Colour, of complex ions

The colour in transition metal complexes can readily be explained in terms of crystal field theory, which accounts for the colour of complex ions from d-d transitions resulting from the splitting of the d sub-level by the repulsive effect of ligands. The type of splitting depends... 

The discharging of the colour by oxalates, tartrates, etc., appears to be caused by the formation of complex ions with the ferric ions of the ionised ferric thiocyanate, which causes further dissociation of the red non-ionised salt and consequent loss of colour. 

Indicator Colour of free indicator Colour of metal-ion complex... 

The model described here to account for colours of transition metal ions is a simplified version of crystal field theory. The theory is based on the idea that the bonding in complex ions is purely electrostatic and that the ligands behave as point negative charges. The most common type of complex ion is octahedral, where six ligands form an octahedron around the metal ion. 

As a class, the complexes of the lanthanides are both beautifully and delicately coloured. There is a symmetry in the colours of the ions in aqueous solution, a symmetry which seems to be accidental. These colours are given in Table 11.9, listed in a way which shows the colour parallel between f" and f " ions. Also shown are the colours of some organo-lanthanide compounds, which do not show anything like the same pattern, colours which support the idea that the colour pattern of the aqua ions is accidental. 

You cannot fail to notice the striking colours of complexes containing transition metal ions. But how do these colours arise White light is made up of all the colours of the visible spectrum. When a solution containing a transition metal ion in a complex appears coloured, part of the visible spectrum is absorbed by the solution. Flowever, that still doesn t explain why part of the spectrum is absorbed by transition metal ions. To answer this question we must look in more detail at the d orbitals in the ions. 

The d orbital splitting depends on the oxidation state of a given ion hence twb complex ions with the same shape, ligands and coordination number can differ in colour, for example... 

Hydrated chromium(III) sulphate exhibits different colours and different forms from which varying amounts of sulphate ion can be precipitated by barium chloride, due to the formation of sulphato-complexes. Chromium(III) sulphate can form alums. 

The many possible oxidation states of the actinides up to americium make the chemistry of their compounds rather extensive and complicated. Taking plutonium as an example, it exhibits oxidation states of -E 3, -E 4, +5 and -E 6, four being the most stable oxidation state. These states are all known in solution, for example Pu" as Pu ", and Pu as PuOj. PuOl" is analogous to UO , which is the stable uranium ion in solution. Each oxidation state is characterised by a different colour, for example PuOj is pink, but change of oxidation state and disproportionation can occur very readily between the various states. The chemistry in solution is also complicated by the ease of complex formation. However, plutonium can also form compounds such as oxides, carbides, nitrides and anhydrous halides which do not involve reactions in solution. Hence for example, it forms a violet fluoride, PuFj. and a brown fluoride. Pup4 a monoxide, PuO (probably an interstitial compound), and a stable dioxide, PUO2. The dioxide was the first compound of an artificial element to be separated in a weighable amount and the first to be identified by X-ray diffraction methods. 

Tin anodes dissolve by etching corrosion in acid baths based on stannous salts, but in the alkaline stannate bath they undergo transpassive dissolution via an oxide film. In the latter the OH" ion is responsible for both film dissolution and for complexing the tin. Anodes must not be left idle because the film dissolves and thereafter corrosion produces the detrimental divalent stannite oxyanion. Anodes are introduced live at the start of deposition, and transpassive corrosion is established by observing the colour of the film... 

A further factor which must also be taken into consideration from the point of view of the analytical applications of complexes and of complex-formation reactions is the rate of reaction to be analytically useful it is usually required that the reaction be rapid. An important classification of complexes is based upon the rate at which they undergo substitution reactions, and leads to the two groups of labile and inert complexes. The term labile complex is applied to those cases where nucleophilic substitution is complete within the time required for mixing the reagents. Thus, for example, when excess of aqueous ammonia is added to an aqueous solution of copper(II) sulphate, the change in colour from pale to deep blue is instantaneous the rapid replacement of water molecules by ammonia indicates that the Cu(II) ion forms kinetically labile complexes. The term inert is applied to those complexes which undergo slow substitution reactions, i.e. reactions with half-times of the order of hours or even days at room temperature. Thus the Cr(III) ion forms kinetically inert complexes, so that the replacement of water molecules coordinated to Cr(III) by other ligands is a very slow process at room temperature. 

A. Direct titration. The solution containing the metal ion to be determined is buffered to the desired pH (e.g. to PH = 10 with NH4-aq. NH3) and titrated directly with the standard EDTA solution. It may be necessary to prevent precipitation of the hydroxide of the metal (or a basic salt) by the addition of some auxiliary complexing agent, such as tartrate or citrate or triethanolamine. At the equivalence point the magnitude of the concentration of the metal ion being determined decreases abruptly. This is generally determined by the change in colour of a metal indicator or by amperometric, spectrophotometric, or potentiometric methods. 

Dyestuffs which form complexes with specific metal cations can serve as indicators of pM values 1 1-complexes (metal dyestuff = 1 1) are common, but l 2-complexes and 2 1-complexes also occur. The metal ion indicators, like EDTA itself, are chelating agents this implies that the dyestuff molecule possesses several ligand atoms suitably disposed for coordination with a metal atom. They can, of course, equally take up protons, which also produces a colour change metal ion indicators are therefore not only pM but also pH indicators. 

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 ... 

This colour change can be observed with the ions of Mg, Mn, Zn, Cd, Hg, Pb, Cu, Al, Fe, Ti, Co, Ni, and the Pt metals. To maintain the pH constant (ca 10) a buffer mixture is added, and most of the above metals must be kept in solution with the aid of a weak complexing reagent such as ammonia or tartrate. The cations of Cu, Co, Ni, Al, Fe(III), Ti(IV), and certain of the Pt metals form such stable indicator complexes that the dyestuff can no longer be liberated by adding EDTA direct titration of these ions using solochrome black as indicator is therefore impracticable, and the metallic ions are said to block the indicator. However, with Cu, Co, Ni, and Al a back-titration can be carried out, for the rate of reaction of their EDTA complexes with the indicator is extremely slow and it is possible to titrate the excess of EDTA with standard zinc or magnesium ion solution. 

It should be noted that this method is only applicable to solutions containing up to 25 mg copper ions in 100 mL of water if the concentration of Cu2+ ions is too high, the intense blue colour of the copper(II) ammine complex masks the colour change at the end point. The indicator solution must be freshly prepared. 

One of the best oxidation-reduction indicators is the 1,10-phenanthroline-iron(II) complex. The base 1,10-phenanthroline combines readily in solution with iron(II) salts in the molecular ratio 3 base l iron(II) ion forming the intensely red l,10-phenanthroline-iron(II) complex ion with strong oxidising agents the iron(III) complex ion is formed, which has a pale blue colour. The colour change is a very striking one ... 

Mercury(II) thiocyanate method Discussion. This second procedure for the determination of trace amounts of chloride ion depends upon the displacement of thiocyanate ion from mercury(II) thiocyanate by chloride ion in the presence of iron(III) ion a highly coloured iron(III) thiocyanate complex is formed, and the intensity of its colour is proportional to the original chloride ion concentration ... 

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