Oxidation states Arbitrary

Oxidation states Arbitrary numbers that can be used as mechanical aids in writing formulas and balancing equations for single-atom ions they correspond to the charge on the ion less metalUc atoms are assigned negative oxidation states in compounds and polyatomic ions. 

The concept of the oxidation state of an element in a chemical compound has a long and confusing history. In the most pretentious form of the concept the oxidation state or oxidation number is the electrical charge localised on the concerned atom in the compound. Confusion arises when we realise that the definition of the atomic domain is arbitrary and the experimental determination of the electrical charge in the once chosen domain is often problematic. 

The compounds in question are classified by oxidation state of the central metal atom. Assignment of oxidation state is normally straightforward when the formula is known, but there are cases where the choice is somewhat arbitrary. Thus bis(cyclopentadienyl)nickel, (CcH Ni, may be considered as the nickel (II) cation complexed by two cyclopentadienyl anions, or as a combination of an uncharged nickel atom with two cyclopentadienyl... 

The mechanistic details of cycle 18.10 have been represented in a somewhat arbitrary fashion, but the essence of the mode of action of transition metal complexes (in particular, complexes of the Group 9 elements Co, Rh, and Ir in the I oxidation state) as homogeneous catalysts for hydrogenation reactions should be clear. 

The analogous selenium and tellurium chemistry has not been explored. The [Tl2Te2]2-anion has been obtained by reacting KTITe with [2.2.2]cryptate in ethylenediamine to give [K([2.2.2]crypt)]2[Tl2Te2]2- en, in which the anion is butterfly-shaped with a 50° fold.328 The assignment of oxidation numbers is somewhat arbitrary in such systems (cf. Section 25.2.6.1.2), but it seems clear that thallium is in a low oxidation state. 

In certain cases the nature of the ligands involved makes definition of the metal oxidation state difficult, e.g. NO, N2R, dithiolenes, etc. For simplification, therefore, arbitrary assignment of ligand charge and therefore metal oxidation state has been made in the text, e.g. NO is... 

The largest group of molybdenum(II) complexes is constituted by the nitrosyls (oxidation states being assigned on the somewhat arbitrary assumption that nitrogen monoxide, as a ligand, is NO+). Tables 6 and 7 give a selection of the compounds discussed. 

The oxidation number, or oxidation state, is a bookkeeping device used to keep track of the number of electrons formally associated with a particular element. The oxidation number is meant to tell how many electrons have been lost or gained by a neutral atom when it forms a compound. Because oxidation numbers have no real physical meaning, they are somewhat arbitrary, and not all chemists will assign the same oxidation number to a given element in an unusual compound. However, there are some ground rules that provide a useful start. 

Redox reactions are better defined in terms of the concept of electron transfer. Thus an atom is said to be oxidized if, as the result of a reaction, it experiences a net loss of electrons and is reduced if it experiences a net gain of electrons. This simple definition can be used to identify oxidation or reduction processes at carbon in terms of a scale of oxidation states for carbon based on the electronegativities of the atoms attached to carbon. The idea is to find out whether in a given reaction carbon becomes more, or less, electron-rich. We will use the following somewhat arbitrary rules ... 

We recommend this scheme of oxidation states only as an aid to identify and balance redox reactions. Also, the terminology redox should not be confused with the mechanism of a reaction, as there is no connection between them. A moment s reflection also will show that virtually all reactions theoretically can be regarded as redox reactions, because in almost every reaction the reacting atoms experience some change in their electronic environments. Traditionally, however, reactions are described as redox reactions of carbon only when there is a net change in the oxidation state of the carbon atoms involved. An indication of just how arbitrary this is can be seen by the example... 

