Ligand field stabilization energies systems

It is possible to describe the site preference of metals for nitrogen or carbon coordination in a Prussian blue analog in terms of ligand field stabilization energies Shriver, Shriver and Anderson, 1965). In application, even this simple treatment requires a number of estimates nevertheless, it affords reasonable agreement with experimentally determined structures and provides a framework for the systemization of Prussian blue type structures. One expectation from this approach is that if one of the two metal ions has more than six d electrons and the other has six or less, the latter will be carbon coordinated and the former nitrogen coordinated. 

Fig. 20.26 Ligand field stabilization energies as a function of for high-spin octahedral systems and for tetrahedral systems Jahn-Teller effects for and configurations have been ignored.

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. 

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