4-2 oxidation state

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). 

Older texts often employ an alternative nomenclature in which the suffixes -ous and -ic are encountered. In general, these labels only apply to the most common oxidation states of the metals, -ic referring to the higher oxidation state and -ous to the lower. Using this nomenclature, copper(ii) is referred to as cupric and copper(i) as cuprous. The system works well if there are only two common oxidation states for a metal ion, but if there are more, the scheme becomes either ambiguous or unwieldy as a variety of prefixes are added. 

It is quite possible for a metal centre to possess a zero or negative oxidation state. Thus, the species [Cr(C0)6] and [Fe(C0)4] are chromium(O) and iron(-2) complexes. We will see in a later chapter that it is not a coincidence that these low formal oxidation states are associated with ligands such as carbon monoxide. 

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. 

When dealing with the kinetic or thermodynamic behaviour of transition-metal systems, square brackets are used to denote concentrations of solution species. In the interests of simplicity, solvent molecules are frequently omitted (as are the square brackets around complex species). The reaction (1.1) is frequently written as equation (1.2). 

The oxidation state (oxidation number) of an element in a compound may have a strong influence on the hazards posed by the compound. For example, chromium from which each atom has lost three electrons to form a chemical compound, designated as chromium(III) or Cr(III), is not toxic, whereas Cr in the +6 oxidation state (CrO , chromate) is regarded as a cancer-causing chemical when inhaled. 

The difference between oxidation state and true charge can be appreciated by considering the wide range of formal oxidation states found in organometallic compoimds. The 

The Relationship Between Oxidation State and the Number of / Electrons 

The oxidation state of the metal center is directly related to the number of valence electrons in orbitals of predominantly metal character. Two simple rules allow one to determine quickly the number of electrons in the metal d-orbitals. 

Iron is another example where oxidation state plays a crucial role. Iron is nearly always in the +2 or +3 oxidation state. In reducing solution, iron(II) is present as a water soluble species which can be easily assimilated by living systems. Iron(II) has the capability of reacting with oxygen to form iron(III), therefore iron(II) has a high chemical oxygen demand (COD). COD is a parameter used to determine water quality. Iron(III) on the other hand, reacts rapidly with water at neutral pH to form highly insoluble iron(III) hydroxides and oxides (which can be removed by filtration). 

Elemental metals (i.e. metals in their zero oxidation state) have their own hazards. Mercury is an example. Elemental mercury has a significant vapour pressure at room temperature. The vapours are easily inhaled and are therefore highly toxic [12]. Contrastingly, most mercurous (Hg ) and mercuric (Hg ) compounds, whilst being highly toxic by ingestion have very low vapour pressures at room temperature and can be more easily handled and stored. 

Some remediation techniques rely on changing the oxidation state of metals in waste streams. Electrochemical techniques in particular rely on this method. 

Clearly, acidic aqueous solution solubilises iron(III) which can be easily assimilated by life forms. Whereas more alkaline conditions result in the 

It also follows that as the pH of a waste stream varies the soluble metal content will also vary. Some processes, especially batch processes, are prone to variation in the chemical content of their waste streams as a direct consequence of intermittent variations in pH. As a result, all metal removal methods must be coupled to effective and reliable pH measurement of the waste stream. 

It is often useful to refer to the oxidation state and d configuration, but they are a formal classification only and do not allow us to deduce the real partial charge present on the metal. It is therefore important not to read too much into oxidation states and d configurations. Organometallic complexes are not ionic, and so an Fe(II) complex, such as ferrocene, does not contain an Fe ion. Similarly, WHeLs, in. spite of being W(VI), is certainly closer to W(CO)5 in terms of the real charge on the metal than to WO3. In real terms, the hexahydride may even be more reduced and more electron rich than the W(0) carbonyl. CO groups are excellent n acceptors, so the metal in W(CO)g has a much lower electron density than a free W(0) atom on the other hand, the W-H bond in 

WH6L3 is only weakly polar, and so the polyhydride has a much higher electron density than the suggested by its W(VI) oxidation state (which assumes a dissection W H ). For this reason, the term formal oxidation state is often used for the value of OS as given by Eq. 2.7. 

There are many situations in which it is useful to refer to the oxidation state and d configuration, but they are a useful classification only and do not allow us to deduce the real partial charge present on the metal. It is therefore important not to read too much into oxidation states and d configurations. 

One very useful generalization is that the oxidation state of a complex can never be higher than the group number of the transition metal involved. 

Titanium can have no higher oxidation state than Ti(IV), for example. This is because Ti has only four valence electrons with which to form bonds and TiMCfi therefore cannot exist. 

Insofar as the catalj c potential of a metal complex is concerned, the formal charge on the metal atom and its ability to bond with the reactant(s) are important We first discuss a way of calculating the formal charge or oxidation state of the metal. We then discuss the ways of counting electrons in the valence shell of the metal. 

It must be remembered that electron counting is a bookkeeping exercise. In other words, the total number of valence electrons of the metal, plus that of the atoms of the ligand that interact with the metal, must remain constant before and after bond formation. 

Electron counting can therefore be done with or without assigning an oxidation state to the metal. As we will see, if done correctly, both methods, i.e., with or without assignment of an oxidation state, would give the same result. 

The oxidation state is assigned and justified on the basis of the relative electronegativities of the central metal atom and the coordinating atoms. The important point to note is that an ionic bonding model is assumed for the purpose of assigning the oxidation state and to that extent the bonding in the metal complex may not correspond to the real situation. 