Numerous transient paramagnetic compounds are known and some of these are also shown in Table IV there is an overlap with Section V, and Table IV does not duplicate material which is more conveniently treated later. The distinction is arbitrary, but we shall defer consideration of transient transition metal-centered radicals, e.g., Pt(I), if their formation is primarily of interest in connection with an organometallic mechanistic study, e.g., the oxidative addition of an alkyl halide to a Pt(0) substrate. The designation of metal oxidation state in Table IV is somewhat formal in many cases it might be more appropriate to describe a complex as derived from a paramagnetic ligand, such as a nitroxide or ketyl. 

Most of the knowledge about aluminate and alkylaluminum coordination stems from X-ray crystallographic studies. The basic idea of this section is to compile a rare-earth metal aluminate library categorizing this meanwhile comprehensive class of heterobimetallic compounds. Main classification criteria are the type of homo- and heterobridging aluminate ligand (tetra-, tri-, di-, and mono alkylaluminum complexes), the type of co-ligand (cyclopen-tadienyl, carboxylate, alkoxide, siloxide, amide), and the Ln center oxidation state. In addition, related Ln/Al heterobimetallic alkoxide complexes ( non-alkylaluminum complexes) are surveyed. Emphasis is not put on wordy structure discussions but on the different coordination modes (charts) and important structural parameters in tabular form. An arbitrary collection of molecular structure drawings complements this structural report. 

In order to keep track of electron shifts in oxidation-reduction reactions, it is convenient to use the concept of oxidation number or oxidation state of various atoms involved in oxidation-reduction reactions. The oxidation number is defined as the formal charge which an atom appears to have when electrons are counted in accordance with the following rather arbitrary rules. 

Fig. 2.9. Construction of an oxidation state diagram. The zero oxidation state has a volt-equivalent of zero (arbitrary). The slopes of the lines have values corresponding to...

Examples of metal complexes containing linear carbon ligands have been characterized for metals from across the transition series (Tables IX, X, and XI) and the structural forms A-D can generally be differentiated on the basis of an examination of the structural parameters. However, the variable precision of the structure determinations, the different size of the various metals and variations in the electrostatic contribution to the M-C bond with metal oxidation state and the nature of the other supporting ligands (e.g., phosphine vs. carbonyl) make detailed comparisons of the molecular parameters within a structural subset somewhat arbitrary. 

In Chapter 5, we learned to write formulas for ionic compounds from the charges on the ions and to recognize the ions from the formulas of the compounds. For example, we know that aluminum chloride is AICI3 and that VCI2 contains ions. We cannot make comparable deductions for covalent compounds because they have no ions there are no charges to balance. To make similar predictions for species with covalent bonds, we need to use the concept of oxidation number, also called oxidation state. A system with some arbitrary rules allows us to predict formulas for covalent compounds from the positions of the elements in the periodic table and also to balance equations for complicated oxidation-reduction reactions. 

The oxidation state of the Ge in the products from such Ge(II) reactions is not always clear and, indeed, may be a matter of arbitrary definition. Three cases may be distinguished ... 

Table 6.4 shows some variation in the actual binding energies, which can be partly related to different materials and partly to the lack of universality applied to aligning the Eg scale. The reference point is typically the C Is line at somewhat arbitrary energies (284.4—285 eV) and some publications do not indicate their referencing procedure at all. However, there are clearly some obvious tendencies to observe in Table 6.4. For example, most materials contain vanadium in mainly the 4-e formal oxidation state molybdenum is usually fitted to obtain two (6-1- and 5-i-) components, that is, its peak maximum is observed between the position of these two oxidation states the Ep values of niobium are somewhat lower, and those of...

Looking at this list it is obvious that the selection of a borderhne between selenium compounds which maybe considered as the derivatives having lower oxidation state and their higher oxidation state analogues is to some extent a matter of formality and, as such, may always be considered as an arbitrary choice. After analysis of a prehminary outline of the chapters in this volume and taking into account the fact that pentavalent, tetracoordinated compounds have not yet been reported and that the first organic member of the selenurane oxide family has just been isolated [4] as a stable chemical species, we have decided to discuss here the recent synthetic apphcation of selenium compounds from the classes c-f, h and j. 

---