Class a and b metals

The advantage of this explanation of thiocyanate coordination is that it does allow a description that fits both classes a and b metal complexes, although there are some examples that do not fit. It is not clear Avhy in Rh(PPhg)2L(CNS) the N isomer should be obtained for L = pip, and the S isomer for L = MeCN (35). Manganese(I) has to be treated as a class a acid, the same approach as is used for it to fit with... 

The class (a) and (b) metals are clustered predictably in the periodic table, with intermediate (borderline) metals being found between these clusters. The exact borders for these classes of metals vary in the published literature because the tendencies used to separate the metals are continuous and a discrete classification is partially arbitrary. 

The widely applied Nieboer and Richardson (1980) tabulation of these metal ions classes is summarized in Table 1.1. The general trend in bond stability of the class (a) metal ions with various ligand donor atoms is O > N > S and that for class (b) metal ions is S>N>0 (Nieboer and Richardson 1980). Borderline metal ions are more complex, having binding tendencies intermediate between class (a) and (b) metals. Interactions between the hard class (a) metal ions and ligands tend to be ionic in... 

Ahrland et al. (1958) classified a number of Lewis acids as of (a) or (b) type based on the relative affinities for various ions of the ligand atoms. The sequence of stability of complexes is different for classes (a) and (b). With acceptor metal ions of class (a), the affinities of the halide ions lie in the sequence F > Cl > Br > I , whereas with class (b), the sequence is F < Cl" < Br < I . Pearson (1963, 1968) classified acids and bases as hard (class (a)), soft (class (b)) and borderline (Table 1.23). Class (a) acids prefer to link with hard bases, whereas class (b) acids prefer soft bases. Yamada and Tanaka (1975) proposed a softness parameter of metal ions, on the basis of the parameters En (electron donor constant) and H (basicity constant) given by Edwards (1954) (Table 1.24). The softness parameter a is given by a/ a - - P), where a and p are constants characteristic of metal ions. 

In their review of the classification of donors and acceptors in inorganic reactions, Williams and Hale (7) pointed out that for reactions in water, class (a) character was exhibited most strongly by lithium and least by caesium, which was indeterminate between classes (a) and (b). Here class (a) character means that the fluoride is more stable in water than the iodide. In general Group IA metals prefer hard ligands, F, O, N their interaction with sulphur and carbon is considered in para. IV. 

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... 

Some polyatomic ions such as and have very low complexing power to either class a or b metals. They are useful as counterions for studying the thermodynamic properties of metal ions (e.g. electrode potentials see Topic E5) unaffected by complex formation. 

The foundations for the concept of chemical hardness lie in the works of Schwarzenbach [1] and Chatt [2]. Independently, they showed that metal ions could be divided into two classes, (a) and (b), depending on the relative affinities for ligands with various donor atoms. 

In terms of the ability of metallic ions to form complexes, Schwarzenbach distinguished two categories classes A and B. 

Species (A) and (B) constitute the main class of unsaturated carbenes and play important roles as reactive intermediates due to the very electron-deficient carbon Cl [1]. Once they are coordinated with an electron-rich transition metal, metal vinylidene (C) and allenylidene (D) complexes are formed (Scheme 4.1). Since the first example of mononuclear vinylidene complexes was reported by King and Saran in 1972 [2] and isolated and structurally characterized by Ibers and Kirchner in 1974 [3], transition metal vinylidene and allenylidene complexes have attracted considerable interest because of their role in carbon-heteroatom and carbon-carbon bond-forming reactions as well as alkene and enyne metathesis [4]. Over the last three decades, many reviews [4—18] have been contributed on various aspects of the chemistry of metal vinylidene and allenylidene complexes. A number of theoretical studies have also been carried out [19-43]. However, a review of the theoretical aspects of the metal vinylidene and allenylidene complexes is very limited [44]. This chapter will cover theoretical aspects of metal vinylidene and allenylidene complexes. The following aspects vdll be reviewed ... 

Thermolysin belongs to a class of proteases (called neutral proteases) which are distinct from the serine proteases, sulfhydryl proteases, metal-loexopeptidases, and acid proteases. Neutral proteases A and B from Bacillus subtilis resemble thermolysin in molecular weight, substrate specificity, amino acid content, and metal ion dependence. Since physiological substrates are most likely proteins, it is difficult to design simple experiments that can be interpreted in terms of substrate specificity and relative velocities. Therefore, studies of substrate specificity and other kinetic parameters must be carried out on di- and tripeptides so that details of the mechanism of catalysis can be obtained and interpreted simply. 

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