Cyanide exchange

Cyanide [57-12-5] Cyanide detoxification Cyanide exchange Cyanide iron blue Cyanide oxidation Cyanides... 

Cya.nideExcha.nge, Acetone cyanohydrin and methyl isobutyl ketone cyanohydrin [4131 -68-4] dissolved in an organic solvent, such as diethyl ether or methyl isobutyl ketone, undergo cyanide exchange with aqueous cyanide ion to yield a significant cyanide carbon isotope separation. The two-phase system yields cyanohydrin enriched in carbon-13 and aqueous cyanide depleted in carbon-13. Fquilibrium is obtained in seconds. 

The cyanide exchange on [M(CN)4]2 with M = Pt, Pd, and Ni is a rare case in which mechanistic comparisons between 3d, 4d, and 5d transition-metal complexes. Surprisingly, the behavior of these metal square-planar centers leads to mechanistic diversity involving pentacoordinated species or transition states as well as protonated complexes. The reactivities of these species are strongly pH-dependent, covering 15 orders of magnitude in reaction rates.85... 

VI. Comparison of the Rates of Inversion and Oxygen and Cyanide Exchange... 

Carbon-13 NMR was utilized to study different aspects of the reactivity of the metal complexes as a function of certain structural features in the selected oxocyano complexes of Mo(IV), W(IV), Tc(V), Re(V), and Os(VI) as depicted in Scheme 1 and illustrated in Figs. 1-4. The NMR spectral properties were similar to those obtained from 13C NMR in general, i.e., very sharp lines indicative of fairly long relaxation times in the order of a few seconds. The large quadrupolar moment ofTc-99 (7 = 9/2, 100% abundance) led to a very broad bound 13C signal (Fig. 5), thus excluding the quantitative study of the cyanide exchange by 13C NMR. However, 16N NMR was successfully used instead. 

As in the case of the oxygen exchange described above in Section IV (see also Fig. 16), well-behaved first-order kinetics were observed for all the cyanide exchange reactions for which the modified exponential form of the McKay equation (8) was used to determine the observed rate constants. 

Fig. 18. The pH dependence of the cyanide exchange rate at 25°C on [Re02(CN)4]3 8 13Cf and 8 15NF are the 13C and 15N chemical shifts of the free HCN/CN and 8 13CB and 8 15NB the chemical shifts of the bound CN. The total complex concentration [Re] = 0.2 m the total cyanide concentration [CN] = 0.3 m and /jl = 1.5-2.8 m KN03 (8). (Adapted with permission from Abou-Hamdan, A. Roodt, A. Merbach, A. E. Inorg. Chem. 1998, 37, 1278-1288. Copyright 1998 American Chemical Society.)...

The significant reactivity difference for cyanide exchange on the Tc(V) and Re(V) of >4000 is in general agreement with that found in previous complex-formation and oxygen-exchange studies (1, 2, 76,... 

The above, together with the fact that a fourfold variation of [CN ] for the [WO(OH)(CN)4]3 complex showed zero-order dependence on free cyanide concentration further points to a dissociative activation for the cyanide exchange process, also in the case of the mono oxo (classic 16-electron) [MO(X)(CN)4]m species. 

A further interesting observation from the cyanide exchange study relates to the fact that the exchange rate constants for the equatorial cyanide in the pentacyano complexes compared to the protonated spe-... 

The different exchange processes described in Sections III-V can be combined to illustrate the reactivity of the different sites in these oxo cyano complexes as a function of pH and the possible interdependence thereof. The three processes that are compared are the inversion along the O-M-O axis (illustrated in Fig. 15 related to proton exchange), the oxygen exchange, and the cyanide exchange. 

The cyanide exchange processes are in most cases the slowest. However, it is interesting to note that in the case of the Mo(IV) center, previously unexplained more-rapid-than-expected oxygen exchange at pH values lower than 6 (Fig. 19b) might be associated with the cyanide exchange (which was found to be of comparable rate). It is quite possible that the rapid decomposition observed for the aqua oxo complex of Mo(IV) at these pH values is due to hydrolysis stimulated by the relative rapid cyanide exchange at this acidity. 

In summary, it is clear from the above-discussed aspects that it was possible by multinuclear NMR (oxygen-17, nitrogen-15, carbon-13, and technetium-99) to successfully study the very slow cyanide exchange and the slow intermolecular oxygen exchange in these oxocy-ano complexes and correlate them both with the proton-transfer kinetics. Furthermore, the interdependence between the proton transfer and the actual dynamic inversion of the metal center was clearly demonstrated. 

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