Formal charge oxidation state

Fig. 7.1 A separation of metal ions and mettaloid ions, As(lll) and Sb(lll), into three categories class A, borderline, and class B ions. The class B index X r is plotted for each ion against the class A index Z /r. In these expression is the metal-ion electronegativity, r its ionic radius, and Z its formal charge. Oxidation states given by Roman numerals imply that simple, hydrated cations do not exist even in acidic solutions. (Reprinted from Nieboer and Richardson (1980), Copyright 2002, with permission from Elsevier Science).

Table 4 presents the calculated results of the effective charges on technetium atoms in technetium compounds, arrived at by using various theoretical approximations. In technetium compounds with M-M bonds and formal technetium oxidation states 2.0 + and lower, Zeff is less than 1.0 +, whatever the... 

The complexes [Mo(bipy)3] can be prepared by reduction of [Mo(bipy)3]3+ with Li2bipy37 or of [MoCl4(MeCN)2] with sodium in presence of bipy.38 Formally, the oxidation state is zero, but there must be considerable distribution of electronic charge in orbitals which are ligand-based. 

The chromium is formally in oxidation state +2 with an electronic ground state term of JE2g (27). In this form the molecule has a scarlet hue and has a characteristic electronic spectrum showing UV-visible ligand-field and UV charge-transfer excitations and a D5d centrosymmetric sandwich structure as shown in Scheme I. 

The formal potential, E0/, contains useful information about the ease of oxidation of the redox centers within the supramolecular assembly. For example, a shift in E0/ towards more positive potentials upon surface confinement indicates that oxidation is thermodynamically more difficult, thus suggesting a lower electron density on the redox center. Typically, for redox centers located close to the film/solution interface, e.g. on the external surface of a monolayer, the E0 is within 100 mV of that found for the same molecule in solution. This observation is consistent with the local solvation and dielectric constant being similar to that found for the reactant freely diffusing in solution. The formal potential can shift markedly as the redox center is incorporated within a thicker layer. For example, E0/ shifts in a positive potential direction when buried within the hydrocarbon domain of a alkane thiol self-assembled monolayer (SAM). The direction of the shift is consistent with destabilization of the more highly charged oxidation state. 

The right lower position of an atomic symbol is reserved for an index (subscript) indicating the number of such atoms in a formula. For example, Sg is the formula of a molecule containing eight sulfur atoms (see Section IR-3.4). For formalisms when oxidation states or charges are also shown, see Section IR-4.6.1. 

With the Ru dimers, [ Ru(bpy)2 2(M-L)], ESR data indicate that these binuclear semiquinone complexes are near the borderline between anion radical complexes and metal-centered mixed-valence species/ The thermal reaction between [W(CO)6] and tetrachloro-l,2-benzoquinone in toluene produces the tris(quinone) tungsten complex/ Features of the molecule indicate that the quinones are coordinated as catecholate ligands and the tungsten ions are in a formal +6 oxidation state. Quinones trapped within a polymeric electrode film can act as electron sinks and/or sources. IJ/I and [FeCCN) ] " " have been utilized as charge-release mediators. 

In common with Enemark—Feltham, the new notation makes no attempt to define the formal charges on the nitrosyl ligand and the formal metal oxidation state but focuses attention on the geometry of the nitrosyl, the metal s coordination number and the total electron count. As De La Cruz and Sheppard have recently pointed out [26] in their extensive analysis of the vihratiOTial data for nitrosyl complexes, the great majority of them conform to 18- and 16-electron rules, and therefore, this parameter establishes whether the molecirle has a closed shell. The total electron count has important chemical implications since it indicates whether the compound is likely to undergo electrochemical conversion or nucleophilic addition in order to achieve an 18-electron configuration. 

It is worth stressing here that the mere fact that a series of complexes in a formally mixed oxidation state shows a band at low energy characteristic of the mixed valencies does not prove localization in the ground state. As the resonance energy increases, the electronic transition does not disappear but merely loses its charge-transfer character, as shown in the sequence (i)-(ii)-(iii) in Figure 3. 

Where it is not feasible to define an oxidation state for each individual member of a group, the overall oxidation level of the group is defined by a formal ionic charge to avoid the use of fractional oxidation states for example, Oy. 

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