Chromium ligand exchange

A metal-nucleotide complex that exhibits low rates of ligand exchange as a result of substituting higher oxidation state metal ions with ionic radii nearly equal to the naturally bound metal ion. Such compounds can be prepared with chromium(III), cobalt(III), and rhodi-um(III) in place of magnesium or calcium ion. Because these exchange-inert complexes can be resolved into their various optically active isomers, they have proven to be powerful mechanistic probes, particularly for kinases, NTPases, and nucleotidyl transferases. In the case of Cr(III) coordination complexes with the two phosphates of ATP or ADP, the second phosphate becomes chiral, and the screw sense must be specified to describe the three-dimensional configuration of atoms. 

R. Brambley, J. Y. Li, and P. A. Lay, Ligand-exchange reactions of chromium(V) Characterization of the ligand-exchange equilibria of bis(2-ethy 1-2-structure of hydroxybutanoato(2-))oxochromate(V) in aqueous 1,2-ethanediol and solution bis(l,2-ethanediolato(2-))oxochromate(V), Inorg. Chem., 30 (1991) 1557-1564. 

M. Rizzotto, A. Levina, M. Santoro, S. Garcia, M. Frascaroli, S. Signorella, L. F. Sala, and P. A. Lay, Redox and ligand-exchange chemistry of chromium(VI/V)-methyl glycoside systems, J. Chem. Soc. Dalton Trans. (2002) 3206-3213. 

Quantum yield data for photochemically initiated ligand exchange reactions of a number of chromium(III) complexes having ligand fields of effective Oh symmetry are given in Table III. These data show that... 

Much less is known about photoinitiated ligand exchange reactions of coordination complexes of cobalt(III) and other de ions than is the case for chromium(III). With the exception of the cobalt(III) ammines, however, available data suggest that photochemical ligand exchange reactions of cobalt(III) and d6 systems involve the thermally equilibrated 17 g and/or 37, states of the complex as intermediates. The reasoning is completely analogous to that described earlier with respect to chromium(III) photochemistry. 

In this review our focus will be largely on thermal ligand exchange and isomerizations. Other reactions, including photochemical ones, will be dealt with only briefly. Most examples will come from studies of cobalt(III) and chromium(III) compounds merely because the chemistry of these metals is more fully developed. 

As discussed above, the ligands that have been typically utilized for the preparation of chromium nitrides are multidentate. Consequently, ligand exchange reactions of such complexes are difficult and rare. Wieghardt and co-workers have reported such a process, however, for the synthesis of a nitrido chromium cyanide complex 43 (Eq. (13)) [18]. Thus, treatment of CrN(salen) 42 with excess sodium cyanide and tetramethyl ammonium chloride results in the formation of a six-coordinate penta-cyano chromium nitride [21]. 

Ligand exchange reactions of monodentate amides, iodides, or alkoxide complexes with other amides, alkoxides as well as sulfides, and carbon ligands has been documented [23,25]. A rather unique reaction of a trimethylsilylmethyl ligand bound to a chromium nitride has been reported thus, reaction of tert-butyl isocyanide with 52 is reported to furnish 53 (Eq. (17)). 

Richard followed the course of the reactions of Cr(CO)6 and Mo(CO)6 with hexamethylborazine by UV-vis spectroscopy but although he observed that the intensity of the absorption maximum of the hexacarbonyls at around 290 nm decreased and a new band at around 350 nm appeared, he was unable to identify the new product. Experiments with other starting materials such as norborna-diene chromium and molybdenum tetracarbonyl or tris(aniline) molybdenum tricarbonyl which readily react with arenes by ligand exchange, also failed. The key to success was to use tris(acetonitrile) chromium tricarbonyl as the precursor, which in dioxan under reduced pressure afforded the desired hexamethylborazine chromium tricarbonyl as a stable crystalline solid in 90% yield. This was the breakthrough and, after we had communicated the synthesis and spectroscopic data of the complex in the January issue 1967 of Angewandte Chemie, Richard finished his work and defended his Ph.D. thesis in June 1967. Six months before, in December 1966,1 defended my Habilitation thesis in front of the faculty and became Privatdozent (lecturer) on the 1st of January 1967. 

The potassium reineckate actinometer developed by Wegner and Adamson (26), in 1966, represent the first actinometer for long-wavelength visible radiation. It is based on a photoinduced ligand exchange in a chromium (111) complex, i.e.. 

Soluble chromium catalysts are obtained from Cr(acac)3 with AlEt3 [55], AlEt2Cl and AlEtCl2 [139]. As with other catalysts ligand exchange takes place and from Cr(acac)3/AlEt3 the complex VIII... 

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