Octacyano complexes mechanism

Redox reactions of octacyano complexes are attractive to kineticists for several reasons, of which their inertness to substitution, stability over a wide pH range, almost negligible protonation in acidic media, and favorable redox potentials are some of the more important. The above-mentioned properties are not rigid and exceptions do occur. Ion association and/or substitution of some cyano ligands has been reported for[Mo(CN)g]<- with[Cr(H20)g] " (37,32), [Fe(H20)6l (33),andTi(IV) (34) and for [W(CN)g] with [Cr(H20)6l (35, 36). There seems to be a difference of opinion regarding the mechanism and product formulation for these anation reactions, especially regarding the intactness or lack thereof of the cyano complexes coordination sphere. This is an area where more research with, for example, trivalent aqua cations is to be done to clarify the ambiguity. No electron transfer occurs in these reactions and any mechanistic details are beyond the scope of this review. 

An impressive number of articles on the redox kinetics of octacyano complexes have been produced during the past two decades. The material in this chapter covers the period between 1969 and 1991. Interested readers may find a good deal on the relatively few older mechanistic studies in reviews on mechanisms of redox reactions (37) and cyanide complexes of the early transition metals (7). A book by Sharpe (2) on... 

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). 

It was shown during this period that photolysis of the octacyano d species of Mo(V) and W(V) produces the reduced octacyanometalate(IV) complexes as main photoproducts, and much research has concentrated on the first mechanistic step of action. On the other hand, photolysis of the primary photoproduct, [MfCNlg] of Mo(IV) and W(IV), forming [M02(CN)4] (and the mono- and the diprotonated [M0(0H)(CN)4] and [M0(H20)(CN)4] , depending on the solution pH) as the final product during exhaustive photolysis, was also extensively studied. Photolysis of both these metals d and d systems has also been studied in organic media as an aid in the elucidation of the photolytic mechanisms. The discussion below concentrates on aspects of the research that has been performed with regard to the photochemistry of these M(V) and M(IV) complexes of Mo and W. 

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