Central atom, oxidation state

This Section deals with the photoinduced reactions in which the central atom oxidation state is preserved and just a tetrapyrrole ligand undergoes a... 

Formula Ligands Central atom oxidation state... 

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. 

The CT inner-sphere, photoinduced redox reaction results in oxidation (or reduction) of the central atom at the expense of the respective reduction (or oxidation) of ligand. The direction of the redox process depends on the CT state character (see Figure 3.1 and section 3.2 in Chapter 3) the central atom reduction is a consequence of the LMCT transition, whereas central atom oxidation is a result of the MLCT transition (Figure 6.7). The photochemical oxidation of one ligand at the expense of another (LLCT) can also be rated among the inner-sphere redox processes (see above). 

The oxidation number is used in coordination compounds and represents the charge an atom would have if all ligands and electron pairs were removed from the central atom. Oxidation numbers are usually identical to oxidation states, and the terms are often used interchangeably. The key difference is that a true oxidation number is represented by a Roman numeral. For example, iron with an oxidation number of +2 could be written as iron(II) or Fe". 

The concept of oxidation states is best applied only to germanium, tin and lead, for the chemistry of carbon and silicon is almost wholly defined in terms of covalency with the carbon and silicon atoms sharing all their four outer quantum level electrons. These are often tetrahedrally arranged around the central atom. There are compounds of carbon in which the valency appears to be less than... 

An important reason for low coordination of iodide ions is that high coordination implies a high oxidation state of the central atom, which often (but not always) means high oxidising power— and this means oxidation of the easily oxidised iodide ligands. Thus the nonexistence of, for example, phosphorus(V) pentaiodide is to be explained by the oxidation of the iodide ligands and reduction of phosphorus to the -(-3 state, giving only PI3, not PI5. 

However, stable complexes where the oxidation state-i6f the central metal atom is 0 are only formed with a very few ligands, notably... 

Oxidation can also occur at the central metal atom of the phthalocyanine system (2). Mn phthalocyanine, for example, can be produced ia these different oxidation states, depending on the solvent (2,31,32). The carbon atom of the ring system and the central metal atom can be reduced (33), some reversibly, eg, ia vattiag (34—41). Phthalocyanine compounds exhibit favorable catalytic properties which makes them interesting for appHcations ia dehydrogenation, oxidation, electrocatalysis, gas-phase reactions, and fuel cells (qv) (1,2,42—49). 

Chelates are often named merely as a complex, eg, cadmium complex with acetylacetone. A common practice ia the Hterature is to give the symbol of the central atom and an abbreviation for the ligand with or without an iadication of ionic charges, oxidation states, stmcture, or counterions, as ia the foUowiag Pb-EDTA, Cacit , Cu(en)2, Co(II)-(phen), [Cu(dipy)2]S04, [Ru(dipy)2(en)], and Na[Co(acac)2]. Ligand abbreviations are given ia Table 1. 

The specific action of a particular metal complex can be altered by varying the hgands or coordination number of the complex or the oxidation state of the central metal atom. 

Almost every metal atom can be inserted into the center of the phthalocyanine ring. Although the chemistry of the central metal atom is sometimes influenced in an extended way by the phthalocyanine macrocycle (for example the preferred oxidation state of ruthenium is changed from + III to + II going from metal-free to ruthenium phthalocyanine) it is obvious that the chemistry of the coordinated metal of metal phthalocyanines cannot be generalized. The reactions of the central metal atom depend very much on the properties of the metal. 

Fig. 5. Dependence of the spin-spin coupling-constant and the F chemical shift on the oxidation state of the central xenon atom.

Now let s compare the oxidation states of the central carbon atom in each of the following compounds ... 

According to the preceding statements certain coordination polyhedra occur preferentially for compounds of transition metals, depending on the central atom, the oxidation state, and the kind of ligand. The general tendencies can be summarized as follows ... 

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