Chromium complexes ligand exchange reactions

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. 

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)). 

Similar reactions proceed in the presence of molybdenum and tungsten complexes as catalysts. These catalysts are more reactive than chromium catalysts but the reaction selectivities are low. The induction period is usually one hour. The induction period is thought to be caused by the ligand exchange reaction of an aryl group and diene with the obvious formation of an intermediate as shown in eq. 

The corresponding [3 + 2 + 2]- and [2 + 2 + 2 + l]-cyclization reactions between these complexes and alkynes are very efficient in the presence of Ni(0). Transmetallation reactions between chromium Fischer carbene complexes and Ni(cod)2 with formation of nickel carbene intermediates is presumed to be responsible for this transformation. The metal-free cycloadducts are easily obtained by ligand exchange reactions at low GO pressures. A [3-h2]-cyclization reaction of a chromium Fischer carbene complex and allene building blocks was accomplished, using... 

Chromium, (ri6-benzene)tricarbonyl-stereochemistry nomenclature, 1,131 Chromium complexes, 3,699-948 acetylacetone complex formation, 2,386 exchange reactions, 2,380 amidines, 2,276 bridging ligands, 2,198 chelating ligands, 2,203 anionic oxo halides, 3,944 applications, 6,1014 azo dyes, 6,41 biological effects, 3,947 carbamic acid, 2,450 paddlewheel structure, 2, 451 carboxylic acids, 2,438 trinuclear, 2, 441 carcinogenicity, 3, 947 corroles, 2, 874 crystal structures, 3, 702 cyanides, 3, 703 1,4-diaza-1,3-butadiene, 2,209 1,3-diketones... 

Chromium complexes acetylacetone complex formation, 386 exchange reactions, 380 amidines, 276 bridging ligands, 198 chelating ligands, 203 carbamic add, 450 paddlewheel structure, 451 carboxylic adds, 438 trinuclear, 441 oorroles, 874... 

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. 

Chromium(III) and cobalt(III) complexes are substitu-tionally inert (no exchange of ligands) under conditions of the experiment. However, chromium(II) and cobalt(II) complexes can exchange ligands very rapidly. One of the products of the reaction is Cr(H20)5CI2+. Is this consistent with the reaction proceeding through formation of... 

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