Cobalt complexes ligand field states

It will not have escaped the reader s attention that the kinetically inert complexes are those of (chromium(iii)) or low-spin d (cobalt(iii), rhodium(iii) or iridium(iii)). Attempts to rationalize this have been made in terms of ligand-field effects, as we now discuss. Note, however, that remarkably little is known about the nature of the transition state for most substitution reactions. Fortunately, the outcome of the approach we summarize is unchanged whether the mechanism is associative or dissociative. 

Term labels appropriate for d3 and d6 configurations in octahedral ligand fields will be used in this review to designate the ligand field or d-d excited states of cobalt(III) and chromium(III) complexes. This... 

The trends toward decreased stability of the very high oxidation states and the increased stability of the II state relative to the III state, which have been noted through the series Ti, V, Cr, Mn, and Fe, persist with cobalt. The oxidation states IV and V are represented by only a few compounds. The III state is relatively unstable in simple compounds, but the low-spin complexes are exceedingly numerous and stable, especially where the donor atoms (usually N) make strong contributions to the ligand field. There are also some important complexes of Co1 this... 

State. When both iron environments contain only iron(II), the resulting salt is not colored (Prussian White). The oxidation state localization in PB has been studied extensively. Structures, electrochemical behavior (electrodes batteries), and uses in medicine (treatment of Cs and of thallium poisoning) of Prussian Blue are mentioned in a review of cyanide complexes. In cobalt-iron Prussian Blue analogues, NaxCo3,Fe(CN)6-zH20 electronic and spin states are controlled by temperature and the ligand field strength around the Co + ions, which in turn is determined by the Co Fe ratio. ... 

The electronic absorption spectrum of the complex shows an intense band at 295 nm (e = 18,000), but ligand field (LF) bands were not observed at lower energy. The intense band was assigned as a benzyl-to-cobalt CT transition, and it was proposed that the photoreaction proceeds through population of such a CT state leading to homolytic cleavage of the cobalt-carixtn bond. 

---