Unit 2.5 Oxidation Numbers

Naming a Coordination Compound. To name a coordination compound, the names of the ligands are attached directly in front of the name of the central atom. The ligands are listed in alphabetical order regardless of the number of each and with the name of a ligand treated as a unit. Thus diammine is listed under a and dimethylamine under d. The oxidation number of the central atom is stated last by either the oxidation number or charge number. 

When several oxidation numbers are found for the same element, they often differ from each other by jumps of one unit. For example, in the case of vanadium the common oxidation numbers form a continuous series from +2 to +3 to +4 to 4-5. Compare this with the halogens (Chapter 19). In the case of chlorine, for example, the common states are —1, +1, 4-3, 4-5, and 4-7 (jumps of two units instead of one unit). 

The equivalent of an oxidising agent is determined by the change in oxidation number which the reduced element experiences. It is the quantity of oxidant which involves a change of one unit in the oxidation number. Thus in the normal reduction of potassium permanganate in the presence of dilute sulphuric acid to an Mn(II) salt ... 

Here the change in oxidation number per atom of iron is from +2 to +3, or by 1 unit of oxidation, hence the equivalent of iron(II) sulphate is 1 mole. Another important reaction is the oxidation of oxalic acid to carbon dioxide and water ... 

The change in oxidation number of two atoms of carbon is from +6 to +8, or by 2 units of oxidation. The equivalent of oxalic add is therefore 2 mole. 

Ionic compounds are named by starting with the name of the cation (with its oxidation number if more than one charge is possible), followed by the name of the anion hydrates are named by adding the word hydrate, preceded by a Greek prefix indicating the number of water molecules in the formula unit. 

In dithiocarbamato complexes such an ambiguity can only occur when at least two dithiocarbamato ligands are bonded to a metal. In that case the question arises whether the compound is a bis (dithiocarbamato) or a thiuram disulfide complex. In these two types of complexes the oxidation number of the metal differs 2 units. 

The first systematic study of the effects of IT bands on the number of nuclei and the oxidation number was carried out for oligo(dihexylferro-cenylene)s, 132 13s (83,84). All the oxidation states of 132-136 were generated by a quantitative chemical oxidation method. Table III shows the dependence of umax, emax, and Av1j2 values of the IT band on the oxidation state and the number of ferrocene units. The Ai/j/2 values... 

To sum up, it is clear that havier elements tend to achieve an oxidation number which is by two units lower than the group number. On the other side, a straightforward explanation for this phenomenon is hard to find. 

But on increasing the oxidation number by one more unit, the enhancement in acidity is very great indeed, there being... 

In terms of electron transfer reactions, transition metal ions can be the one- or two-electron type. The two-electron ions transform into unstable states on unit change of the metal oxidation number. In the outer-sphere mechanism, two-electron transfer is a combination of two one-electron steps. 

Notice that the oxidation number per atom is multiplied by the number of atoms in the formula unit. 

Electrochemical Reactions. Consider a simple galvanic cell, composed of two metal electrodes, zinc and copper, immersed in two different aqueous solutions of unit activity—in this case, 1.0 M ZnS04 and 1.0 M CUSO4, respectively, connected by an electrical circuit, and separated by a semipermeable membrane (see Figure 3.8). The membrane allows passage of ions, but not bulk flow of the aqueous solutions from one side of the cell to the other. Electrons are liberated at the anode by the oxidation (increase in the oxidation number) of the zinc electrode ... 

Degree of oxidation (number of aldehyde groups contained in every 100 anhydroglucose units) Nitrogen content of product % Hygroscopicity (absorbed water) %... 

The Ni3S2 fragments in the two complexes (263) and (265) have a trigonal bipyramidal geometry with the two S atoms in the apical positions and the three nickel atoms in the equatorial plane, as found in other organometaUic compounds which contain the same Ni unit.1939 The formation of the enneanuclear complex (264), on the contrary, is exceptional and no other complex of this stoichiometry was isolated with analogous tertiary phosphines, with selenium, or with metals other than nickel. It is not possible to assign any definite oxidation number to nickel in complex (264). The lack of two electrons with respect to the situation of nine nickel(II) ions was reported to be essential for the existence of complex (264) because the oxidation is spontaneous and its reduction invariably leads to the decomposition of the cluster compound.19 ... 

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