Cobalt ligand field stabilization energy

Cobalt. Cobalt-substituted aluminophosphates have been the most extensively investigated MeAPO-n systems. In comparison to the first row transition metals, which usually prefer octahedral coordination geometry, the Co + ions rather easily adopt tetrahedral stereochemistry with the oxide ions as the ligands. This can be explained by the fact that the ligand field stabilization energies for Co + ions (d ) disfavor the tetrahedral coordination relative to octahedral one to a lesser extent than for most of the other d configurations. The characteristic royal blue color of the obtained CoAPO products is a good indication that the cobalt incorporation into the aluminophosphate framework has been achieved. 

In aqueous solution, the complexes of most metal cations exist in dynamic equilibrium with their components. If we disturb this equilibrium, another one is instantly formed. It is quite otherwise with robust complexes which persist for hours (or even days) under conditions favourable to their decomposition any biological properties that they may have are strikingly different from those of their components. Robust complexes are formed where metal ions have 3,4 (low spin), 5, or 6 d electrons provided that formation of the complex involves large values of ligand-field stabilization energy. Metals most prone to form robust complexes are the transition metals platinum, iridium, osmium, palladium, rhodium, ruthenium, also (but not so frequently) nickel, cobalt, and iron. The halide and, particularly, the cyanide anions most readily form robust complexes with these transi-... 

The crystal held model can also be used to account For the stability of particular oxidation states. In aqueous solution Co(lll) is unstable with respect to reduction by water to form Co(Il)- Although there are several energy terms involved, this may be viewed as a reflection of the high third ionization energy of cobalt. If various moderate-to-strong field ligands are present in the solution, however. theCo(IIl)ion is perfectly stable. In fact, m some cases it is difficult or impossible to prevent the oxidation of Co(ll) to Coflll). 

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