Oxides ligand coordination

It is very common for inorganic chemists to neglect or ignore the presence of solvent molecules coordinated to a metal centre. In some cases, this is just carelessness, or laziness, as in the description of an aqueous solution of cobalt(ii) nitrate as containing Co ions. Except in very concentrated solutions, the actual solution species is [Co(H20)6] . In other cases, it is not always certain exactly what ligands remain coordinated to the metal ion in solution, or how many solvent molecules become coordinated. Solutions of iron(iii) chloride in water contain a mixture of complex ions containing a variety of chloride, water, hydroxide and oxide ligands. 

These structural data are in agreement and support EXAFS data for MOP (214) as well as for xanthine oxidase (in both oxidized and reduced forms) (198, 215), but the coordinated water ligand was iden-... 

Oxidation of Coordinated Diimine Ligands in Basic Solutions of Tris( diimine )iron(III),-ruthenium(III), and -osmium(III)... 

Classification exclusively in terms of a few basic mechanisms is the ideal approach, but in a comprehensive review of this kind, one is presented with all reactions, and not merely the well-documented (and well-behaved) ones which are readily denoted as inner- or outer-sphere electron transfer, hydrogen atom transfer from coordinated solvent, ligand transfer, concerted electron transfer, etc. Such an approach has been made on a more limited scale. Turney has considered reactions in terms of the charges and complexing of oxidant and reductant but this approach leaves a large number to be coped with under further categories. 

NO disproportionation has been shown to be promoted by the Mn(II) tropocoronand complex Mn(TC-5,5) (82) (Eq. (38)), and the reaction was found to involve three equivalents of NO leading to formation of N20 and O-coordinated nitrito ligand. The electron balance is provided by oxidation of Mn(II) to Mn(III). The mononitrosyl complex Mn(TC-5,5)(NO) was proposed to react with NO to produce an unstable cis-dini-trosyl, Mn(TC-5,5)(NO)2, which is then poised to form an N-coordinated hyponitrito (0=N-N=0) ligand from which oxygen transfer occurs to another NO (82a). The intermediacy of a hyponitrito ligand parallels other proposed mechanisms for metal complex promoted NO disproportionation (5a-d). 

Table IV lists a series of octahedral (phenolato)chromium(III) precursor complexes that contain one or three oxidizable coordinated phenolato pendent arms (146, 154). These complexes display characteristic electrochemistry Each coordinated phenolato ligand can undergo a reversible one-electron oxidation. Thus complexes with one phenolato moiety exhibit in the C V one reversible electron-transfer process, whereas those having three display three closely spaced (AE1/2 250 mV) ligand-centered one-electron transfer processes, Eqs. (7) and (8).

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. 

How could one distinguish experimentally in the interaction of a hydrous oxide surface with a fatty acid, whether the interaction is due to hydrophobic bonding or to coordinative interaction (ligand exchange of the carboxyl group with the surface functional groups of the hydrous oxide) ... 

As mentioned in the introduction, early transition metal complexes are also able to catalyze hydroboration reactions. Reported examples include mainly metallocene complexes of lanthanide, titanium and niobium metals [8, 15, 29]. Unlike the Wilkinson catalysts, these early transition metal catalysts have been reported to give exclusively anti-Markonikov products. The unique feature in giving exclusively anti-Markonikov products has been attributed to the different reaction mechanism associated with these catalysts. The hydroboration reactions catalyzed by these early transition metal complexes are believed to proceed with a o-bond metathesis mechanism (Figure 2). In contrast to the associative and dissociative mechanisms discussed for the Wilkinson catalysts in which HBR2 is oxidatively added to the metal center, the reaction mechanism associated with the early transition metal complexes involves a a-bond metathesis step between the coordinated olefin ligand and the incoming borane (Figure 2). The preference for a o-bond metathesis instead of an oxidative addition can be traced to the difficulty of further oxidation at the metal center because early transition metals have fewer d electrons. 

The rate is 400-fold faster than for the free ligand. In both cases intermolecular attack of OH at the phosphorus center is involved. Hydrolysis of the corresponding Co(III) complex is through 100% Co - O bond cleavage, thus preventing a test of the effect of metal coordination. Oxidation of Ir(H20) + yields binuclear Ir(IV) and Ir(V) species although little is known of their chemistry. 3 Electrochemical reduction in nonaqueous solution of Ir(bpy) -"... 

HCHO and PH3 proceeds in the presence of K2PtCl4 at room temperature and affords the crystalline product in an essentially quantitative yield in 2.5 h [4]. Palladium compounds are also active in the catalysis [5]. In these reactions the active species is believed to be zero valent. Two mechanistic possibilities have been proposed as illustrated in Scheme 2. The first elemental process involved in the catalytic cycle is oxidative addition of a P-H bond, which is well precedented [6]. In one of the mechanistic possibilities the processes that follow the oxidative addition are the insertion of the C=0 bond into H-M species and P-C reductive elimination, the latter of which is also precedented [7]. In the other, the coordinating phosphide ligand makes a nucleophilic attack [8] at the formaldehyde carbon forming zwitterionic species. 

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