In complexes where the metal is bonded only to stable molecules assigning the oxidation state is simple. A few typical examples of such organometallic complexes are Cr(CO)g, NiCPRj), CrfCgH j, and PfiCjH XPRj). In all these complexes toe ligands are stable neutral molecules without any charge. The metals must therefore be in the zero oxidation state. 

The compounds discussed in this section are restricted to nickel, as the existence of palladium (I) or platinum (I) compounds has not been established. Since nickel(I) contains nine d-electrons, analogies with copper(II) might be expected, but these do not arise there are, however, certain similarities with cobalt(O), particularly in the tendency to dimerization. 

The best-known of the nickel(I) complexes is the cyanide, K4[Ni2(CN)6], first isolated by Bellucci (19, 20), and later by Nast and Pfab (188). The salt is diamagnetic in solution and in the solid state (168, 244), and the 

In a preliminary X-ray investigation, Nast and Pfab (188) were able to say that the anion is flat and binuclear they suggested the structure shown in Fig. 1A. Later, however, the same authors (206) found that the structure was more complicated than they had previously imagined. The anions are stacked in layers with a certain degree of randomness. It is fairly certain that the anions have centers of symmetry, and the nickel-nickel distance 

Another member of this series is bis(cyclopentadienylnickel carbonyl), (CsHaNiCO it is dimeric, diamagnetic, and must, therefore, contain a nickel-nickel bond (79). The dipole moment, quoted (79) as 0 0.38 Debye unit, indicates that the molecule must be very nearly centro-sym-metric in benzene. The infrared spectrum, however, shows two carbonyl stretching frequencies in the solid state and in solution, but the vapor at 100°C shows only one band (173, 199). The wave-numbers are shown in Table IV. 

The observed frequencies are influenced by the solvent, and the splitting appears to increase with the polar nature of the solvent. It may be that the free molecule indeed has a center of symmetry but is slightly distorted 

Peroxide Value, The method for determination of peroxide concentration is based on the reduction of the hydroperoxide group with HI or Fe +. The result of the iodometric titration is expressed as the peroxide value. The Fe method is more suitable for detemuning a low hydroperoxide concentration since the amount of the resultant Fe +, in the form of the ferrithiocyanate (rhodanide) complex, is determined photometrically with high sensitivity (Fe-test in Table 14.27). The peroxide concentration reveals the extent of oxidative deterioration of the fat, nevertheless, no relationship exists between the peroxide value and aroma defects, e. g. rancidity (already existing or anticipated). This is because hydroperoxide degradation into odorants is influenced by so many factors (cf. 3.7.2.1.9) which mdkc its retention by fat or oil or its further conversion into volatiles unpredictable. 

Carbonyl Compounds. The analysis of the compounds responsible for the rancid aroma defect of a fat or oil is of great value. Volatile carbonyls (cf. 3.7.2.1.9) are among such compounds. 

In a simple test, such as benzidine, anisidine or heptanal values, the volatile aldehydes are not separated from fat or oil, rather the reaction with the group-specific reagents is carried out in the fat or oil. In addition to the odorous aldehydes, the flavorless oxo-acylglycerols and oxo-acids can be 

The thiobarbituric acid test (TBA) is a preferred method for detecting lipid peroxidation in biological systems. However, the reaction is nonspecific since a number of primary and secondary products of lipid peroxidation form malonaldehyde which in turn reacts in the TBA test. In food containing oleic and linoleic acids, the TBA-test is not as sensitive as the Fe +-test outlined above. 

The gas chromatographic determination of individual carbonyl compounds appears to be a method suitable for conparison with findings of sensory panel tests. Analytical methods for the odorants causing aroma defects is still in the early stages of development because only a few fats or fat-containing foods have been examined in such detail that the aroma substances involved are clearly identified. 

As shown, the Co 2p spectrum (a) exhibits two peaks, identified at BE 779.82 and 795.04 eV, which are associated to the doublet 2p3/2 and 2pi/2, respectively. The energy difference of the doublet is 15.22 eV, in agreement with the literature [6], which is assigned to the LaCoOs mixed oxide. 

The satellite peak around 790 eV assigned to the Co species was not detected at the surface, which indicates that this method was efficient to diffuse cobalt ions in the La203 stmcture for the formation of the LaCo03 perovskite [7]. 

The O Is spectmm showed on peak at 528.5 eV (Fig. 11.6c), revealing the existence of two contributions. The first one centered at 528.26 eV is assigned to the oxygen in lattice, and the other one at 529.96 eV is attributed to the adsorption of oxygen species as hydroxyls or carbonate. 

Element Component spin-OTbit Binding energy (eV) Area (CPS.eV) FWHM (eV) Surface composition (molar %) 

The surface composition of the LaCoOa perovskite is presented in Table 11.2. The Co/La ratio was 0.47, lower than the stoichiometric value (1.0) for the LaCoOa structure, probably due to the segregation of La ions at the surface and the formation of La(OH)3. 

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Metal derivatives of terminal alkynes, RC2H. Transition metals form complex acetylides (e.g. (M(C = CR) ]- ) often containing the metal in low oxidation states. 

Exists as the (Hg —Hg) ion. Other polymercury cations, e.g. Hgj (Hg plus AsFj), Hg4 etc., are also known. All positive oxidation state compounds of Hg are readily reduced to the metal, mercury chlorides... 

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