Cyanide, metal cations

A species that bonds to a metal cation to form a complex is known as a ligand. Any species that has a lone pair of electrons has the potential to be a ligand, but in this section, we confine our description to a few of the most common ligands ammonia, compounds derived from ammonia, cyanide, and halides. We describe additional examples in Chapter 20 which addresses the chemistry of the transition metals. 

Considerable practical importance attaches to the fact that the data in Table 6.11 refer to electrode potentials which are thermodynamically reversible. There are electrode processes which are highly irreversible so that the order of ionic displacement indicated by the electromotive series becomes distorted. One condition under which this situation arises is when the dissolving metal passes into the solution as a complex anion, which dissociates to a very small extent and maintains a very low concentration of metallic cations in the solution. This mechanism explains why copper metal dissolves in potassium cyanide solution with the evolution of hydrogen. The copper in the solution is present almost entirely as cuprocyanide anions [Cu(CN)4]3, the dissociation of which by the process... 

Quite in contrast to, e.g., [CoCp2]+, borabenzene metal cations show a pronounced affinity toward hard nucleophiles such as amines, OH-, and to some extent even F- and H20. Qualitatively this affinity increases in the order CoCp2]+ 36 < 1 < 61 (69). [CoCp(C5H5BPh)]+ (1) adds tertiary amines at boron. With pyridine, the pyridinioboratacyclohexadienyl complex 70 is formed (K = 174 5 liters mol-1, in MeCN, 20°C), which can be isolated from CH2C12 as PF6- salt (69). The similar rhodium and iridium cations 36 and 37 form the stable cyanide adducts 71 and 72 (69). 

Adsorption of cyanide anions can be affected by adsorption of cations. In the solutions containing nonspecifically adsorbed anions, the nature of alkali metal cations was found to influence the measured value of the electrode capacitance at potentials more negative than —0.6 V (versus standard hydrogen electrode (SHE)). At < —l.OV adsorption of CN ions was enhanced in the presence of Li+ and Na+ cations, and inhibited in the presence of Cs+ ions [81]. A combined SERS and density-functional theory has been applied to study cyanide adsorption at Au electrode [82]. The authors have arrived at the conclusion that the polarity of Au—CN bonds falls between that of Au—Cl and Au—Br surface bonds. The binding strength for three different gold surfaces decreased in the order ... 

The building block [Au(CN)2] has been revealed as a versatile unit in the formation of heteronuclear arrays which, in some cases, display unusual and interesting luminescent properties, such as vapochromic behavior. In many of these complexes the cyanide group uses the carbon and nitrogen ends as Lewis bases able to coordinate two different metal cations (i.e., Au-CN-M) [82-86]. These ideas may be illustrated by the reaction of K[Au(CN)2] with MX2 salts (M = Cu, Ni, Co, Mn ... 

It is well known that ACN reacts with active metals (Li, Ca) to form polymers [48], These polymers are products of condensation reactions in which ACIST radical anions are formed by the electron transfer from the active metal and attack, nucleophilically, more solvent molecules. Species such as CH3C=N(CH3)C=N are probably intermediates in this polymerization. ACN does not react on noble metal electrodes in the same way as with active metals. For instance, well-re-solved Li UPD peaks characterize the voltammograms of noble metal electrodes in ACN/Li salt solutions. This reflects a stability of the Li ad-layers that are formed at potentials above Li deposition potentials. Hence, the cathodic limit of noble metal electrodes in ACN solutions is the cation reduction process (either TAA or active metal cations). As discussed in the previous sections, with TAA-based solutions it is possible that the electrode surfaces remain bare. When the cations are metallic (e.g., Li+), it is expected that the electrode surfaces become covered with surface films originating from atmospheric contaminants reduction if the electrodes are polarized below 1.5 V (Li/Li+). As Mann found [13], in the presence of Na salts the polarization of metal electrodes in ACN solutions to sodium deposition potentials leads to solvent decomposition, with evolution of H2, CH4 and sodium cyanide (due to reaction with metallic sodium). 

