Hexacyano-complexes

Similarity with cobalt is also apparent in the affinity of Rh and iH for ammonia and amines. The kinetic inertness of the ammines of Rh has led to the use of several of them in studies of the trans effect (p. 1163) in octahedral complexes, while the ammines of Ir are so stable as to withstand boiling in aqueous alkali. Stable complexes such as [M(C204)3], [M(acac)3] and [M(CN)5] are formed by all three metals. Force constants obtained from the infrared spectra of the hexacyano complexes indicate that the M--C bond strength increases in the order Co < Rh < [r. Like cobalt, rhodium too forms bridged superoxides such as the blue, paramagnetic, fCl(py)4Rh-02-Rh(py)4Cll produced by aerial oxidation of aqueous ethanolic solutions of RhCL and pyridine.In fact it seems likely that many of the species produced by oxidation of aqueous solutions of Rh and presumed to contain the metal in higher oxidation states, are actually superoxides of Rh . ... 

The Ni /CN system illustrates nicely the ease of conversion of the two stereochemistries. Although, as already pointed out, there is no evidence of a hexacyano complex, a square pyramidal pentacyano complex is known ... 

C20-0015. Explain why hexacyano complexes of metals in their +2 oxidation state are usually yellow, but the corresponding hexaaqua compounds are often blue or green. 

A critical survey of the formation constant data available for the hexacyano complexes of the... 

There are many other Prussian blue analogs for which the visible spectrum is complex and not understood. Heavy metal ferricyanides and hexacyano complexes of other transition metals with less than d configurations are in this class due to the presence of ligand-metal charge transfer processes. 

Crespo, I., Barriga, C., Rives, V. and Ulibarri, M. A. (1997). Intercalation of iron hexacyano complexes in Zn,Al-hydrotalcite. Solid State Ionics 101, 729. 

Since these hexacyano complexes are hydrated salts, aqueous solutions are used. To prepare the saturated Sm[Fe(CN)6] solution 10 mL of 1 M neutral SmCl3 solution is first added, followed by the same amount of 1 M K3[Fe(CN)6] solution, to 500 mL of distilled water kept at 65°. In this way a supersaturated solution is obtained that (if everything is clean) can remain in this metastable state for hours before the first crystallites separate. To accelerate the procedure, microcrystals of Sm[Fe(CN)6] 4H20 are prepared by mixing 1 M solutions of 1 mL of SmCl3 and 1 mL of K3[Fe(CN)6], The tiny seed crystals that are obtained are used to initiate precipitation in the growth solution. After 10 minutes, still at 65°, the solution is filtered and 50 mL of hot dis-... 

From a coordination viewpoint, there are two views that support the idea of an inner-sphere coordinated cyano bridge on the Cr(VI) core. First, four-coordinated Cr(VI) or Cr(V) changes to six-coordinate Cr(IV) so that there is room for coordination sphere expansion. Second, it has been shown (118) that, in spite of the high negative charge on octa-and hexacyano complexes, they can be used as nucleophiles in substitution of an axial water ligand of mesotetra(4-A/ -methylpyridyl)por-phinediaquocobalt(III) ions and that the stability constants of these products are comparable to those of NCS . 

X /X" reduction potentials in the bound and ion-pair systems. The actual reduction potentials have been measured for some of the (NH3)5Ru "[NCM (CN)5] complexes [83] and estimated for some of the Ru (NH3)5(X ) " complexes. For these complexes, the effective reorganizational free energies are Xp = (5 0 1.4) x 10 cm , independent of whether X is a halide or a hexacyano complex. Similarly, a plausible correlation line can be drawn through the data in Figure 3 with a slope of one and an intercept of —5 x 10 cm. In the Co complexes, the coordinated halide is probably more difficult to oxidize, while the metal is easier to reduce [73, 94] than in the corresponding ion pair, and the effect of the reduction potentials may not be particularly important. A very rough estimate suggests that Xx/Xp ay... 

Fe (CN)6] , when included in 17, is involved in an electron transfer reaction with iodide which cannot take place in the absence of the macrocyclic species. In fact, the E°(Fe 7Fe ) value for the hexacyano complex is considerably lower than E°(l2/I ), so that the electron transfer from I to Fe center is thermodynamically disfavored. However, in the presence of 17, in an acidic aqueous solution (e.g., pH = 4.4), the redox potential of Fe /Fe couple inside the supercomplex is increased to such an extent that the reaction (see Eq. 1), which is essentially iso-ergonic, proceeds to an equilibrium situation K = 1 M , at 25 °C). 

For compound types where the coordination number (C) tends to remain more constant than the electronic configuration (E)f an AEQ matrix instead of an ACQ matrix can be used. Except for the substitution of an electronic configuration (E) axis for a coordination number (C) axis, an AEQ matrix is entirely analogous to an ACQ matrix. Tables IX and X illustrate AEQ matrices for the hexacyano complexes and the di-TT-cyclopentadienyl complexes, respectively, of the first row transition metals. 

Again, the metal-carbon stretching frequencies are markedly different in these two hexacyano complex ions. It is, therefore, relatively easy to differentiate coordination isomers such as fCo(NH3)6] [Cr(CN)6] and [Cr(NH3)6] [Co(CN)6]. Coordination isomers involving the same central metal may be more difficult to identify from their infrared spectra. Thus, [Pt(NH3)4] [PtClJ and [Pt(NH3)3Cl] [Pt(NH3)Cl3] may exhibit very similar spectra. This is also expected for polymerization isomers such as [Co(NH3)3(N02)3] and [Co(NH3)e] [Co(N02)e] because the central metal is the same in both compounds. However, minor differences may possibly be seen in the far-infrared region where the skeletal vibrations of these complexes appear. 

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] " ... 

Other photosynthesis applications include the labilization of [CN] from the hexacyano complexes followed by thermal anations of the resulting aquo complex" ... 

Table 111-36. Vibrational Frequencies and Band Assignments of Hexacyano Complexes (cm" )... 

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