Red complex

The normal form in which nickel is weighed in analysis. There is metal-metal bonding in the solid. The red complex is precipitated from alkaline solution. 

If nickel(II) cyanide, Ni(CN)2, is dissolved in excess potassium cyanide, the orange-red complex salt K2Ni(CN)4. HjO can be crystallised out this contains the stable square-planar [Ni(CN)4] anion. 

Nickel also is deterrnined by a volumetric method employing ethylenediaminetetraacetic acid as a titrant. Inductively coupled plasma (ICP) is preferred to determine very low nickel values (see Trace AND RESIDUE ANALYSIS). The classical gravimetric method employing dimethylglyoxime to precipitate nickel as a red complex is used as a precise analytical technique (122). A colorimetric method employing dimethylglyoxime also is available. The classical method of electro deposition is a commonly employed technique to separate nickel in the presence of other metals, notably copper (qv). It is also used to estabhsh caUbration criteria for the spectrophotometric methods. X-ray diffraction often is used to identify nickel in crystalline form. 

Niobic Acid. Niobic acid, Nb20 XH2O, includes all hydrated forms of niobium pentoxide, where the degree of hydration depends on the method of preparation, age, etc. It is a white insoluble precipitate formed by acid hydrolysis of niobates that are prepared by alkaH pyrosulfate, carbonate, or hydroxide fusion base hydrolysis of niobium fluoride solutions or aqueous hydrolysis of chlorides or bromides. When it is formed in the presence of tannin, a volurninous red complex forms. Freshly precipitated niobic acid usually is coUoidal and is peptized by water washing, thus it is difficult to free from traces of electrolyte. Its properties vary with age and reactivity is noticeably diminished on standing for even a few days. It is soluble in concentrated hydrochloric and sulfuric acids but is reprecipitated on dilution and boiling and can be complexed when it is freshly made with oxaHc or tartaric acid. It is soluble in hydrofluoric acid of any concentration. 

Thiocyanates are rather stable to air, oxidation, and dilute nitric acid. Of considerable practical importance are the reactions of thiocyanate with metal cations. Silver, mercury, lead, and cuprous thiocyanates precipitate. Many metals form complexes. The deep red complex of ferric iron with thiocyanate, [Fe(SCN)g] , is an effective iadicator for either ion. Various metal thiocyanate complexes with transition metals can be extracted iato organic solvents. 

Bromide ndIodide. The spectrophotometric determination of trace bromide concentration is based on the bromide catalysis of iodine oxidation to iodate by permanganate in acidic solution. Iodide can also be measured spectrophotometricaHy by selective oxidation to iodine by potassium peroxymonosulfate (KHSO ). The iodine reacts with colorless leucocrystal violet to produce the highly colored leucocrystal violet dye. Greater than 200 mg/L of chloride interferes with the color development. Trace concentrations of iodide are determined by its abiUty to cataly2e ceric ion reduction by arsenous acid. The reduction reaction is stopped at a specific time by the addition of ferrous ammonium sulfate. The ferrous ion is oxidi2ed to ferric ion, which then reacts with thiocyanate to produce a deep red complex. 

Low-spin, octahedral complexes are formed by ligands such as bipy, phen and CN , and their stability is presumably enhanced by the symmetrical configuration. [Fe(bipy)3] + and [Fe(phen)3] + are stable, intensely red complexes, the latter being employed as the redox indicator, ferroin , due to the sharp colour change which occurs when strong oxidizing agents are added to it ... 

Fluorination of NiCl2 -t- KCl produces red K2NiFe which is strongly oxidizing and will liberate O2 from water. Dark red complexes of the type [Ni (L)](C104)2 (H2L is a sexidentate oxime) have been obtained by the action of cone HNO3 on [Ni (H2L)](C104)2 and are stable indefinitely under vacuum but are reduced in moist air. 

In the pH range 7-11, in which the dye itself exhibits a blue colour, many metal ions form red complexes these colours are extremely sensitive, as is shown, for example, by the fact that 10 6 — 10 7 molar solutions of magnesium ion give a distinct red colour with the indicator. From the practical viewpoint, it is more convenient to define the apparent indicator constant K ln, which varies with pH, as ... 

The indicator used is fast sulphon black F which is virtually specific in its colour reaction with copper in ammoniacal solution it forms coloured (red) complexes with only copper and nickel, but the indicator action with nickel is poor. 

Discussion. An excellent method for the colorimetric determination of minute amounts of cobalt is based upon the soluble red complex salt formed when cobalt ions react with an aqueous solution of nitroso-R-salt (sodium 1-nitroso-2-hydroxynaphthalene-3,6-disulphonate). Three moles of the reagent combine with 1 mole of cobalt. 

The acidity of benzylic protons of aromatics complexed to transition-metal groups was first disclosed by Trakanosky and Card with (indane)Cr(CO)3 [61]. Other cases are known with Cr(CO)3 [62], Mn(CO)3 [63], FeCp+ [64, 65], and Fe(arene)2+ [31, 66] but none reported the isolation of deprotonated methyl-substituted complexes. We found that deprotonation of the toluene complex gives an unstable red complex which could be characterized by 13C NMR ( Ch2 = 4.86 ppm vs TMS in CD5CD3) and alkylated by CH3I [58] Eq. (13) ... 

