High Oxidation State Complexes

There is an interesting paradox in transition-metal chemistry which we have mentioned earlier - namely, that low and high oxidation state complexes both tend towards a covalency in the metal-ligand bonding. Low oxidation state complexes are stabilized by r-acceptor ligands which remove electron density from the electron rich metal center. High oxidation state complexes are stabilized by r-donor ligands which donate additional electron density towards the electron deficient metal centre. 

The problems associated with the storage and handling of larger amounts of fluorine at high pressures can be avoided by the use of fluorine gas generators which are based on stable solids. Two practical approaches have been demonstrated,7 8 one of which is commercially available.9 Both approaches are based on high oxidation state complex fluoro anions of transition metals such as nickel, copper, or manganese. 

The earliest high-oxidation-state complex of nickel reported was the heteropoly(molybdate) (132, 133) complex [NilvMo90 32]6. which contains nickel(IV) in an octahedral Ni06 coordination environment. There is no evidence for the corresponding nickel(III) species but further work on nickel(IV) complexes of this type has been reported recently (134). Nickel(III) can be prepared in a six-coordinate oxygen donor environment (135) as a tris chelate with 2,2 -bipyridine-l,T-dioxide (bpy02). The complex has a rhombic EPR spectrum and a reduction potential of 1.7 V, from which an estimate of the reduction potential of the ion [Nini(H20)6]3+ of 2.5 V (versus nhe) has been calculated. 

Another dinuclear, high oxidation state complex, [MnIV(salpn)( j.2-0)]2, 63, contains a MnIVMnIV center, an oxidation state level even higher than those known for catalases. Nevertheless complex 63 has catalase activity (1000 turnovers without decomposition) and follows a catalase pathway (Scheme 7) similar to that of L. plantatarum catalase (Scheme 1) [109]. 

Until recently there has been surprisingly little interest in high oxidation state complexes of terpy. Meyer and co-workers have demonstrated that the ruthenium(IV) complex [Ru(terpyXbipy)0] is an effective active catalyst for the electrocatalytic oxidation of alcohols, aromatic hydrocarbons, or olefins (335,443,445,446). The redox chemistry of the [M(terpy)(bipy)0] (M = Ru or Os) systems has been studied in some detail, and related to the electrocatalytic activity (437,445,446). The complexes are prepared by oxidation of [M(terpy)(bipyXOH2)] . The related osmium(VI) complex [Os(terpyXO)2(OH)] exhibits a three-electron reduction to [Os(terpyXOH2)3] (365,366). The complex [Ru(terpy)(bipyXH2NCHMe2)] undergoes two sequential two-electron... 

In general, high-oxidation state complexes have far fewer than 18 electrons becanse extreme steric crowding would result if enough ligands were bonded to the metal center to form either a 17- or 19-electron species. Similarly, dimerization to generate cluster-type complexes is often prevented for steric reasons see Dinuclear Organometallic Cluster Complexes). 

Numerous examples of homoleptic complexes in high or low formal oxidation states are known. In general, the high oxidation state complexes are best prepared by chemical or electrochemical oxidation of the normal oxidation state compounds, followed by further reaction in situ or precipitation with a suitable inert anion. In this respect, perchlorate is ideal as both oxidant and precipitant, but the complexes obtained are frequently violently explosive. Similarly, the low oxidation state complexes are best obtained by chemical or electrochemical reduction of available compounds (or normal oxidation salts in the presence of an excess of bpy). Commonly used reductants have included dissolving metals (zinc, sodium, lithium, magnesium) and the complexes Li(bpy) and Li2(bpy). Isolated examples are known of the synthesis of low oxidation state complexes by reaction of M(0) complexes with bpy or by metal vapor synthesis. 

The carbene carbon atom in high oxidation state complexes of Schrock-type car-benes, by contrast, shows nucleophilic behavior. The bonding mode of these two types of carbene complexes may be schematically represented as shown in Scheme 8.3. 

The usual representation of Schrock-type nucleophilic carbenes as electron rich at carbon can be especially misleading in the case of the Tebbe reagent and related complexes. These high oxidation state complexes are electron-deficient and electrophilic at the metal center, and it is unlikely for polarization of the metal-carbon bond to remove even more electron density from the metal under these circumstances. Thus, the reactivity of the Tebbe reagent is more closely related to the electrophilicity and oxophilicity of the metal center than to the nucleophilicity of a polarized carbene carbon that is, the reactivity is due to carbonyl polarization upon complexafion, not attack of the alkylidene carbon on an unactivated, electrophilic carbonyl carbon. 

In this case, LMCT excitation forms R and a Co(ii) metal center. Note, this is formally a redox reaction. An example of LMCT reactivity in a high oxidation-state complex occurs on irradiation of Cp2TiCl2. The Cp — Ti CT state leads to the intermediate formation of Cp and CpTiCla radicals. ... 

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