Charge transfer transitions cobalt complexes

The electronic structure, in particular the electronic spectroscopic properties, of the whole class of cobalt amine complexes may be reduced to a discussion of the central Co-Ne core. This disregards, of course, the charge-transfer transition that in these compexes typically occurs around 250 nm. The geometrical structure is either octahedral or is defined in terms of a subgroup of the cubic point group Oh, where the... 

The solid-state spectrum in dry lithium fluoride is similar to the solution spectrum, indicating that no structural transformation has occurred in solution. However, weak shoulders or bands around 14,500 and 7700 cm" (Fig. 2) are observed. The absorption maxima around 27,000 cm is attributed to charge transfer transition, as it appears in all complexes of both cobalt(II) and nickel(II) with this ligand. 

Photoreduction of cobalt(III) complexes can occur under a variety of conditions. Irradition of the charge transfer bands of these systems results only in decomposition with production of cobaltous ion and oxidation of one of the ligands. In some instances photoreduction can be initiated by irradiation of the ligand field transitions. Irradiation of ion pairs formed by these complexes with iodide ion with ultraviolet light also leads to reduction of the complexes. Finally, irradiation of iodide ion in the presence of the complexes leads to reduction. 

Robert Plane I would like to comment on the photochemistry of these complexes, particularly the chromium which I think are well chosen for at least two of Dr. Gray s reasons. First, in the case of chromium, unlike cobalt, the charge transfer band is well separated, so that one can study the d-d transitions, at least in certain systems where one has six ligands all alike, and there is no Jahn-Teller splitting, and one has a fairly good idea as to the assignment of bands. 

Many reducing cations in aqueous solutions can photoreduce water in low yields when irradiated in their charge transfer bands. Similarly, oxidizing cations can photo-oxidize water. Several hydride complexes of transition metals such as cobalt decompose on irradiation to yield molecular hydrogen... 

A chemistry of cobalt-sulfide-thiolate molecular clusters comparable with that of iron systems has also begun to emerge. Treatment of [Co4( -SPh)6(SPh)4]2- with HS- in acetone affords the octanuclear cluster [Co8(ji4-S)6(SPh)8]4 isolated as its Pr4N+ salt.988 In MeCN solution the complex is red-purple with intense sulfur-core charge transfer bands which obscure the Co" d-d transitions. This behaviour contrasts with that of both mono- and poly-nuclear cobalt"-thiolate complexes, which all display LMCT bands below 440 nm and have well-developed v2 and v3 features. The [Co8(/j4-S6)]4+ core sustains reversible one-electron oxidation and reduction (E]l2 = —0.54, — 1.18 V, MeCN) and chemical reduction with sodium acenaphthylenide in THF gives [Co8(/r4-... 

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