Soft electron donors

Although the subject of stability of complexes will be discussed in greater detail in Chapter 19 it is appropriate to note here some of the general characteristics of the metal-ligand bond. One of the most relevant principles in this consideration is the hard-soft interaction principle. Metal-ligand bonds are acid-base interactions in the Lewis sense, so the principles discussed in Sections 9.6 and 9.8 apply to these interactions. Soft electron donors in which the donor atom is sulfur or phosphorus form more stable complexes with soft metal ions such as Pt2+ or Ag+, or with metal atoms. Hard electron donors such as H20, NH3( or F generally form stable complexes with hard metal ions like Cr3+ or Co3+. 

The thiocyanate ion provides an interesting test of these ideas. In the SCN ion, the sulfur atom is considered to be a soft electron donor, whereas the nitrogen atom is a much harder electron donor. Accordingly, Pt2+ bonds to SCN- at the sulfur atom, whereas Cr3+ bonds to the nitrogen atom. Uncharged metal atoms are considered to be soft electron acceptors, and they form complexes with soft ligands such as CO, 11, and PR3. We will see many examples of such complexes in later chapters. On the other hand, we would not expect complexes between uncharged metal atoms and NH3 to be stable. 

The strength of the complexation is a function of both the donor atom and the metal ion. The solvent medium is also an important factor because solvent molecules that are potential electron donors can compete for the Lewis acid. Qualitative predictions about the strength of donor-acceptor complexation can be made on the basis of the hard-soft-acid-base concept (see Section 1.2.3). The better matched the donor and acceptor, the stronger is the complexation. Scheme 4.3 gives an ordering of hardness and softness for some neutral and ionic Lewis acids and bases. 

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. 

Earlier it was described how PH3 is a much weaker base than NH3. That is certainly true when the interaction of these molecules with H+ is considered. However, if the electron pair acceptor is Pt2+, the situation is quite different. In this case, the Pt2+ ion is large and has a low charge, so it is considered to be a soft (polarizable) Lewis acid. Interaction between Pt2+ and PH3 provides a more stable bond that when NH3 bonds to Pt2+. In other words, the soft electron acceptor, Pt2+, bonds better to the softer electron donor, PH3, than it does to NH3. The hard-soft interaction principle does not say that soft Lewis acids will not interact with hard Lewis bases. In fact, they will interact, but this is not the most favored type of interaction. 

The behavior of 3 toward ether or amines on the one hand and toward phosphines, carbon monoxide, and COD on the other (Scheme 2), can be qualitatively explained on the basis of the HSAB concept4 (58). The decomposition of 3 by ethers or amines is then seen as the displacement of the halide anion as a weak hard base from its acid-base complex (3). On the other hand, CO, PR3, and olefins are soft bases and do not decompose (3) instead, complexation to the nickel atom occurs. The behavior of complexes 3 and 4 toward different kinds of electron donors explains in part why they are highly active as catalysts for the oligomerization of olefins in contrast to the dimeric ir-allylnickel halides (1) which show low catalytic activity. One of the functions of the Lewis acid is to remove charge from the nickel, thereby increasing the affinity of the nickel atom for soft donors such as CO, PR3, etc., and for substrate olefin molecules. A second possibility, an increase in reactivity of the nickel-carbon and nickel-hydrogen bonds toward complexed olefins, has as yet found no direct experimental support. 

E = electron donor constant of L y = a constant, characteristic of the metal and related to a softness parameter... 

The principle of hard and soft Lewis acids and bases, proposed by Pearson (1963), is useful to describe these reactions. A Lewis acid is any chemical species that employs an empty electronic orbital available for reaction, while a Lewis base is any chemical species that employs a doubly occupied electronic orbital in a reaction. Lewis acids and bases can be neutral molecules, simple or complex ions, or neutral or charged macromolecules. The proton and all metal cations of interest in subsurface aqueous solutions are Lewis acids. Lewis bases include H, O, oxyanions, and organic N, S, and P electron donors. A list of selected hard and soft Lewis acids and bases found in soil solutions is presented in Table 6.1. 

Neutral extracting agents possessing oxygen-donor atoms (hard bases) in their structure easily coordinate trivalent lanthanide and actinide cations, but do not discriminate between the two families of elements, because the ion-dipole (or ion-induced dipole type) interactions mostly rely on the charge densities of the electron donor and acceptor atoms. As a result, the similar cation radii of some An(III) and Ln(III) and the constriction of the cation radius along the two series of /elements make An(III)/Ln(III) separation essentially impossible from nitric acid media. They can be separated, however, if soft-donor anions, such as thiocyanates, SCN-, are introduced in the feed (34, 35, 39, 77). 

Conversely, neutral extracting agents possessing nitrogen electron-donor atoms in their structure (soft bases) will easily discriminate between An(III) and Ln(III) even from nitric acid feeds, thanks to covalently hinted An(III)-N interactions, the best example being the terdentate bis-triazinyl-pyridines (BTPs) or the tetradentate bis-triazinyl-bis-pyridines (BTBP). 

The idea of hard and soft acids and bases (HSAB) is a development of the concept of Lewis acids and bases, so that acids are electron acceptors and bases are electron donors. 

The products of acid-base interactions such as those shown in Equations (5.36) through (5.39) are not properly considered as salts because they are not ionic compounds. Because in many cases these products are formed from two neutral molecules, they are more properly considered as addition compounds or adducts held together by the formation of coordinate covalent bonds. In that connection, they are similar to coordination compounds except that the latter ordinarily involve the formation of coordinate bonds to metal ions by the electron donors ligands). There are some useful generalizations that correlate to the stability of bonds during this type of acid-base interaction, and these are largely summarized by the hard-soft acid-base principle. 

Trihalides of the Group VA elements are pyramidal (C3v) with an unshared pair of electrons on the central atom. Typically, the molecules are Lewis bases, and they form acid-base adducts and metal complexes. In accord with the hard-soft interaction principle, these species are better electron pair donors toward soft electron pair acceptors. Therefore, most of the complexes of these EX3 molecules contain second and third row transition metals or first row metals in low oxidation states. 

The thermodynamics and kinetics of NCS-, CN-,871 Cl-, Br-, NCS- and I- 872 substitutions at [Rh(TPPS)(H2 O)] have been reported the reactions involved are shown in equation (169), and the parameters determined are summarized in Table 60. Spectrophotometric titrations showed two inflection points as OH- is added to [Rh(TPPS)(H20)2]3-, and the consecutive pKx values (7.01 and 9.80 at 20 °C) correspond to the pKk values for fac- and mer-[RhCl3(H20)3], suggesting that the TPPS6- anion and 3 Cl- ligands are comparable electron donors toward the Rh center.872 The trends in the equilibrium constants (Table 60) imply that Rhni is a soft (class B) add in these complexes the NCS- ion is presumed to be sulfur bonded, although no direct evidence is presented to support this assumption. 

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