Class A metal

With the exception of actinium, which is found naturally only in traces in uranium ores, these elements are by no means rare though they were once thought to be so Sc 25, Y 31, La 35 ppm of the earth s crustal rocks, (cf. Co 29ppm). This was, no doubt, at least partly because of the considerable difficulty experienced in separating them from other constituent rare earths. As might be expected for class-a metals, in most of their minerals they are associated with oxoanions such as phosphate, silicate and to a lesser extent carbonate. 

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. 

Until recently, there was little published material dealing with the chemistry of the Group V triad. This agrees with their classification as "class a metal ions. Using new synthetic routes similar to those for the preparation of the titanium complexes, however, a number of V(III), V(IV), V(VI), Nb(IV), Nb(V), Ta(IV), and Ta(V) complexes have been prepared (,54). 

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. 

The mechanisms proposed over the last 50 years for the Fischer-Tropsch synthesis, principally on the basis of studies using heterogeneous catalyst systems, may be divided into three main classes (a) metal-carbide mechanisms (b) hydroxyl carbene, =CH(OH), condensation mechanisms and (c) CO insertion mechanisms. 

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. 

Exactly why this is, we are not certain. One can simply say that 0H is a hard base, whereas (Pt(II) is a class A metal or soft acid, and this hard-soft combination is unstable. It is also possible to suggest that the repulsive interaction between the filled d-orbitals on Pt(II) and the filled -orbitals on 0H make it a poor reagent. The same is true of F which is also a poor reagent. Other halide ions have low energy, vacant d-orbitals which can accept electrons and decrease the effect of the filled -orbitals. This makes these halide ions better reagents than F . 

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... 

As a result of there being so many kinds of interactions that involve the donation and acceptance of electrons, the electronic theory of acids and bases pervades the whole of chemistry. In the 1950s, Ahrland, Chatt, and Davies had classified metals as class A metals... 

Class A metal ions have the following preferences for ligands ... 

Manganese(II) is quite distinctly a class (a) metal This is manifest, for example, in the preference for O donor rather than N donor ligands, so that amino acid compounds become simply those of substituted carboxylates, and there is currently little evidence for the N bonding of peptides that leads to the interesting chemistry of the peptide compounds of the later transition metals. Of P and As ligands but little is known, although recent work suggests the possibility that this may become... 

Table 4.1. Experimental values of

This is the dominant oxidation state and the most stable. In neutral or acidic aqueous solution, very pale pink [Mn(H20)6] + is present, which is resistant to oxidation, while in basic media, the hydroxide Mn(OH)2 formed is easily oxidized by air (Figure 1). Manganese (II) is a hard acid and is quite distinctly a class (a) metal. This is manifest, for example, in the preference for O donors rather than N-donor... 

Later workers have correlated the classification of elements in class (a) and class (b) with Pearson s principle of hard and soft acids and bases (HSAB principle) (see Hard Soft Acids and Bases) on the basis that class (a) metal ions are hard acids and class (b) are soft acids. Borderline elements in the Ahrland-Chart-Davies classification tend to be harder in the higher oxidation states and softer in their lower oxidation states. 

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.

In view of the foregoing situation and the uncertainties of whether or not O-cyanates do indeed exist or whether all the many A-cyanato complexes have been correctly characterized, any explanations concerning the behavior of the cyanate group would be premature. INDO calculations showed that almost equal electron densities existed at nitrogen and oxygen (383), so there is no apparent reason why both ends should not be involved in coordination. Attention has been drawn (86) to the fact that many of the metals found to form O-cyanates have vacant or only partly filled dn orbitals to interact with filled tt orbitals of the cyanate group. Whether these are centered on the oxygen or whether that atom forms the better a bond with these mostly class a metals remains to be seen. 

The preceding sections have shown that the behavior of the thiocyanate and selenocyanate groups in homogeneous complex anions is identical in both cases the nitrogen atom coordinates to the class a metal, and the sulfur or selenium atoms coordinate to the class b metal. Although fewer comparable cyanate complexes are known, it is quite clear that the coordination of the cyanate group is not governed by the same factors. 

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