Pentacyanoferrates and Nitrosyls

There have been rather few simple rate constant studies on substitution in pentacyanoligandferrates recently, though much effort is being put into demonstrating whether the intimate mechanism is limiting dissociative, D, or interchange, I. The pentacyanoferrate moiety is also currently of great interest in relation to MLCT and MMCT (intervalence CT) spectra, and their relation to electron transfer kinetics. 

Limiting rate constants for loss of pyrazine or of piperidine from their respective pentacyanoferrate(II) derivatives are affected to only a very small extent by the acetone content of binary aqueous mixtures.Probably here, as in the earlier case of the 4-cyanopyridine complex in aqueous alcohols, this minor effect on rate constants conceals large but almost equal effects on the initial and transition states. This proved impossible to assess, due to the authors failure to find salts of appropriate solubility for the requisite measurements and transfer chemical potential derivations. It may be that the recently characterized transition metal(II) salts could lead to an answer. 

The dependence of rate constant on incoming nucleophile concentration has been established, at 298 K in 40% methanol, for reaction of the 4-cyanopyridine complex [Fe(CN)5(4-CNpy)] with an extensive range of nucleophiles. The range 

Competition experiments again feature prominently in another discussion of the possible role of transient five-coordinate [Co(NH3)5] in induced and in spontaneous aquation of pentaaminecobalt(III) derivatives. The operation or nonoperation of the D mechanism at various cobalt(III) centers and at penta-cyanoferrate(II) still requires a few experiments providing unambiguous results. Its operation at molybdenum(O)- and tungsten(0)-penta or tetracarbonyl complexes seems more firmly based. The question of its operation at pentacyanoferrates(III) does not seem to have caused much concern. The only recent paper which mentions kinetics of such a reaction, replacement of 2-methyl imidazolate in [Fe (CN)5(2-Meimid)] , reports that the limiting first-order rate constant is 2.3 x 10 s at 298 K, but is more preoccupied with redox catalysis by traces of iron(II) than with simple substitution. 

8 Inert-Metal Complexes Other Inert Centers 

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