The hexamethylbenzene complex could be similarly deprotonated [54, 57] but the red complex obtained is then stable and its x-ray crystal structure could be recorded [55], showing a dihedral angle of 32° between the cyclohexadienyl plane and the exocyclic double bond (Fig. 5). This complex can also be cleanly obtained by the reaction of dioxygen with the 19e wostructural complex Fe CpfCgMeg) as shown in the preceding section. 

Complexes of molybdenum in the lower valence-states of -t 2 and + 3 have been produced only in the past two years. For the Mo(II) species, the usual starting-material is Mo2(acetate>4. Reaction of this with KS2COEt in THF gives two products, a green complex tentatively assigned as [Mo2(Etxant>4], which solvates to form the red complex [Mo2(Etxant)4(THF)2]. The structure of the latter complex was elucidated by X-ray analysis 169). Steele and Stephenson 170) were also able to synthesize a red, crystalline solid (methanol solution), which they formulated as [Mo(Etxant)2]2 (XI), and reacted this with Lewis bases, e.g., pyridine, to form [Mo(Etxant)2L]2- Thus, there appears to be a difference between the two compounds formulated as [Mo2(Et-xant)2]2 that... 

The rate of oxidation with Ce(IV) perchlorate depends on the method of preparation . The material from certain preparations gives a deep red complex, containing two equivalents of Ce(IV) to one molecule of H2O2, which decomposes in second order fashion-presumably by means of two concerted one-equivalent oxidations of the substrate. Other preparations give no complex and decompose peroxide much faster. The difference is thought to lie in the degree of association of the oxidant cf. the Ce(IV) oxidation of iodide ion, p. 359). 

The reaction of the red complex to give species II is first-order, but the rate is dependent on acidity, viz. 

The method is based on the oxidation of ferrous ions to ferric ions, which form a deep red complex with thiocyanate. 

The dehydrohalogenated product reacts with 85% sulfuric acid to produce a red complex of unknown composition with an absorption maximum at 555 mfi (Figure 1). No other organic insecticide now in use produces any color under similar conditions. Therefore, the method is specific for methoxychlor. Fats and waxes, however, yield strong brown colors which will completely mask the methoxychlor reaction. In the method described this interference has been reduced to a point where it introduces an error of less than 1% when the methoxychlor concentration is between 5 and 50 micrograms. Quantities of methoxychlor of less than 1 microgram may be determined by this method. 

Reaction between IrHCl2(PPh3)2 and Hg(o-tolyl)2 yields the red complex (45), in which the Ir—Hg bond has been confirmed by 31P—199Hg coupling in 31P NMR spectroscopy. Addition of CO and I2 gives [Ir(o-tolyl) Hg(o-tolyl) Cl(CO)(PPh3)2] and [Ir(o-tolyl)Cl(HgI)(PPh3)2], respectively.54... 

The characterization and crystal structure of the dimer [Pt2( -dppm)3] (dppm = bis(diphenyl-phosphino)methane), first reported as a deep red complex in 1978, was described by Manojlovic-Muir et al. in 1986.11 The structure, the first of its type, is made up of two parallel and almost eclipsed trigonal-planar platinum moieties bridged by three diphosphine ligands. The Pf Pt separation is 3.0225(3) A, too long to be considered a bond.11 [Pt2(//-dppm)3] catalyzes the hydrogenation/reduction of carbon dioxide with dimethylamine to give dimethylformamide12 (Equation (1)) and the reverse reaction.13... 

To examine the cavitational effect of ultrasound on bright-red complex of aluminon adsorbed on Al(OH)3, in our experiments, 25 ml of 0.005 M aluminium sulphate was treated with 5 ml of 1% aluminon (triammonium aurine-tricarboxylate). This adsorption complex was sonicated for 10, 20 and 30 min, while the control sample was agitated for the same duration with a magnetic stirrer. The turbidity in sonicated sample increased, as time of sonication increased compared to the unsonicated condition (Table 9.18 and Fig. 9.4). In sonicated sample, the colour of the adsorption complex was dark compared to the control sample and the settlement of the adsorption complex was also slower due to the smaller size particle of the complex. 

The red complex is more dangerously explosive than the ketenide itself, now described as mildly explosive in the dry state when heated strongly or struck [1]. It was previously formulated as a 2 1 complex [2],... 

The orange-red complex formed with oxygen in benzene decomposes vigorously on friction or heating in air. 

W-W distance (2.48A) reflecting the double bond character. Oxidation of 1 by 02, H202, N02 or Ag(I) salts gives dark red complexes (structure ), which are the ditungsten(V) analogs of (W-W dis-... 

The now familiar alternatives of visual and potentiometric detection are available. A number of organic dyes form coloured chelates with many metal ions. These coloured chelates are often discernible to the eye at concentrations of 10 6-10 7 mol dm 3 and can function as visual indicators. Most metal ion indicators will also undergo parallel reactions with protons bringing about similar colour changes. Hence, a careful consideration of pH is prudent when selecting an indicator. Some typical indicators appear in Table 5.9. Of these, eriochrome black T, which forms red complexes with over twenty metal ions, is amongst the most widely used. Its behaviour will serve as a general example of indicator function. 

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