of oxide films [233]. It therefore cannot be mled out that, as the concepts concerning the anodic behavior of metal components of HTSCs in various media are developed, new approaches will be found. The development of combined methods that include anodic oxidation can also be expected, by analogy with hydrothermal-electrochemical methods used for obtaining perovskites based on titanium [234,235], even at room temperature [236]. 

The preceding method is sufficient for naming binary ionic compounds containing metals that exhibit only one oxidation number other than zero (Section 4-4). Most transition metals and the metals of Groups IIIA (except Al), IVA, and VA, exhibit more than one oxidation number. These metals may form two or more binary compounds with the same nonmetal. Ta distinguish among all the possibilities, the oxidation number of the metal is indicated by a Roman numeral in parentheses following its name. This method can be applied to any binary compound of a metal and a nonmetal. 

E3.41 Low oxidation number d-metal oxides can lose electrons through a process equivalent to the oxidation of the metal atoms, with the result that holes appear in the predominately metal band. The positive charge carriers result in their p-type semiconductor classification. NiO is an example of this p-type semiconduction. Early transitional metal oxides with low oxidation number such as TiO and VO have metallic properties owing to the extended overlap of the d orbitals of the cations. See Section 24.6b for more details. 

The specific catalytic activity of different oxides varies in the very wide range of about 10 times. It is determined mainly by the oxygen bonding energy on the oxide surface depending on the electronic structure of metal cation. The oxides of transition metals with the number of d-electrons of cation 3, 7, 8, and 9 possess the highest activity. 

With the requirement of electronic conductivity, oxides containing cations with mixed valence and, in particular, reducible cations are preferable. Oxides containing transition metals are therefore appropriate alternatives. There are indications based on conductivity measurements that Ti02 could be a possible candidate [83], but no direct measurements of hydrogen permeability have been reported. Tita-nates, in general, however, are interesting because there are a number of materials classes that accommodate oxygen vacancies and may dissolve protons. 

Ions and oxides of transition metals which may exist in different valence states have been shown to oxidize thiols. Most of the studies so far available on this topic deal with the oxidation by ferric ions careful investigations with many other metals have been carried out as well. The catalytic effect of these metal ions on the auto-oxidation of thiols has been pointed out (see section IV). The intervention of metals in a number of redox enzymes in which the metal is bound to a thiol group at the active site of the enzyme has been also suggested. 

Oxides of transition metals, mainly Cr, Mn, Co, Ni, Fe, Cu, and V are employed in the oxidation of organic compounds. Deep oxidation reactions over these metal oxides are considered to be catalyzed by lattice oxygen. A common feature of these metal oxides is the presence of multiple oxidation states. During catalysis, the metal may be reduced by the hydrocarbon and reoxidized by oxygen. It may cycle between two or more oxidation states thus operating in a redox cycle (Mars-van Krevelen mechanism) [12]. However, the actual mechanism of a working catalyst may involve many steps in a number of consecutive or parallel reactions. Because of its low volatility and low toxicity, MnOjc has received the attention of many researchers. 

An alternative mechanism starts from the coordination of an amine, and the successive deprotonation gives a metal amide species (Scheme 8b). Coordination of a C-C multiple bond to this metal center is followed by migratory insertion into the M-N bond. The newly formed M-C bond is cleaved by protonolysis to regenerate the active metal species. The advantage of this pathway is that it does not require the change of oxidation number of metal, and it looks similar in mechanism to hydroamination of other group metals (for group 4 metals, metathet-ical reaction takes place at the step of C-N bond formation) and partially similar in mechanism to oxidative amination of late transition metals. However, so far, most hydroamination reactions catalyzed by late transition metals can be explained by the mechanisms discussed in Sects. 3.1 and 3.2.2. If the activation of the C-C... 

The equilibrium is more favorable to acetone at higher temperatures. At 325°C 97% conversion is theoretically possible. The kinetics of the reaction has been studied (23). A large number of catalysts have been investigated, including copper, silver, platinum, and palladium metals, as well as sulfides of transition metals of groups 4, 5, and 6 of the periodic table. These catalysts are made with inert supports and are used at 400—600°C (24). Lower temperature reactions (315—482°C) have been successhiUy conducted using 2inc oxide-zirconium oxide combinations (25), and combinations of copper-chromium oxide and of copper and silicon dioxide (26). 

HDPE resias are produced ia industry with several classes of catalysts, ie, catalysts based on chromium oxides (Phillips), catalysts utilising organochromium compounds, catalysts based on titanium or vanadium compounds (Ziegler), and metallocene catalysts (33—35). A large number of additional catalysts have been developed by utilising transition metals such as scandium, cobalt, nickel, niobium, molybdenum, tungsten, palladium, rhodium, mthenium, lanthanides, and actinides (33—35) none of these, however, are commercially significant. 

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