Oxidation state of a metal

Entries where the oxidation state of a metal has been specified occur after all the entries for the unspecified oxidation state, and the same or similar entries may occur under both types of heading. Thus cyanide appears under Chromium complexes, Chromium(O) complexes, Chromium(I) complexes, etc. More specific entries, such as Chromium, hexacyano-, may also occur. Similar ligands may also occur in different entries. Thus a carboxylic acid-metal complex may occur under Carboxylic acid complexes, under entries for specific carboxylic acids, and under the specific metal. Coordination complexes may also be listed in the Cumulative Formula Index. 

Oxidation state is a frequently used (and indeed misused) concept which apportions charges and electrons within complex molecules and ions. We stress that oxidation state is a formal concept, rather than an accurate statement of the charge distributions within compounds. The oxidation state of a metal is defined as the formal charge which would be placed upon that metal in a purely ionic description. For example, the metals in the gas phase ions Mn + and Cu are assigned oxidation states of +3 and +1 respectively. These are usually denoted by placing the formal oxidation state in Roman numerals in parentheses after the element name the ions Mn- " and Cu+ are examples of manganese(iii) and copper(i). 

The different oxidation states of a metal can have dramatically different chemical properties, which in turn affect their biogeochemical forms and significance. For example, almost 4 g/L ferrous iron, Fe(II), can dissolve in distilled water maintained at pFi 7.0. However, if the water is exposed to air and the iron is oxidized to Fe(III) essentially all the iron will precipitate, reducing the soluble Fe concentration by more than eight orders of magnitude. Oxidation state can also affect a metal ion s toxicity. For instance, the toxicity of As(III) results from its ability to inactivate enzymes, while As(V) interferes with ATP synthesis. The former is considerably more toxic to both aquatic organisms and humans. 

The thermodynamically stable oxidation state of a metal in a given environment is a function of the prevailing oxidation potential. The value of the potential is given by the Nemst equation, which is described in Chapter 5 for the generic reduction half-cell ... 

This is an important consideration in two ways for if a compound is to be useful it may be required that (1) it does not dissociate all its ligands but that (2) it is still able to exchange some of them. Fig. 5 shows that for a fixed oxidation state of a metal the stability of X, where X is... 

A set of oxygen donor atoms, providing both a and tt donation to a metal center, is not appropriate to stabilize any low oxidation state of a metal.19 This is, however, a synthetic advantage since very reactive, unstable, low-valent metalla-calix[4]arenes can be generated in situ and intercepted by an appropriate substrate. In the absence of a suitable substrate, the reactive fragment, however, can collapse to form metal-metal bonded dimers. The formation of metal-metal bonds has been, however, so far observed in the case of Group V and VI metals only. The most complete sequence so far reported has been for tungsten, molybdenum, and niobium. 

It must be possible to change the oxidation state of a metal. Normally the metal changes oxidation state by 2 units, but two metal atoms can change by 1 unit each as shown in Eqs. (22.10) and (22.11). 

Note that there are a number of different methods for indicating the oxidation state of a metal ion, especially transition metal ions that have variable oxidation states. As an example, the iron ion in its +2 oxidation state may be written as Fe +, Fe(II), Fe , or iron(ll). In this text, the methods are used interchangeably. 

An empirical method for correlating the oxidation state of a metal ion with the coordination geometry and the bond lengths Bond valence sum analysis has been used in characterizing the structural features of vanadium-dependent haloperoxidases . ... 

A study with OsBr(TPP)PPh3 as a resting state and iodosylbenzene as the oxidant was not very promising [239]. Furthermore, dioxoosmium(VI) porphyrins 0s02(P), despite their record in oxidation state of a metal within the porphyrin, are only weak oxidants. 

Many of the more spectacular examples of biological processes associated with metal ions are concerned with the oxidation or reduction of organic substrates. We saw in the previous chapter the ways in which changes in the oxidation state of a metal ion... 

Inorganic systems featuring linkage isomerism induced by changing the oxidation state of a metal are also known.145-561 For instance, a sulfoxide is O-bonded to ruthenium n) in its stable form (Ru-OSR2). On reduction of the metal to the divalent state, the initially obtained O-bonded species rearranges to afford the stable S-... 

The average oxidation state of a metal in a catalyst during reaction was found to be related to the presence of carbonaceous deposits on the surface. As the feed for propane ODH was depleted in O2, the catalyst was readily reduced (Mul et al., 2003) and amorphous carbon-containing deposits formed. This behavior was corroborated by UV-vis DRS (Mul et al., 2003 Puurunen and Weckhuysen, 2002) and by combination of UV-vis DRS and Raman spectroscopy (Kuba and Knozinger, 2002 Nijhuis et al., 2003). 

An oxidative addition reaction occurs when there is an increase in oxidation state of a metal that is accompanied by a simultaneous increase in its coordination number. Numerous examples of oxidative addition (sometimes referred to as oxad) reactions have already been presented for nonmetallic elements including the following ... 

In most redox enzymes, the different oxidation states of a metal center can be produced by poising the ambient redox potential of the sample. However, the intermediate oxidation states of the Mn complex in PSII have very high reduction potentials (in the range of 0.8-1.2 V vs. the... 

The ability of transition metals to adopt a number of different oxidation states is a very important feature of transition metal chemistry. In order to determine the oxidation state of a metal in a complex, a simple formalism is adopted where the oxidation state is the charge remaining on the central metal atom when all the ligands are removed in their closed shell configuration . While this... 

Biological molecules are often switched on and off by controlling the ligation environment or oxidation state of a metal center. The selective binding of ions [104, 197] and modulation of the binding environment of metal centers [79, 92, 198 204] have also been demonstrated as mechanisms for switching. The second of these examples is based on chemical modulation of the valence state of a metal or its... 

Turn now to complex formation and a half-reaction involving two oxidation states of a metal in solution (charges omitted for simplicity) ... 

Oxidation states met amongst complexes of transition metal elements rf-electron counts for the particular oxidation states of a metal appear below each oxidation state. [Oxidation states that are relatively common with a range of known complexes are in black, others in grey.]... 

Modifying the relative stabilities of different oxidation states of a metal... 

Just as we can tune the reducing power of Ag by manipulation of the solution species or precipitates present, we can also alter the relative stabilities of two oxidation states of a metal, both of which are subject to removal by precipitation or complexation. As an example, consider the Mn /Mn couple, for which equation 7.34 is appropriate for aqua species. 

Some organic ligands, notably 1,10-phenanthroline and 2,2 -bipyridine (Table 6.7), stabilize the lower of two oxidation states of a metal. This is apparent from the values of FF for the appropriate half-reactions in Table 7.1. The observation is associated with the ability of the phen and bpy ligands to accept electrons. Iron(II) complexes of bpy and phen are used as indicators in redox reactions. For example, in a redox titration of Fe with powerful oxidizing... 

In reactions between a C-H compound, RH, and a metal complex, M, to produce R-M-H, the oxidation state of a metal ion is changed and it is two units higher in the organyl hydride than it was in the initial metal compound. Alkanes, arenes, alkenes and monosubstituted acetylenes enter into such processes. The reactions frequently occur in solution at room temperature, but sometimes heating is required. Certain reactions are stimulated by irradiation. An increase in temperature or light is essential for the abstraction of several ligands from the initial complex and the formation of a coordinatively unsamrated species capable of effecting the oxidative addition of the C-H compound to itself. 

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