Mixed Cyano Complexes

Chan and Tobe investigated the reaction between trans-[CaC 2 pa) and cyanide and found that the concentrated aqueous solution was disproportionated to [Co(en 3] and some unidentifiable products, whereas in dilute solution hydrolysis of the cyanide ion occurred. This su ests that the design to replace a coordinated ligand such as Cl in a complex by CN is undesirable. However, mixed cyano complexes were obtained in the 1960 s. 

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It is generally difficult to prepare a cobalt(III) complex containing different kinds of ligands that are considerably separated in the spectrochemical series. The several complexes containing cyanide ion, which occupies the highest position in the series, and ammonia, which is ranked in a middle position, have been prepared by methods to prevent disproportionation reactions.3 The following procedure gives a mixed cyano complex with carbonate ion. 

The redox potentials of the afore mentioned complex couples decrease strongly wnth increasing acid concentration 13). While activity and ion-pairing (774) effects may be important factors, the effect as a whole is not clearly understood. It is noteworthy that the redox potential of mixed cyano complexes, e.g. Fe(bipy)2(CN)2, increases with increasing acidity (775). 

The photochemistry of octacyanometallates, and of mixed cyano-dii-mine complexes of the type [W(CN)6(diimine)]2 and [MO(CN)3(bpy)] M = Mo, W, has been reviewed (183). The authors pay particular attention to the role of the counterion in this type of reaction they also call attention to questions which were, at the time of writing, unresolved. A mainly structural and redox review of octacyano-, nitridotetracyano-, and oxotetracyano-metallates (Nb, Ta Mo, W Tc, Re) contains some kinetic and mechanistic information on thermal and photochemical substitution in these complexes, with the main conclusion being that much more needs to be done on such reactions (184). 

Structural models, which are synthesized to imitate features of the proposed structure of the active site. These may be used to demonstrate the chemical conditions, which allow such structures to exist, to investigate their chemical properties and to give a better understanding of the spectroscopic characteristics of the native proteins. Examples of these include the mixed carbonyl/cyano complexes of iron, used to verify the infrared spectra to the hydrogenases (Fig 7.4) (Lai et al. 1998) and the nickel-thiolate complexes which have low redox potentials like the hydrogenases (Franolic et al. 1992). 

Mixed-valence complexes terminated in other than ammine and cyano ligands include [([9]aneS3)ClRu(/x-243)RuCl([9]aneS3)] which exhibits a strong intermetallic interaction consistent with a Robin-Day class III (delocalized) species. The synthesis of the Ru Ru complex [ P(OMe)3 2(MeCN)Ru(u-S2)(/u-NH2NH2)Ru P(OMe)3 2(MeCN)] + has been reported. Oxidation of the (M = Ru or Os) complexes [ 4 -(4-MeC6H4 tpy)M(u-L)M 4 -(4-... 

Comparison between the half-wave potentials (equations 2 to 4) of [Cr(CNR)6](PF6)2, e.g. for R = Bu , -1.04, -0.28 and 0.84 V (versus SCE),22 with those for [Cr(CNPh)6](PF6)2, i.e. -0.35, 0.25 and 1.00 V,20 shows that alkyl and aryl isocyanides favour respectively the higher and the lower oxidation states as expected from the greater a-donor and weaker jr-acceptor capabilities of the alkyl over the aryl isocyanides. Similarly, the phosphines in the mixed ligand complexes (Table 3), 23 relative to isocyanide ligands, stabilize the Cr111 oxidation state. The great difference in the relative stabilities of Cr—C bonds in the cyano and phenyl isocyanide complexes is indicated by the magnitude of the shift (ca. 2.0 V) between the Cr(CN) "/Cr(CN)r (-1.130 V) and the Cr(CNPh)i+/Cr(CNPh)i+ reduction potentials.28... 

Only a limited number of cyano and isocyano complexes and mixed carbonyl, nitrosyl and phosphine cyano complexes of nickel(O) have been described so far.16... 

K4Ni(CN)4 is a yellow solid which is extremely sensitive to oxygen and moisture. It can be prepared by reducing K2Ni(CN)4 in liquid NH3 with an excess of potassium17 and decomposes in water with H2 evolution. CN ions can be easily replaced by ligands with greater jt acceptor properties such as CO, NO and phosphines. Mixed carbonyl and nitrosyl cyano complexes have been prepared by direct reactions (equations 1 and 2).18,19... 

The cyano complexes of cobalt(—I) are restricted to nitrosyl and carbonylnitrosyl mixed ligand complexes, all of which obey the 18-electron rule. 

The pentacyanonitrosylferrates, (the nitroprussides ) are of interest. Cyano complexes in general do not easily form mixed complexes by replacement of CN" groups and, in the hexacyano-complexes of iron, only one CN" can be replaced by NH3, HgO, CO, NOg (nitro) or NO (nitroso). Acidification of a K4Fe(CN)g--KN02 mixture gives first the pentacyanonitro-ferrate(II) ion, [Fe(CN)5N02] " ... 

Prussian blue analogs are here defined as polynuclear transition metal cyanides of the composition M [M (CN)6]i xHzO a retallizing with a cubic unit cell. They are easily obtained as sparsely soluble precipitates by mixing solutions of a cyano complex M (CN)e with an appropriate salt of The compounds prepared by using the hexacyanometalate in the form of the most common potassium salt invariably contain different amounts of potassium, which in some cases can be exchanged by cesium... 

In addition to the phosphineplatinum(II) fulminate complexes mentioned already, many other mixed complexes have been prepared and their vibrational spectra studied." Equations (2)-(6) illustrate typical preparations. In each case the fulminato complex formed closely resembles the analogous cyano complex. 

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