Colour point groups

More complex physical properties may require the specification of three or more colours . In this case the general term colour symmetry is used, and the lattices and point groups so derived are the colour lattices and colour point groups. 

Figure 2.7 Projection of the great rhombicuboc-tahedron 48-vertex regular orbit cage of the 0[, point group. The rotational axes are distinguished as [C4], [C3] and [C2] coloured circles on the unit sphere. The 48 vertices are divided into sets of 8 (a), 6 (b) and 4 (c) about these axes using the colour coding displayed in the diagrams.

Fig. 4.19 Point groups of octahedral metal carbonyl complexes M(CO)5, M(CO)5X, trawv-M(CO)4X2 and cw-M(CO)4X2. Colour code metal M, green C, grey O, red group X, brown.

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. 

In acid-base titrations the end point is generally detected by a pH-sensitive indicator. In the EDTA titration a metal ion-sensitive indicator (abbreviated, to metal indicator or metal-ion indicator) is often employed to detect changes of pM. Such indicators (which contain types of chelate groupings and generally possess resonance systems typical of dyestuffs) form complexes with specific metal ions, which differ in colour from the free indicator and produce a sudden colour change at the equivalence point. The end point of the titration can also be evaluated by other methods including potentiometric, amperometric, and spectrophotometric techniques. 

Some examples of metal ion indicators. Numerous compounds have been proposed for use as pM indicators a selected few of these will be described. Where applicable, Colour Index (C.I.) references are given.12 It has been pointed out by West,11 that apart from a few miscellaneous compounds, the important visual metallochromic indicators fall into three main groups (a) hydroxyazo compounds (b) phenolic compounds and hydroxy-substituted triphenylmethane compounds (c) compounds containing an aminomethyldicarboxymethyl group many of these are also triphenylmethane compounds. 

It was pointed out in Chapter 1 that, after the azo class, anthraquinone derivatives form the next most important group of organic colorants listed in the Colour Index. The major application groups are vat dyes, disperse dyes and acid dyes (Table 1.1). 

Colour and Structure. The azo chromogen is one of the many that can be described as falling within the donor-acceptor group. The donor-acceptor system is shown in Figure 2.7 with a simple mono azo dye. Cl Disperse Red 1, to illustrate the point. The donor part of the molecule, as its name implies, contains donor groups such as amino and alkylamino, hydroxy and aUcoxy. Conversely, the acceptor part contains elec-non-acceptor groups such as nitto, cyano etc. These donor and acceptor groups may... 

Cr(XH3)s(XO.,)]R .—These salts were discovered by Christensen in 1SS1. It is still doubtful whether the salts are true nitrito-salts with the group O—NO in the nucleus, or whether they are nitro-compounds they are yellow in colour, which points to the possibility that they are nitro-eompounds. Only one series is known, whereas in the eobalt-ammino-compounds both nitro- and nitrito-salts have been isolated (see p. 144). 

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