Cyano complexes stability

Electroplating. Aluminum can be electroplated by the electrolytic reduction of cryoHte, which is trisodium aluminum hexafluoride [13775-53-6] Na AlE, containing alumina. Brass (see COPPERALLOYS) can be electroplated from aqueous cyanide solutions which contain cyano complexes of zinc(II) and copper(I). The soft CN stabilizes the copper as copper(I) and the two cyano complexes have comparable potentials. Without CN the potentials of aqueous zinc(II) and copper(I), as weU as those of zinc(II) and copper(II), are over one volt apart thus only the copper plates out. Careful control of concentration and pH also enables brass to be deposited from solutions of citrate and tartrate. The noble metals are often plated from solutions in which coordination compounds help provide fine, even deposits (see Electroplating). 

The low stability of two-coordinate complexes with respect to other possible structures is well illustrated by (he cyano complexes. Although sifver(f> and gold( ) form discrete bislcyano) complexes, solid KCufCNk possesses a chain structure in which the coordination number of the copperfl) is 3. 

The trans effect illustrates the importance of studying the mechanisms of complex substitution reactions. Before continuing with a discussion of mechanisms, the distinction between the thermodynamic terms stable and unstable and the kinetic terms labile and inert should be clarified. Consider the following cyano complexes [Ni(CN)4]2-, [Mn(CN)6]3-, and [Cr(CN)6]3-. All of these complexes are extremely stable from a thermodynamic point of view is yet kinetically they are quite different. If the rate of exchange of radiocarbon labeled cyanide is measured, we find that despite the thermodynamic stability, one of these complexes exchanges cyanide ligands very rapidly (is labile), a second is moderately labile, and only [Cr(CN)6]3 can be considered to be inert ... 

Analogous cyano complexes can be prepared. Treating the bis chelate complex [Pt(dppm)2]2+ with 2 equivalents of NaCN leads to stabilization of the r 1-dppm complex (151) which can be used to prepare heterobimetallic bridging complexes with added M (M= Ag+/I, HgCl2, Rh2Cl2(CO)4) (equation 433).1461 Similar complexes can be formed with dialkyl and diaryl substituents on platinum in place of cyanide.1462... 

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. 

Conversely, the calibration for the cyanide determination poses no problem. Free cyanide is found, for example, in untreated liquors from electroplating processes. Since these samples also contain transition metal ions, part of the cyanide exists in complexed form, whereby the various metal cyano complexes differ significantly in their stability. 

Beck, M. T., Critical survey of stability constants of cyano complexes. Pure Appl. Chem., 59, (1987), 1703-1720. Cited on page 436. 

These spectral features and other evidence presented in this paper (97) leave no doubt that the Tl(III) cyano complexes exist, that they have the stated composition, and that they are extremely strong and kinetically inert. In fact, the cyano complexes are stronger than any other known monodentate complexes of thallium(III). The only possible known competitor as a ligand, the iodide ion, forms the complex TII4 with the overall stability constant log = 35.7 (99), i.e., several orders of magnitude lower than that of T1(CN)4" (see Table III). The distribution of thallium among the various Tl(CN), "-species is shown in Fig. 5. Stepwise stability constants of MX complexes often decrease with increasing n because of statistical, steric, and coulombic factors... 

There is some evidence that back-donation plays an important role in cyano complexes (149). For the isoelectronic Au(I), Hg(II), and Tl(III) ions, the back-donation should be most efficient (and hence the complexes should be strongest) for gold and least efficient for thallium because of the increasing charge on the metal ion. Thus, if back-donation is a major effect, the stability constants for the cyano complexes should decrease in the order Au > Hg > Tl. Unfortunately, only one stability constant, namely /32, is (approximately) known for gold(I), but there is no doubt that the 2-values follow the predicted trend log 82 = 39 (for Au) > 32.7 (for Hg) > 26.5 (for Tl) (97, 150,151). 

Relatively little work has been done on the redox reaction between thallium and halide/pseudohalide ions (75, 93, 97,110, 328-330). Let us consider the qualitative order of stability of thallium(III) in the form of TlXp " complexes, where X = Cl, Br, I, SCN, CN, Thallium(III) forms strong complexes with all these ligands on the other hand, it can oxidize the X ions to X2. It is well known that the thallium(III) chloride complexes are perfectly stable for an indefinite period of time. The corresponding bromide complexes are usually stable, but at low Br/Tl ratios Tl(III) can be reduced by Br the reduction is easily prevented by adding excess of bromine. The iodo and thiocyanato complexes are approximately equally unstable toward redox reaction Tl(III) is rapidly reduced by the anion.Finally, the cyano complexes... 

However, the cyanide ion does have the ability to stabilize metal ions in low formal oxidation states, and it presumably does this by accepting electron density into its n orbitals. The fact that cyano complexes of zerovalent metals are generally much less stable (in a practical as opposed to a well-defined thermodynamic or chemical sense) than similar metal carbonyls has often been taken to show the poor 7t-acidity of CN-, but it should be noted that the cyano compounds, e.g., [Ni(CN)4]4-, are anionic and might thus tend to be more reactive for this reason alone. In some instances cyano... 

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