Cobalt oxide coordination number

A simplified reaction scheme is shown in Fig. 26.5 Again, the ability of rhodium to change its coordination number and oxidation state is crucial, and this catalyst has the great advantage over the conventional cobalt carbonyl catalyst that it operates efficiently at much lower temperatures and pressures and produces straight-chain as opposed to branched-chain products. 

X-ray absorption near edge structure (XANES) is useful in determining the coordination number and the oxidation state of metal ions (Sankar et al, 1983). In Figs. 2.16 and 2.17 we show the XANES of Co and Cu in some compounds as well as catalysts. The ls-3[Pg.99]

It is a type of oxidative addition reaction. In these reactions a low valent transition metal complex reacts with a molecule to give a product in which both the oxidation number and coordination number of the metal are increased. In the given reaction H has entered as the hydride ion and hence the oxidation number of cobalt has increased from 0 to + 1. 

Although the vast majority of cobalt(III) complexes are six-coordinate, several groups of five and even four-coordinate complexes are known in which the cobalt may be considered as having the formal oxidation state(III). These complexes represent the extreme case where the thermodynamic effects are so great that no ligand, even H2O, can form a stable bond to the cobalt, i.e., there is a decrease in coordination number. 

In the high-concentration cobalt systems, cobalt ions are reported to be present in several configurations, with the Co" dimer being the most active [71]. Some non-varivalent metal salts, such as ZrO(OAc)2, are reported to promote cobalt catalyzed oxidations. This may be the result of the influence of the promoter on coordination number and monomer-dimer equilibria [72, 73]. 

From this conclusion Werner postulated perhaps the most important part of his theory that in this series of compounds cobalt exhibits a constant coordination number of 6, and as ammonia molecules are removed, they are replaced by chloride ions which then act as though they are covalently bound to the cobalt rather than as free chloride ions. To de.scribe the complex chemistry of cobalt, one must therefore consider not only the oxidation state of the metal but also its coordination number. 

Nineteenth-century chemists were puzzled by a certain class of reactions that seemed to violate valence theory. For example, the valences of the elements in cobalt(III) chloride and in ammonia seem to be completely satisfied, and yet these two substances react to form a stable compound having the formula C0CI3 6NH3. To explain this behavior, Werner postulated that most elements exhibit two types of valence primary valence and secondary valence. In modem terminology, primary valence corresponds to the oxidation number and secondary valence to the coordination number of the element. In C0CI3 6NH3, according to Wemer, cobalt has a primary valence of 3 and a secondary valence of 6. 

The extent of reduction from EXAFS was obtained by looking at the fractional coordination munber of the nearest Co-0 coordination shell. The bulk cobalt oxides have Co-0 coordination of 6 (Aox). The Co-0 coordination number for the reduced catalysts will be lower, depending on the extent of reduction. For a completely reduced catalyst, will be zero. The degree of reduction is thus given by the expression 1 -... 

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