Metal oxidation numbers

Formally, the metal oxidation number x increases to x+2, while the coordination number n of ML, increases to n+2. If such oxidative addition reactions are intended to be the first step in a sequence of transformations, which eventually will lead to a functionalization reaction of C-X, then the oxidative addition product 2 should still be capable of coordinating further substrate molecules in order to initiate their insertion, subsequent reductive elimination, or the like [1], This is why 14 electron intermediates MLu (1) are of particular interest. In this case species 2 are 16 electron complexes themselves, and as such may still be reactive enough to bind another reaction partner. 

This chapter is devoted to electrochemical processes in which chemical reactions accompany the initial transfer of one electron. This is actually a pretty common situation with organic reactants since the radical or ion-radical species resulting from this initial step is very often chemically unstable. Although less frequent, such reactions also occur with coordination complexes, ligand exchange being a typical example of reactions that may accompany a change in the metal oxidation number. 

In terms of electron transfer reactions, transition metal ions can be the one- or two-electron type. The two-electron ions transform into unstable states on unit change of the metal oxidation number. In the outer-sphere mechanism, two-electron transfer is a combination of two one-electron steps. 

An example of a complex of type 318 is a nickel chelate, while structure 319 represents an ICC of platinum and palladium. These complexes are of renewed interest due to their controversial data concerning metal oxidation number (0, +2, or +4), in particular for Ni, Pt, and Pd, which depends on the resonance structures of the ligand systems shown as 318b and 318c ... 

This situation has been widely discussed [592a-c] and has not been finally resolved [592d]. At the same time, there is experimental data (reactions with halogens) in favor of the existence of dithiol structures of type 318b containing M — S bonds (M = Ni, Pd, Pt) with metal oxidation number +4 [592e,f]. 

Among the electrochemical syntheses related to the change of metal oxidation number, we emphasize obtaining acetylacetonates of divalent iron, cobalt, and nickel [551,623]. The method of alternating-current electrochemical synthesis was applied to isolate Ji-complexes of monovalent copper with allylamines, allylimines, and ally-lurea from the salts of divalent copper [624-628], We note that the same method was used for preparation of analogous ji-complexes with copper(II) halides (X = Cl, Br) [629a]. Other electrochemical syntheses with participation of metal salts and complexes are described in monographs [201,202] and literature cited therein. 

At the same time, it is necessary to take into account that the approach described has a number of exceptions, related for example to the nature of other ligands forming pseudohalide complexes. A series of classic examples of inversion of the bond M — N —> M — S —> M — N have been reported [6,8,11,42-44,59] and are presented in Sec. 2.2.3.5. In this respect, we especially emphasize the capacity of other ligands for soft or hard metals, related with symbiotic [60] and anti-symbiotic [61] effects. Thus, Pearson [61] emphasized that soft ligands, which are placed in a trans position to SCN ion, contribute to N-binding of thiocyanate ions, and hard bases contribute to S-coordination of these ambidentate ligands. Metal oxidation number (Table 1.4) is important in this problem and it regulates soft hard properties of complex-formers. 

The change of metal oxidation number in quinone complexes can also be reached by electrochemical methods. For example, the electrochemical oxidation [(5.20), (5.21)] of [MIV(DBCat)3]2 (M = Mn, Tc, Re) yields products having different oxidation state of the central atom [171,172] ... 

A battery requires two half cells, each of which must involve two oxidation states of an element. Thus, in the Daniell cell, one of the half cells consists of copper metal (oxidation number = 0) in contact with... 

Electron counting (by any method) does not imply anything about the degree of covalent or ionic bonding it is strictly a bookkeeping procedure, as are the metal oxidation numbers that may be used in the counting. Physical measurements are necessary to provide evidence about the actual electron distribution in molecules. Linear and cyclic organic n systems interact with metals in more complicated ways, as discussed in Chapter 5. 

Group Number Metals Oxidation Number Exampies... 

As in the reactions of the metal salts with the epoxy oligomers, the first stage of the reactions of the different metal complexes with them is the formation of the associated molecules. The subsequent decay of the complexes gives the active particles which are the initiators of the polymerization [546-548]. The metal acetylacetonates interact with the epoxy oligomers on the mechanism accompanied by the reduction of the metal oxidation number [Eq. (7)] [473,549]. 

Alkali metals Oxidation number always = +1 Li in LiCI, LijO, LiH KinKOH, K3CO3, KNO3... 

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