Anodic oxidation of metals

The metatheis of acetates with the alkali alkoxides (method 5) can be used for the preparation of the methoxides and ethoxides of all three elements and is the only reaction leading to Hg(OR)2 [1623]. The trans-esterification of Zn(OMe)2 (method 6), according to [1121], can be carried out only in the presence of LiOR (forming soluble bimetallic complexes). The direct electrochemical synthesis on the anodic oxidation of metals in alcohols has been described for Zn(OEt)2 and a series of cadmium derivatives (Cd(OR)2 — obtained in the presence of such donor ligands as Dipy, Phen, and Dmso [98]) (method 2). 

At present the most perspective and ecologically pure route appears to be the anodic oxidation of metal in alcohols in the presence of a conductive additive (method 2), and industrial technology has been developed in Russia for Ti(OBu )4 [948]. 

Recently, the anodic oxidation of metals (method 2) was also applied for the preparation of the niobium and tantalum derivatives of M(OR)5 series (R= Me, Et, Pr, Bu ) [1478, 1616, 1639]. It should be mentioned that a crystalline oxoisopropoxideTa20(OPri)g,iPrOH (Fig. 4.1 c), was isolated from the PrOH-based electrolyte. It is destroyed on heating in vacuo, yielding Ta(OPri)J. 

The main route to rhenium alkoxides is the interaction of halids and oxy-halids with alkali alkoxides or alcohols in presence of amines (method 5). As the important starting reagents can serve also Re2(CO),0 and Re207 (method 3) The preparation ofrhenium (V) and (VI) oxoderivatives by the anodic oxidation of metal in alcohols has also been described (method 2) (see Table 12.22). The bimetallic alkoxides ofrhenium and heavy transition metals can most efficiently be obtained by interaction ofrhenium (VII) oxide with the alkoxides ofthese elements in refluxing toluene ... 

Passive layers of various metals have semiconducting properties others have in-solating properties. As usual, this is a consequence of the band gap. The anodic oxides of metals like Fe, Cr, Ni, Co and Cu show semiconducting properties, whereas the valve metals like Al, Ta, Zr, Hf and Ti form electronically insolating... 

Anodic oxidation of metallic tungsten in methanol afforded a mixture of [W(OMe)6] and [WO(OMe)4], and reaction of [W(NMe2)6] with pinacol afforded die tris-pinacolate (2-) [W(OCMe2CMe20)3]. Treatment of WCle with one or two equivalents ofthe silsesquinoxane R7Si709(0H)3 afforded the bis(silsesquinoxanato) complexes (7). 

Oxides are by no means new materials to electrochemists [63-76] and have been actively studied in connection with electrocatalysis and anodic oxidation of metals. In particular, a great deal of information has accumulated on the electrochemistry of two- (and more-) component perovskites and also of spinels, both of which are structurally similar to HTSC oxides. 

The anodic oxidation of metals and alloys can also be performed in certian melts. For instance, a KCl-NaCl-CuCl-LaCla melt was used in which the additional copper and lanthanum ions were generated by the simultaneous galvanostatic dissolution of... 

To date the acid-base behavior of several hydrous oxide systems, Au,41 Ir,4 Rh,42 Ni,43 and Pt27 has been discussed in considerable detail. It was also suggested recently that, as will be discussed later, anionic species may be involved in the onset of anodic oxidation of metals such as Pt,44 Pd,28 Ni,45 and Au28 where a significant decrease in the initiation or onset potential (RHE scale) for surface oxidation is observed on increasing the solution pH. 

Similar results are obtained using the model of VERWEY /163/ for anodic oxidation of metals, in which the limiting step is assumed to be the motion of ions inside the oxide film where they must surmount a barrier between two interstitial positions. 

Yurkinsky, V.P. (1988) Anodic oxidation of metals in nitrate melts, Rasplavy 2, 74-82... 

Pickett, C.J. and Fletcher, D. (1976) Anodic oxidation of metal carbonyls in Irilluoroacetic acid stabilities of some 17e cations. Journal of the Chemical Society, Dalton Transactions, 636-638. 

This section on film growth will be restricted to direct film formation processes in which passivation is the result of the direct reaction between the metal surface and the aqueous solution. Other processes of film formation, such as dissolution/precip-itation (dissolution of metal ions and subsequent precipitation of an oxide, oxi-hydrox-ide, or hydroxide) and anodic deposition processes (anodic oxidation of metal ions in the solution and deposition on the surface), are not treated here. 

Considered initially as, in general, a simple extension of the direct reaction with alcohols for less active metals by application of an anodic potential, the anodic oxidation of metals turned to be a much more complicated process. At present, at least three different oxidation mechanisms have been proposed for different groups of metals ... 

For these reasons, we preferred a technique using an undivided cell with a sacrificial anode made of a rearhly oxidized metal. Anodic oxidation of metals has been previously shown to provide organometallic compounds from either preformed carbanions [22], or carbanions formed in situ by cathodic reduction of organic halides... 

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