Cyanide trans effect

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. ... 

In the aquation sequence, the precursors to the uncharged molecule l,2,3-Cr(CN)3(H20)3 all have cyanide groups trans to another cyanide, whereas, in the tricyano complex and its daughters, the cyanide ligands are always opposite water molecules. The magnitude of the trans effect is sufficient to make the reaction essentially stereospecific, so that only cis isomers are formed in the aquation sequence. 

A similar effect conceivably accounts for the higher Pi value (—0.74 V) (weaker net electron-donation) of the cyanide ligand estimated [15] at trans- FeH(dppe)2 with the strong donor trans-hydride, in comparison with that (—l.OV) [10] obtained at Cr(CO)5 and also proposed at trans- MoL(dppe)2) (L = CO, N2) with the strong net electron-acceptor L ligand, in spite of the lower electron-richness of the former Fe site Eg = 1.04 V) relatively to the latter (Eg = —0.11 or —0.13 V) molybdenum centers. 

Flow of electrons away from the iron will place a positive charge (effective) on Fe. The trans cyanide group in 8 (L = C6H5CH2NC, L" = CN) competes for the same set of orbitals and since it is a strong electron donor toward iron, it renders iron least positive and therefore enhances structure 8. Since the inductive effect is presumably small and since one aromatic ring in II appears more reactive toward N02 than the others, hypothesis 2 is the more probable one. A... 

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