Iron ammine complexes

Direct Titrations. The most convenient and simplest manner is the measured addition of a standard chelon solution to the sample solution (brought to the proper conditions of pH, buffer, etc.) until the metal ion is stoichiometrically chelated. Auxiliary complexing agents such as citrate, tartrate, or triethanolamine are added, if necessary, to prevent the precipitation of metal hydroxides or basic salts at the optimum pH for titration. Eor example, tartrate is added in the direct titration of lead. If a pH range of 9 to 10 is suitable, a buffer of ammonia and ammonium chloride is often added in relatively concentrated form, both to adjust the pH and to supply ammonia as an auxiliary complexing agent for those metal ions which form ammine complexes. A few metals, notably iron(III), bismuth, and thorium, are titrated in acid solution. 

Good separation from iron is achieved by formation of solutions of stable Ni11 and Co11 ammine complexes, whilst any Fe11 leached is oxidized and precipitates as Fem oxyhy dr oxides. 

At high initial [Fe2+] in the absence of added substrates, the stoichiometric ratio [Fe3+]oo/[L(H20)Rh00H2+]0 approaches 2.0. At lower concentrations of Fe2+, the overall reaction produces less Fe2+ because some of the newly formed L(H20)Rh02+ decomposes, presumably by loss of NH3 from the ammine complex and intramolecular ligand oxidation in the macrocyclic compounds, as observed for similar complexes of high-valent nickel, cobalt, and iron (58,118-120). Competition experiments were carried out at sufficiently high [Fe2+] to ensure that no L(H20)Rh02 + was lost in self-decay. 

Evidently a low ammonia concentration (0.003 Af) and a high ammonium chloride concentration (1.5 to 2 M) are favorable for efficient separation of metal ions that do not form ammine complexes (Ca" ", Mg" ). In fact, a single precipitation has been found to be effective under these conditions.To separate iron(III) from copper, zinc, and nickel, high concentrations of both ammonia and ammonium chloride produced excellent results in a single precipitation. 

It is observed in the experiment that the iron nail immediately creates a copper deposit in a blue colored copper sulfate solution (see E8.1), whereby this does not happen in the violet colored ammine complex solution. A trace of copper deposit can only be observed after it has been dipped into the complex solution for a while (see E9.6). It is possible to verify this hypothesis with the help of a second reaction, the metal hydroxide precipitation (see E9.6) a greenish blue deposit is commonly observed in the blue solution of hexaaquacopper ions, but not in the solution of tetraamminecopper ions. Apparently, copper ions and water molecules are not very tightly bonded in aqua complexes, but copper ions and ammonia molecules in ammine complexes are there is a weak stability of aquacopper ions, but a great stability of tetraamminecopper complexes. The stability constants can be taken and interpreted if one wants a quantitative explanation of these phenomena. 

Halo-ammine complexes show halide - Ru charge transfer bands which have been characterized by circular dichroism studies. Calculations predict that the Ru—N bond strength increases in the order ciHRuX2(NH3)4]+< [RuX(NH3)5] +< [Ru(NH3)50H2] +< [Ru(NH3)6] ", with Ru—NHj Irons to X weaker than when cis to X. The magnetic and ESR properties of IRuCl(NH3)5]Cl2 have been reiwited. - ... 

Asare [50-52] studied the adsorption of Cu(H), Ni(II), and Co(II) onto titania, hematite, alumina, and quartz in ammoniacal solutions and found that the conventional sigmoidal adsorption curve was replaced by an adsorption profile that increased initially with increased pH, declined in adsorption as the ammine complexes formed, and then increased at high pH as the hydroxide ligands replaced the ammonia ligands. This effect was also reported by Luo and Huang [53], who studied Cu(H) adsorption onto iron(III), aluminum(Hl), and tin(IV) oxides in ammonia solution for the pH range 5-9. 

The most important publications dealing with multicomponent systems were those of Jannik BJerrum on copper(II)-ammine complexes (15-17) and of Holler on iron(III)- thiocyanato complexes (18). 

In the reaction with superoxo-complexes of the type [LCo-/A(NH2,Oa)Co-L] + where L = (NHa)4, (bipy>2, or (phen>2 and with [(Ha sCo Oa Co-(NH3)6] +, the 1 1 redox reactions take place via the a-form, the jS-complex accounting for less than 5% of the rate constant, although the proton in the latter form might have been expected to help in the bridging of the O2" to the 02 in the electron-transfer step. Where there is the possibility of electron delocalization in the ligand system, the rate of transfer is substantially faster than in the ammine complexes, an order which parallels the iron(n) reductions of these superoxo-complexes. 

Iron(III).— The reactions of iron(ni) with platinum(n) ammine complexes in the presence of halide ions have been described ... 

Because ammine ligands are neutral molecules, the oxidation state of each metal is the same as the charge on the complex. Iron loses two of its eight valence electrons to reach the +2 oxidation state, leaving six electrons for the d orbitals. Likewise, cobalt in its +3 oxidation state has six d electrons. 

After the resolution of 1-2-chloro-ammino-diethylenediamino-cobaltie chloride many analogous resolutions of optically active compounds of octahedral symmetry were carried out, and active isomers of substances containing central cobalt, chromium, platinum, rhodium, iron atoms are known. The asymmetry is not confined to ammines alone, but is found in salts of complex type for example, potassium tri-oxalato-chromium, [Cr(Ca04)3]K3, exists in two optically active forms. These forms were separated by Werner2 by means of the base strychnine. More than forty series of compounds possessing octahedral symmetry have been proved to exist in optically active forms, so that the spatial configuration for co-ordination number six is firmly established. 

Although iron, cobalt, and nickel occur in the same triad in Group VIII., the three elements differ considerably in their ability to form addition compounds with ammonia. Iron forms few ammino-salts, most of which are unstable, and its tendency to complex-salt formation of the ammine type appears in the complex cyanides and not in the ammines themselves. 

In the ammines, In valent metals, such as cobalt(lll) and cliromuim(lll) and iron(lII), possess a coordination number of 6. this number being the sum of tlie unit replacements on the metal in tlie complex ion. Since a regular octahedron has six corners equidistant from the center, it is assumed that the metal occupies tlie center and each of the six replacing groups... 

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