Cyanide exchange substituted complexes

Cyanide anation in DMSO has proved useful in the preparation of cyano complexes (Scheme 29). 25 Cyanide displaces coordinated DMSO cleanly but deprotonates rather than displaces coordinated water. The preparation of trans- and ds-[Cr(CN)2(NH3)4]C104 starts from [Cr(DMSO)2(NH3)4]3+ or [CrCl(DMSO)(NH3)4]2+. The insolubility of trans-[Cr(CN)2(NH3)4]C104 permitted its isolation from solution, but it was necessary to separate the cis isomer by ion exchange. Rapid trans to cis isomerization of bww-[Cr(DMSO)2(NH3)4]3+ occurs on the addition of CNT before anation produces the dicyano complexes stepwise, both apparently via c -[CrCN(DMSO)(NH3)4]2+, which in an uncommon rearrangement gives a mixture of trans- and cis-dicyano complexes. Substitution of DMSO by CN" in water is stereoretentive. 

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 reaction of the aqua complex and KCN in aqueous solution gave no cyano complex, but unidentified materials. On the other hand, KCN substituted the coordinated chloride ion in [CoCl(gly)(tacn)]N03 m DMSO by the cyanide ion. Yellow crystals of [Go(CN)(gly)(tacn)]Br were obtained after ion-exchange purification... 

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