Very recently Geus and co-workers [44, 45] have applied another method based on chemical complexes. This is the complex cyanide method to prepare both monocomponent (Fe or Co) and multicomponent Fischer-Tropsch catalysts. A large range of insoluble complex cyanides are known in which many metals can be combined, e.g. iron(n) hexacyanide and iron(m) hexacyanide can be combined with iron ions, but also with nickel, cobalt, copper, and zinc ions. Soluble complex ions of molybdenum(iv) which can produce insoluble complexes with metal cations are also known. Deposition precipitation (Section A.2.2.1.5) can be performed by injection of a solution of a soluble cyanide complex of one of the desired metals into a suspension of a suitable support in a solution of a simple salt of the other desired metal. By adjusting the cation composition of the simple salt solution, with a same cyanide, it is possible to adjust the composition of the precursor from a monometallic oxide (the case when the metallic cation is identical to that contained in the complex) to oxides containing one or several foreign elements. 

Hexacyanoferrate(II). K4[Fe(CN)6] has been used to photosensitize Ti02. The acidity constants forH4[Fe(CN)6] are pKi = 2.54, p 2 = T08, pA 3 = 2.65, p 4 = 4.19. Association constants have been pubhshed for alkali metal cations ion pairing with hexacyanoferrate(II). Substitution at hexacyanoferrates(II) is very difficult, though it can be catalyzed by metal ions such as Hg +. Such catalysis can be augmented by surfactants such as sodium dodecyl sulfate (SDS), and indeed SDS-catalysis of Hg +-catalyzed replacement of cyanides in [Fe(CN)6]" ... 

Some compounds contain polyatomic ions that behave much like monatomic anions. Compounds that contain these ions are called pseudobinary ionic compounds. The prefix pseudo- means false these compounds are named as though they were binary compounds. The common examples of such polyatomic anions are the hydroxide ion, OH , and the cyanide ion, CN. The ammonium ion, NH4+, is the common cation that behaves like a simple metal cation. 

This review analyses the vibrational spectra of complex cyanides and relates them to recently obtained structural data. In no structure is there a strong bond, and there may be no bond at all between the CN ligands and the metal cations. The H bonds are often very weak and when they are stronger, as in organic salts, they do not seem to influence the spectra. This invalidates Kharitonov s hypothesis which attempts to ex-... 

A complex in general is any species formed by specific association of molecules or ions by donor-acceptor interactions (see Topic C9). In aqueous solution the most important complexes are those formed between a metal cation and ligands, which may be ions (e.g. halides, cyanide, oxalate) or neutral molecules (e.g. ammonia, pyridine). The ligand acts as a donor and replaces one or more water molecules from the primary solvation sphere, and thus a complex is distinct from an ion pair, which forms through purely electrostatic interactions in solvents of low polarity (see Topic El). Although complex formation is especially characteristic of transition metal ions it is by no means confined to them. 

Complexation of transition metal cations ( Sc, Fe, ) by one cyanide radical ... 

Complexation of Transition Metal Cations by One Cyanide Radical... 

The deposition process of a metal is a special case of electrochemical kinetics. Details can be found in a recently published monography [1]. It is connected with the stepwise transfer of a cation from the electrolyte into the metal layer with its specific crystalline structure (cf. Fig. 1). The metal cation in the plating electrolyte exists as a complex MeL with ligands, either molecules of the solvent (e.g. H2O) or special complexing agents such as ammonia (NH3) or cyanide (CN ). After transport of this complex by diffusion or migration to the electrode (Step 1), it is adsorbed on the electrode surface accompanied by a partial loss of the ligand molecules and a partial reduction (Step 2, Eq. 1)... 

UV absorbing complexes Cyanide Inorganic cations Involves the formation of metal-ligand complexes The resultant negatively charged complexes require an appropriate electrolyte and an EOF modifier... 

Strong acids and strong bases react, and oxidants and reductants react. So, just because a chemical is safe by itself in water, doesn t mean it won t react with something else. Some particularly weU-known examples of never down the sink chemicals are salts of azides (Ns ) and perchlorates (C104 ) that can form explosive compounds with metal cations. Sulfides (S ) and cyanides (CN ) will form poisonous gases in contact with acids. (It is not likely that you ll encounter these anions in introductory courses.)... 

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