Class B metals

Gold Compounds. The chemistry of nonmetallic gold is predominandy that of Au(I) and Au(III) compounds and complexes. In the former, coordination number two and linear stereochemistry are most common. The majority of known Au(III) compounds are four coordinate and have square planar configurations. In both of these common oxidation states, gold preferably bonds to large polarizable ligands and, therefore, is termed a class b metal or soft acid. 

There is also clear evidence of a change from predominantly class-a to class-b metal charactristics (p. 909) in passing down this group. Whereas cobalt(III) forms few complexes with the heavier donor atoms of Groups 15 and 16, rhodium(III), and more especially iridium (III), coordinate readily with P-, As- and S-donor ligands. Compounds with Se- and even Te- are also known. Thus infrared. X-ray and nmr studies show that, in complexes such as [Co(NH3)4(NCS)2]" ", the NCS acts as an A -donor ligand, whereas in [M(SCN)6] (M = Rh, Ir) it is an 5-donor. Likewise in the hexahalogeno complex anions, [MX ] ", cobalt forms only that with fluoride, whereas rhodium forms them with all the halides except iodide, and iridium forms them with all except fluoride. 

The nature of the donor atoms in the chelating agent. Ligands which contain donor atoms of the soft-base type form their most stable complexes with the relatively small group of Class B metal ions (i.e. soft acids) and are thus more selective reagents. This is illustrated by the reagent diphenylthiocarbazone (dithizone) used for the solvent extraction of metal ions such as Pd2+, Ag+, Hg2+, Cu2+, Bi3+, Pb2+, and Zn2 +. ... 

Ligands which form stronger complexes with Class (a) metals are described as hard and those which form stronger complexes with Class (b) metals are called soft. Hard metals form more stable complexes with hard ligands and soft metals form more stable complexes with soft ligands. A listing of hard and soft metals and ligands is presented in Table 9-4. 

Metallation and oxymetallation reactions have been observed with the salts of only a few metals, namely mercury(II) (66, 67), thallium(III) (66,67), lead(IV) (66, 67), palladium(II) (100), gold(III) (63), and platinum-(II) (29). These facts correlate well with what Chatt (1) has termed class b, and Pearson (130) has called "soft acid character. Soft acids are characterized by low charge, large size, and, often, d electrons in their outer shell. No class b metal is known, in fact, which contains fewer than five d... 

However, with soft electron pair acceptors such as Pt2+, Ag+, and Ir+, phosphines are stronger Lewis bases than are NH3 and amines, so phosphines and arsines interact better with class B metals than do amines. Generally, phosphines and arsines form stable complexes with second- and third-row transition metals in low oxidation states. 

Many observations concerning these trends had been made over the years, and in the 1950s S. Ahrland, J. J. Chatt, and M. Davies presented a classification of metals based on their preferred interaction with donor atoms. Class A metals are those that interact preferentially when the donor atom is in the first row of the periodic table. For example, they prefer to bond to N rather than P donor atoms. Class B metals are those which interact better when the donor atom is in the second row of the periodic table. For example, a class B metal would bond better to P than to N. The following table summarizes the behavior of metal atoms according to this classification. 

Most terrestrial invertebrates have limited access to water and feed on solid matter. As a consequence, they take up most of their nutrients by ingestion of foodstuffs that are also the vehicle for ingestion of contaminants. Many of the class a , metals that are taken up are found in membrane-bound granules in the cells of the hepatopancreas, although uncertainties remain as to the initiation of granule formation. Other metals, such as the class b metal cadmium, may be in the granule or may be bound to a metallothionein type protein. 

Complexes of class a metals are more ionic, while those of the class b metals are more covalent. Generally, the metals that form tetrahedral complexes by using sp hybrid orbitals are class a types. Those forming square planar complexes by using dsp hybrid orbitals are normally class b types. 

Sekido, E., Saito, K., Naganuma, Y., and Kumazaki, H. (1985) Liquid-liquid extraction of some class B metal ions with thiacrown ether 1,4,8,11 -tetrathiacyclotetradecane, A nal. Sci, 1(4), 363-368. 

For the following limited discussion of ions in water it is advisable to define class (a) and class (b) in the earliest historical sense. Class (a) metal ions form halides whose stability in water is of the order MFn > MCln > MBrn > MI . Class (b) metal ions form halides whose stability is in the reverse order. Table I classifies metal ions in this way. This definition clearly leaves out of consideration (on experimental grounds) acceptor properties, especially of neutral species, which could not be studied in this way and to which we return later. We now need some quantitative experimental assessment of the degree of class (a) or (b) character. For simplicity we shall use —AG°aq/per ligand for the first group of more or... 

The heat of ionization in aqueous solution, AH q, represents the enthalpy change for the following reaction M(+anq) -f- nX<-aq) = MXn(aq). Although much AHaq data exist for class (b) metal chlorides, bromides and iodides, few data are available for class (b) fluorides and class (a) halides in general. This is because MXn(aq) in these cases is not a stable species. It is therefore difficult to compare class (a) and (b) halides in aqueous solution in a manner which is entirely consistent with AHion(g). It is easy to show, however, that in aqueous solution most metal ions, which are class (b) by... 

From Table 3 it is apparent that Hg has the order MI2 > MBr2 > > MC12, and is therefore clearly a class (b) metal. Sr has the reverse order and is class (a). (It should be pointed out that any consideration of the entropy change for SrX2[Pg.257]

Class B metal ions exhibit the opposite preference for ligands ... 

Modifying the Active Conformation of Biomolecules. Class B metals such as Hg have been shown to alter the steric conformation of proteins via interactions with sulfur atoms, particularly disulphide bonds. For example, Hg insertion between the two sulfurs involved in a disulfide bond can significantly alter the shape of the protein and reduce or abolish its activity. This has been demonstrated with ribonuclease, for example. 

Table 4.1. Experimental values of

The observation 571) that class a metals apparently show a different solvent dependence of thiocyanate coordination to class b metals was discounted by Marzilli 51 ) on the grounds that cobalt(III) in these systems has become sufficiently soft for it to be regarded as similar to palladium(II). The problem of the relative hardness or softness of metals was discussed in the previous section of this review from which it is clear that the association of cobalt(III) with palladium(II) in these terms must be viewed with extreme caution. 

Table XLI summarizes the structures of known, homogeneous, anionic selenocyanate complexes. Although fewer examples exist than for the corresponding thiocyanate complexes, it is apparent that a similar pattern exists for the two sets of complexes. Class a metals are coordinated by the nitrogen atom, whereas selenium is the donor for the class b metals. In mixed metal complexes containing bridging seleno-oyanate groups the nitrogen atom coordinates to the harder or class a metal, and the selenium atom bonds to the softer or class b metal, as was observed with the corresponding thiocyanate complexes.

---