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Octacyano complexes

Apart from the above-mentioned theoretical applications, there is also the possibility of practical applications. The octacyano complexes of Mo(V) and W(V), for example, find excellent use in the oxidation of organic substances which require a mild oxidizing agent. It was also recently observed that derivatives of the dioxotetracyano complexes of Mo(IV) and W(IV) can take up dioxygen, which leads to the activation of the O2 molecule. The complexes are thus candidates as catalysts in oxidation reactions with dioxygen as well as oxygen-transfer reactions. [Pg.242]

These and other applications in for example the field of analytical chemistry have led to an increasing number of articles during the past two decades. Although this chapter does not cover all aspects, it gives a fairly comprehensive account of research output on the octacyano complexes. It is moreover the first review covering the chemistry of 0x0- and nitridotetracyano complexes, which has developed extensively during the past decade. [Pg.242]

Octacyano-metal ions, being discrete MLg moieties, can have different spatial arrangements. Those that have been considered include, with point symmetry in parentheses, the square antiprism (D 82m), dodecahedron (D2 -42m), cube (Oh-mSm), hexagonal bipyramid (Dg -6lmmm), puckered hexagonal bipyramid 4,4- [Pg.242]

Calculations, based on VSEPR theory so as to minimize repulsion between ligands in some of these afore-mentioned forms, have shown that repulsion energy coefficients for the square antiprism, dodecahedron, and cube are about the same but that the antiprism should be the most stable configuration for a MLg polyhedron (S). This principle has also been applied to K4[Mo(CN)g] 2H20, for which, in contrast [Pg.243]

Shape parameter [(n-But)4N]a = [MoiCNisP K4Mo(CN)8. = 2H20 (HPic)4 = [MoCCNig] MFP HSM [Pg.245]


Also of interest are the octacyano complexes, (M(CN)g] (M = Mo, W), whieh are commonly prepared by oxidation of the M" analogues (using MnO,) or Ce" ) and whose structures apparently vary, aceording to the environment and counter cation, between the energetically similar square-antiprismatic and dodecahedral forms. [Pg.1025]

Mo(IV), octacyano complexes, photochemistry, 40 287-289 Mo(V), octacyano complexes, photo-chemistry, 40 283-287... [Pg.190]

Octacyano complexes (continued) organic compounds, 40 276-280 oxyanions of Groups VIB and VIIB, 40 269-274... [Pg.212]

Molybdenum forms many other complexes. Of particular interest are the octacyano complexes, containing eight cyanide ions. CN. coordinated to a single tetravalent of pentavalenl molybdenum ion. MolCN)) and Mo(CN)MOg . the latter being exceptionally stable, and both form oclacyanomolybdic acids H, Mo(CN)R 3H-0 and R((Mo[Pg.1039]

Of the rather limited number of tetravalent molybdenum and tungsten derivatives known, wre shall mention here only molybdenum disulfide (M0S2), which occurs as the ore molybdenite and the octacyano complexes Mo(CN)i 4 and W(CN) 4 which, along with their pentavalent counterparts (p. 333), represent two of the very few examples of octa-covalency. [Pg.335]

The aim of this chapter is to summarize the chemistry of octacyano complexes and oxo- and nitridocyano complexes of molybdenum, tungsten, niobium, tantalum, technetium, and rhenium, with special emphasis on structural and kinetic properties. [Pg.241]

These complexes are excellent models for theoretical studies. The octacyano complexes of molybdenum and tungsten are stable and inert toward substitution reactions and therefore very suitable for theoretical studies of redox reactions and application of the Marcus theory. The photoreactivity of these systems is also proving to be important. The 0X0- and nitridocyano complexes of Mo(IV), W(IV), Tc(V), Re(V), and Os(VI) are very good candidates for kinetic studies of substitution reactions with both mono- and bidentate ligands and are of interest especially in view of the large variations in the observed reactivity. [Pg.241]

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. [Pg.249]

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... [Pg.249]

It is interesting that the specific alkali metal ion catalysis in Eq. (84) is also accompanied by first-order dependence on [M(CN)8 ], which suggests that this catalysis phenomenon should be ascribed to the presence of an octacyano complex rather than ion association between alkali metal and thiosulfate ions. In the case of [MofCNlg] ", the ratedetermining step, Eq. (86), is followed by a fast radical dimerization,... [Pg.269]

Contrary to the facile oxidation of sulfite ions, that of selenite and tellurite ions by octacyano complexes do not proceed at all except if a catalyst such as OSO4 is employed. The kinetics for the Os(VIII)-cata-lyzed oxidation of SeOa (94) and TeOa " (95) ions by octacyanotungs-tate(V) and -molybdate(V) ions in alkaline medium has thus been studied. The experimental rate laws are the same and show first-order... [Pg.270]

An interesting application for the oxidation of organic compounds is of electrochemical nature. Octacyano complexes have been used to monitor redox enzymes such as lactate oxidase (from Pediococcus sp.) and sarcosine oxidase (from Arthrobactersp.) in a suitable electrochemical system (114). Two equivalents of [M(CN)g] can, for example, be oxidized at the electrode surface to [M(CN)g], which in turn can oxidize the flavoproteien to its oxidized form. This in turn reacts with, for example, L-lactic acid to produce pyruvic acid. [Pg.280]

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. [Pg.313]

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). [Pg.313]

Scheme 6. Reaction scheme for the formation of the octacyano complexes of Mo(IV)... 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. [Pg.314]


See other pages where Octacyano complexes is mentioned: [Pg.137]    [Pg.211]    [Pg.238]    [Pg.257]    [Pg.1633]    [Pg.11]    [Pg.333]    [Pg.241]    [Pg.242]    [Pg.242]    [Pg.243]    [Pg.244]    [Pg.247]    [Pg.264]    [Pg.266]    [Pg.268]    [Pg.283]    [Pg.287]    [Pg.290]    [Pg.940]   


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