Soft metal ions, complexation

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+. 

Iodides of soft metal ions such as mercury (II) are essentially un-ionized in dimethylsulfoxide 54). This feature is due both to the strength of the Hg-I bond and the weakness of the Hg-DMSO bond which appears to occur through the sulfur atom of the DMSO molecule, as is known for the palladium (II) complex 5S). 

The main feature for cation recognition by tetra-bridged phosphorylated cavitands arises from the cooperative effect of the four phosphorus groups and the aromatic molecular cavity. In the phosphorus(IV) cavitands guest binding will be achieved through O (P=0) or S (P=S) coordination with different affinity for hard or soft metal ions. On the other hand, transition metal rim complexes described above can act as host for metal cation. 

Versatility of coordination in polynuclear complexes with the soft metal ions Ctf, Ag1 and Atf 566... 

Thioether analogs of the crowns have been known since the 1930s,103 but metal ion complexes of these ligands have not been investigated with the intensity found for the crowns and polyaza macrocycles. As anticipated from the soft nature of the heteroatom, the sulfur macrocycles show a preference to bind transition metals rather than alkali and alkaline earth ions. Studies have... 

S-Acetals are very readily hydrolyzed in the presence of soft metal ions and large rate accelerations e.g. ca. 106-fold compared with the proton) are found. In the case of S-acetals such as (136) only a small extent of pre-equilibrium complex formation occurs428 even with Hgu, but for substrates such as (137), 1 1 complex formation readily occurs with weaker binding metal ions such as Ag1. Kinetic terms reflecting binding of two Ag1 ions to the substrate (137) are detectable.429 Both A1 and A2-like schemes have been proposed for these S-acetal reactions and the topic was reviewed17 in 1977. 

Soft metal ions also form coordinate bonds with the water molecules from the solvent. As a result, the metal ions are hydrated. This coordination partially breaks down when the complex forms. As a result, AH2 > 0, but of moderate size. The complex formation is a disordering process in the solvent as the hydration of the metal ion breaks down. Hence, AS2 > 0. The magnitude of AH is larger than that of AH2, while the magnitude of A5) is larger than the... 

Misono, M. Saito, Y. Evaluation of softness from the stability constants of metal-ion complexes. Bull. Chem. Soc. Jpn, 1970, 43 (12), 3680-3684. 

However, the dithioketones may be readily prepared by reaction of the metal complexes of the monothio derivatives, 3.11, with hydrogen sulfide (Fig. 3-21). In this case, the metal probably plays a dual role, both in polarising the ligand and stabilising the product. Clearly, the use of a soft metal ion such as lead(it) or platinum(u) is to be preferred in a reaction of this type. 

Soft metal ions are not restricted to complexation by heteroatom donors. Their electron-rich nature and large, uncontracted orbitals allow some very interesting complexation behaviour with n-aciA ligands at the boundary between supramolecular and organometallic chemistry. A large range of transition metals... 

The binding mode of uracils and thymines in neutral and deprotonated forms has been reviewed up to 1987 [13]. They coordinate hard, and relatively few soft metal ions, through 0(4) (preferentially) and 0(2). Uracil (thymine) behaves as a weak dibasic acid in alkaline media with the more basic site N(3) at pKa 9.69 (10.16), as compared to N(l) at pKa 14.2. At high pH the monoanions of uracil and thymine bind the metal ions preferentially via N(l). However, the N(3) linkage isomer of the Ptn complex has also been obtained [24]. The relatively few examples of complexes with soft metal ions, containing monodentate uracilate anions, are due to the high tendency of the ligand to bind additional metal ions to form polynuclear species [13]. 

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