Octacyano complexes formation

The reactions of the molybdenum(IV) and tungsten(IV) complexes with cyanide ions proceed via the same scheme as those of other mono-dentate ligands (see Scheme 3) with the formation of the pentacyano complex (155,156). This very fast reaction is followed by a much slower reaction with the production of the octacyano complex in the presence of an excess of cyanide ions. 

Although the octacyano complexes of Mo(IV) and W(IV) have been known for many years (see Section IIIA) and more convenient methods for synthesizing these complexes have been described, little is actually known about the mechanism of the formation of these complexes. A recent kinetic study of the formation of these complexes, however, suggested the overall reaction presented in Scheme 6 (155,156). 

Scheme 6. Reaction scheme for the formation of the octacyano complexes of Mo(IV)...

In this reaction scheme, the formation of the pentacyano complex is a relatively fast reaction, with rate constants of about 116 and 2.9 Af" sec" for the molybdenum (20°C) and tungsten (25°C) complexes, respectively, whereas the formation of the octacyano complex from the pentacyano complex is a relative slow reaction, with a half-life of several minutes at a cyanide ion concentration of 1 Af for both the molybdenum and the tungsten complexes. The formation of the octacyano complex from the pentacyano complex is third order in the cyanide ion concentration 155,156). This suggests that the rate-determining step is the reaction of the heptacyano complex with cyanide ions. It seems, however, that the pentacyano complex is a necessary intermediate in the synthesis of the octacyano complex. This proposed reaction scheme makes it possible for the first time to explain why the octacyano complex of rhenium(V), which is also a d species, is still unknown in spite of several attempts (and claims of success) by different groups in the past (see Section IIA) to synthesize this complex The reactive complexes [Re0(H20)(CN)4]" and [ReO(OH)(CN)4] do not exist at a pH > 8, at which there are enough free cyanide ions since the values of [Re0(H20)(CN)4]" are only 1.4 and 4.2. The formation ofthe intermediate [ReOtCNlg] (see Scheme 6) is thus not possible. Thus one cannot proceed beyond the tetracyano complex in this way. 

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