Nickel tetracyanonickelate

Silver halides can be dissolved in a solution of potassium tetracyanonickelate(II) in the presence of an ammonia-ammonium chloride buffer, and the nickel ion set free may be titrated with standard EDTA using murexide as indicator. 

The procedure involved in the determination of these anions is virtually that discussed in Section 10.58 for the indirect determination of silver. The anion to be determined is precipitated as the silver salt the precipitate is collected and dissolved in a solution of potassium tetracyanonickelate(II) in the presence of an ammonia/ammonium chloride buffer. Nickel ions are liberated and titrated with standard EDTA solution using murexide as indicator ... 

Alkali metal salts of such tetracyanonickelate(II) anion may be crystallized from such solutions as hydrates, K2 [Ni(CN)4 3H2O upon evaporation of the solution. In strong cyanide solution, a pentacyano complex anion, red penta-cyanonickelate(ll), [Ni(CN)5] forms. Strong acids decompose cyanonickelate salts, precipitating nickel cyanide. 

The tetracyanonickelate(II) ion is the most extensively studied square planar complex of nickel(II). TTie electronic spectrum is characterized by d-d bands at 31 000-32 000 cm-1 (eM 500-800) and charge transfer bands (eM = 6000-15 000) at 33 000-37 000 cm-1. In Figure 22 the electronic and MCD spectra of K2Ni(CN)4 are shown. In the MCD spectrum of the low energy band (d-d), since C terms are zero because the ground state is lAlg, the presence of the... 

If the solution of tetracyanonickelate(II) is heated with sodium hypobromite solution (prepared in situ by adding bromine water to sodium hydroxide solution), the complex decomposes and a black nickel(III) hydroxide precipitate is formed (difference from cobalt ions) ... 

An excess of potassium tetracyanonickelate(II) is necessary to prevent reduction to potassium tetracyanonickelate(O). The desired nickel (I) complex is the only reduction product if at least a twofold excess of the nickel(II) complex is present. 

The exchange of cyanide ion between the Ni(CN)4 ion and the bulk solution was complete within 30 seconds, which was too fast to be followed properly by the method used. A study of the Ni - Ni(CN)4 exchange was complicated by formation of the precipitate (Ni(CN)2) in the mixture of nickel(II) and tetracyanonickelate ions. The results indicated two non-equivalent metal ions in the solid, which confirmed that its structure is indeed Ni[Ni(CN)4]. The nickel(II) exchange between Ni(CN)4 and four nickel(II) complexes indicated a direct bimolecular interaction. 

Nishikiori. S. Takahashi-Ebisudani. Y. Iwamoto, T. Novel series of clathrate compounds of the three-dimensional metal complex hosts (yV-methyl-l,3-diaminopropane) cad-mium(II) tetracyanonickelate(II), (V,V-dimethyl-1,3-diaminopropane) cadmium(II) tetracyanonickelate(II) and (2-hydroxyethylinethylamine) cadmium(II) tetracyano-nickelate(II). J. Inclusion Phenom. Mol. Recognit. Chem. 1990, 9(2), 101-112. 

Potassium tetracyanonickelate(II) may be prepared by adding an excess of potassium cyanide to a solution of a soluble nickel salt and bringing the solution to crystallization. However, the preliminary precipitation of nickel... 

Potassium tetracyanonickelate(II) crystallizes in orange-red monocliniccrystals that have one molecule of water of hydration. This water is lost when the compoimd is dried at 100°. The salt is extremely soluble, even in cold water. It is decomposed by the action of the mineral acids with precipitation of nickel cyanide. Potassium tetracyanonickelate(II) is readily decomposed by the addition of sodium hypobromite, whence hydrated nickel peroxide is thrown down as a black precipitate. 

Because the complex is an anion, the metal is indicated as nickelate(O). Putting these parts together and naming the cation first, we have potassimn tetracyanonickelate(O). 

THE CRYSTAL STRUCTURES OF a, co-DIAMINOALKANECADMIUM(II) TETRACYANO-NICKELATE(II)-AROMATIC MOLECULE INCLUSION COMPOUNDS. I. 1,4-DIAMINO-BUTANE CADMIUM( I I) TETRACYANONICKELATE(II)-2,5-XYLIDINE (1/1) ... 

Nickel is found in the rare +1 oxidation state in Rb4[Ni2(CN)J. There are two crystallographically independent [Nij(CN)6] anions which contain planar Ni(CN)a units linked together by a strong Ni-Ni bond, so that the co-ordination at each nickel atom is approximately square-planar. The mean angle between the nickel co-ordination planes is 86°. The two Ni-Ni distances are 2.32 and 2.29 A, not significantly different from twice the Pauling covalent radius of nickel (2.30 A). The mean Ni-C distance is 1.91 A. Two crystal structures containing the tetracyanonickelate anion have been described. - ... 

The compound as initially formulated by Hofmann was Ni(CN)2 NH3 C6H6. However, it is now recognized that the actual formulation is Ni(NH3)2 Ni(CN)4 2C6H6. There are two kinds of nickel atoms, one that exhibits square planar four-coordination [tetracyanonickelate(II)] and one that exhibits an octahedral array of nitrogen atoms around the nickel. The host has a layered structure, with the NH3 units protruding into the layers. The cavities thus formed are illustrated in Fig. 3. The benzene is completely enclosed, and the compound is a true clathrate. Because of the limitation in space, only molecules smaller than substituted benzenes can be entrapped. 

C12H67FO20S, Tri-n-butylsulphonium fluoride hydrate, 27, 781 Cl 2H88Ni,026 f n-Propylamine clathrate hydrate, 39B, 466 Cl2H118N4O41, Trimethylamine clathrate hydrate, 33B, 260 Cl3H22FeN6N3, Ferrocene - thiourea clathrate, 44B, 571 Cl4H18CdN8Ni, catena-M Ethylenediaminecadmium(lI) catena-tetra-M cy-ano-nickelate(II) - bis(pyrrole), 41B, 701 ClgHigCdNeNi, Diamminecadmium tetracyanonickelate benzene clathrate, 40B, 1096... 

The second approach is much like the preceding method. The precipitate in which the anion to be determined is engaged is treated with a reagent that stoichiometri-cally liberates a cation that is titrated with a standard solution of EDTA. Hence, a halide (except the fluoride) or the thiocyanate ion is precipitated as the corresponding silver salt. The latter is treated with a solution of potassium tetracyanonickelate K2 [Ni(CN)4]. The silver salt dissolves into this solution with liberation of one nickel ion Ni + for two ions X . Ni + is titrated with EDTA in the presence of murexide. The reaction constituting the basis of the process is... 

Cyanide ions yield a green precipitate of nickel cyanide Ni(CN)2 soluble in excess of cyanide ions by the formation of the tetracyanonickelate(II) complex (or nickelo-cyanide anion) [Ni(CN)4] . The complex is moderately stable. It is decomposed by adding mineral acids, which liberate hydrocyanic acid and precipitate nickel cyanide. In a second stage, nickel cyanide itself decomposes into Ni + and hydrocyanic acid ... 

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