High and Low Oxidation States

The species with the lowest known oxidation states of the metal (M and M ) have already been described under iron derivatives. All other species are relatively less 

Marchon and coworkers have prepared a methoxytitanium(III) complex. An unusual feature of the complex is a nearly linear Ti-O-CHj bond (171(1)°) and a short Ti-O bond (1.77(1) A). These features were associated with strong jc-bonding to titanium because of the electron deficient nature of the Ti ion. The Ti -Np bonds are as long as the Ti -Np bonds in [Ti(0EP)0] . Guilard and coworkers have reported the structure of two six-coordinate vanadium(II) complexes (the axial ligands are bis-tetrahydrofuran or bis-dimethylphenylphosphine). These have normal trans geometries with modestly long equatorial bonds V-Np = 2.048(4) A. Axial bonds are V-O = 2.174(4) or V-P = 2.523(1) A. The common oxidation state for these two metals is V and Ti.  

Weiss and coworkers have characterized a number of molybdenum complexes in which the oxidation state of Mo is lower than the normal value of -t 5. First is a series of Mo complexes five-coordinate [Mo(TTP)0] which has Mo-Np = 2.111(5) A and Mo- Ct = 0.64 A. The remaining three complexes are six-coordinate species with trans axial ligands. [Mo(TTP)Cl2] [Mo(TIP)(N2C6H5)2] and [Mo(TPP)(a-C6H5)Cl] ) have, respectively, Mo-Np = 2.074(23), 2.071(15), and 2.070(7) A. 

A number of high-valent metalloporphyrin complexes have been reported. Again high-valency is taken relative to normal species. Many of these species are stabilized by the presence of oxygen donors in the form of oxo or alkoxo ligands. One such set is a series of binuclear p-oxo complexes where the metals (Ru, Os, Mn) have the formal oxidation state +4. All complexes of this class are six-coordinate and include [Ru(0EP)(0H)]20 [Ru(TPP)(0C6H4CH3)]20 ), [Ru(OEP)C1]20 ), 

Mononuclear high oxidation state species are rarer. Hill and coworkers have reported the structures of two six-coordinate Mn complexes [Mn(TPP)(OCH3)2] and [Mn(TPP)(NCO)2] . The equatorial Mn-Np bond distances, 2.012(27) and 1.970(2) A, respectively, are slightly shorter than those of Mn complexes. However, the axial bond distances, Mn-O = 1.839(2) and Mn-N = 1.926(11) A, are substantially shorter than those of six-coordinate Mn complexes. These distances are entirely consistent with those expected for Mn d complexes (S = 3/2). 

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The X-ray crystal structures of a variety of high- and low-oxidation-state nickel complexes are now known. These are introduced throughout the text in Section IV,A and the important points are summarized in... 

Table 1 Examples or the Stabilization of High and Low Oxidation State Aquo and Hydroxo Complexes...

There are three types of SOD. All SODs have transition metal ions in their reaction center, and the enzymes in both high and low oxidation states react readily (k = 2 x 109 dm3 mol1 s 1 Klug et al. 1972 Fielden et al. 1974 Pick et al. 1974) with 02 " [written with Cu as an example reactions (62) and (63)] which allows the enzyme to be recycled. 

The oligopyridines are ideally suited to such roles they possess a filled highest occupied molecular orbital (HOMO) and a vacant lowest unoccupied molecular orbital (LUMO) of suitable energies for interaction with metal d orbitals (283). They are thus capable of stabilizing both high and low oxidation states of metal ions. 

The ferrocenyl ligand is able to tolerate both high and low oxidation states of the central metal, as may be seen from the series of half-sandwich tungsten-ferro-cenyl complexes CpW(CO)3-Fc [178, 184], CpW(NO)2-Fc [177], and CpWO -Fc [177] (Scheme 5-26). 

The difference and the ratio are between high and low oxidation states, raiiorelativity intensity of the white line. for solution samples is dependent upon the relative concentration of Ce and Ce (Ref. 36). The calculation was done with the Herman Skillman program, for example, configurations 2p - 4f 6d - and 2p, 4f 4d° for Ce and Ce, respectively were used only the difference in the transition energy was considered. 

J.A. McCleverty (1999) Chemistry of the First-row Transition Metals, Oxford University Press, Oxford - A valuable introduction to metals and solid compounds, solution species, high and low oxidation state species and bio-transition metal chemistry. 

This chapter illustrates the complementarity of photochemical and radiation chemical techniques to elucidate elementary pathways in mechanistically rich systems. Some of the mechanistic conclusions that have resulted from these studies in aqueous media are presented. Extreme (both high and low) oxidation states of transition-metal complexes are included. Reactivity with respect to electron transfer reactions and small-molecule activation are addressed. 

We and others have used pulse radiolysis methods to clarify a number of complex photochemical mechanisms. In the course of these studies we have also been able to learn a great deal of new chemistry, including the electronic absorption spectra, thermodynamics, and reaction mechanisms of highly reactive transition-metal centers in both unusually high and low oxidation states. As these data pertain to aqueous media, they contribute in an important way to future work on solar photoconversion in water (the ideal medium from both economic and environmental points of view) and to catalysis in aqueous media in general. 

Isocyanide appears to be a stronger a-donor than CO, and also a stronger Ti-acceptor than CN and is capable of forming complexes in high and low oxidation states. From Tables 21 and 22 one can see that the number of terminal isocyanides is nearly double that of terminal cyanides. As for cyanides, the terminal bonding mode MC3 of isocyanides is far more common than the two bridging modes of MC4 and MC5. 

The electrochemistry of iron porphyrins in nonaqueous media has been discussed in several reviews [2, 7, 10, 12], and only a few of the major trends of iron porphyrin electrochemistry will be summarized in the current paper. Both high and low oxidation states of the metal ion can be accessed upon reduction or oxidation of iron porphyrins and the overall electron-transfer mechanism of these metalloporphyrins is shown in Sch. 3, where [(P)Fe "]" represents the initial compound in the absence of an associated anionic ligand. 

TM compounds in high and low oxidation states with metal-carbon double and triple bonds have been the subject of a systematic theoretical study. Table 7 shows the calculated metal-ligand bond lengths of several tungsten carbene and carbyne complexes at the HF and MP2 levels of theory. The HF optimizations were carried out using two different basis sets. Basis set I has DZ quality at tungsten (no additional 6p... 

The crystal structure and spectroelectrochemistry of the complex fac-[Re(CO)3(bpy)(COMe)] have been reported These indicate that there is an orthogonal arrangement of dxy/7i (bpy) and dxy/Ti (acetyl) orbitals. The excited state properties of both high- and low-oxidation state rhenium complexes have been reviewed. ... 

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