Covalent species, oxidation numbers

Classical complexes are identified [1112] as those species in which the central metal ion possesses a well-defined oxidation number and a set of ligands with a discrete electron population. Non-classical complexes , in contrast, involve highly covalent and/or multiple metal-ligand bonding resulting in indistinct oxidation numbers for both participants. 

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. 

Examples MnO " with Mn +7 and Cr04 with Cr +6 Species containing rf-block elements with high oxidation numbers tend to exhibit covalent bonding. 

Dollimore [5] has discussed some aspects of the influence of the central atom on the thermal stabilities of solid coordination compounds. The most fully characterized compounds are those of Co, Cr and Pt. Central atoms of relatively small radius but high oxidation number coordinate most effectively. The possible influences of melting and dependences upon reaction conditions increase the difficulties of identification of the factors which control reactivity. Stabilities are influenced [5] by the electronic structure of the coordinated ion, whether this is a normal or a penetration compound. In the latter species, covalent bonding involves 3dHs4p orbitals, whereas in a normal coordination compound the 4s4pMd orbitals... 

Oxidation State The system of oxidation states (or oxidation numbers) has been devised to give a guide to the extent of oxidation or reduction in a species the system is without direct chemical foundations, but is extremely useful being appropriate to hope ionic and covalently bonded species. 

The N2 molecule contains a triple bond and is very stable with respect to dissociation into atomic species. However, nitrogen forms a large number of compounds with hydrogen and oxygen in which the oxidation number of nitrogen varies from -3 to +5 (Table 21.2). Most nitrogen compounds are covalent however, when heated with cer-... 

It seems unlikely that either of these extreme ionic descriptions is physically realistic. Probably, in each case there is an essentially covalent M—N a bond which may be more or less polarized, one way or the other depending on the exact nature of the metal and its attached ligands. Probably the NO-description comes close to the truth in some cases where it also gives intuitively reasonable formal oxidation numbers. In [Coen2Cl(NO)]+ and [Co(NH3)5NO]2 +, for example, if NO is treated as a coordinated NO- ion, the presence of Co(in) is then implied and this seems quite consistent with the large body of cobalt(m) ammine chemistry. The idea that these species contain Co(i) donating to NO+ appears a little bizarre. 

Oxidation numbers are useful in identifying oxidation-reduction reactions involving covalent species. The terms oxidation and reduction have been defined in terms of transfer of electrons (page 121). However, in reactions of covalent substances the concept of oxidation-reduction is less clear. For example, does oxidation-reduction occur in the following reaction (oxidation numbers are shown) ... 

Formal charge and oxidation number. For the following species (a) locate all formal charges that are not zero, (b) determine the oxidation number of each atom, (c) compare the covalency of each atom having a formal charge with the normal value. Make a useful generalization based on your comparison. 

Although use of radio and stable isotope labels involving the trio of covalently-bonded nitrogenous functions in 3 and in 78, provided evidence that isocyano is the precursor of the isothiocyano and formamido groups [30, 81], it remains to be shown that a biosynthetic equivalent of the in vitro chemically-proven fusion process between isocyano and free sulfur (e.g., cf. Introduction) exists in the cells of sponges. In marine biota, various ionic forms of sulfur in a number of oxidation states, as well as organo-polysulfides are known. However, any association with the isonitrile group and a sulfated species has yet to be established. 

---