Cyanide labilizing effect

The fundamental examples of this class of complexes, for example, cyanides, nitrates, thiocyanate, and the isocyanato derivatives are well known, and with the exception of the interesting, strong labilizing effect of the latter, which leads to fast decarbonylation to [Re2(/r-NCO)2(CO)8], they do not have a special chemistry and are sometimes difficult to prepare. In any case, these difficulties have been removed by the use of Re(CO)5FBF3, which readily reacts with these types of anionic ligands to form the corresponding Re complexes. A few structural analogs of (14) and (15), Re(ER2)(CO)5,... 

The trans effect illustrates the importance of studying the mechanisms of complex substitution reactions. Before continuing with a discussion of mechanisms, the distinction between the thermodynamic terms stable and unstable and the kinetic terms labile and inert should be clarified. Consider the following cyano complexes [Ni(CN)4]2-, [Mn(CN)6]3-, and [Cr(CN)6]3-. All of these complexes are extremely stable from a thermodynamic point of view is yet kinetically they are quite different. If the rate of exchange of radiocarbon labeled cyanide is measured, we find that despite the thermodynamic stability, one of these complexes exchanges cyanide ligands very rapidly (is labile), a second is moderately labile, and only [Cr(CN)6]3 can be considered to be inert ... 

The cyanide complex Pt(CN)4 has a stability constant of in aqueous solution, but the CN exchange rate is too fast to measure by radioisotope methods. Using CNMR methods the exchange rate has been found to follow the rate law rate = k2[Pt(CN)4 ][CN ], and rate constants measured in the 26M s range. The experimental values for AH and AS tire 17 2kJmol and -178 7JK" mol It can be concluded that CN as a ligand for platinum(II) shows a high trans effect and forms thermodynamically stable yet kinetic y labile complexes. ... 

Owing to the desirability of converting the more labile forms of vitamin Bi2 to cyanocobalamin by cyanide treatment, it has been common to extract Bi2 from crude materials with KCN solutions. Skeggs, Driscoll, Chamey, and Wright (39) found the most effective treatment with cyanide to be extraction with 0.01% KCN (10 mg. of KCN for 10 y.g. of vitamin B12 activity) at 60 C. for 30 minutes. Higher temperatures are necessary to release bound forms of vitamin B12. Vitamin Bu has a maximum of stability at pH 4-5 an acetate buffer pH 4.6 is recommended when higher temperatures are used. 

Whereas the great majority of aquo-chromium(m) complexes are inert to formation reactions, the tetra-(p-sulphonatophenyl)porphine complex is remarkably labile—stopped-flow techniques are required to monitor replacement of the co-ordinated waters of this complex by, for instance, chloride, cyanide, or pyridine. Analogous cobalt(in) complexes are also substitution labile. This has been attributed to the possibility of a redox mechanism for substitution. In view of the difficulty of reducing chro-mium(ni) to chromium(n) such a mechanism seems unlikely here, so that the porphine ligand may be promoting reactivity to substitution by electronic effects. ... 

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