Hard metal ions, complexation

The chemistry of Th(IV) has expanded greatly since the mid-1980s (14,28,29). Being a hard metal ion, Th(IV) has the greatest affinity for hard donors such as N, O, and light haUdes such as F and CF. Coordination complexes that are common for the t7-block elements have been studied for thorium. These complexes exhibit coordination numbers ranging from 4 to 11. 

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+. 

Compartmental ligands (16) provide extensive opportunities for multiple metal ion complexation. An example of a mixed donor ligand incorporating different metal ions is the macrocyclic trinucleating hgand (63), which is capable of complexing two soft donor metal centers in addition to a hard alkali or alkaline earth metal. ... 

Consistent with the definition of hard and soft metal ions see Hard Soft Acids and Bases) based on the nature of the stable complexes that they form with different ligand donor atoms, these metal ions behave similarly toward donor atoms of nucleic acids and their derivatives (Figure 8). Hard monovalent cations (M+) usually interact with nucleic acid polyanions only in a diffuse ion atmosphere manner, whereas hard and borderline polyvalent (M- +) cations can form both outer- and inner-sphere complexes. Soft metal ions tend to form inner-sphere complexes, however. Hard metal ions (class A) prefer O-donor ligands (usually phosphate oxygens), while soft ones (class B) prefer N-donor atoms of the nucleic acid bases as well as S atoms... 

The crown ethers and cryptands generally form more stable complexes with hard metal ions, such as the group 1, 2, and lanthanide ions. The introduction of softer heteroatoms into the macrocycle can change the selectivity dramatically. There is much interest in tuning macrocycles in this way to favor complexation of certain metal atoms, but O-donor macrocycles will dominate the discussion here. 

The unsubstituted para-t-butyl calixarenes themselves complex metals via their aryloxide groups. Since aryloxide complexes are frequently oligomeric, the simple calixarenes do not give monomeric complexes. Aryloxides are hard ligands, therefore they readily form complexes with oxo-philic hard metal ions such as alkali metals, early transition metals, lanthanides, and actinides. Complexation is often inferred because the calixarene acts as a carrier for the metal ion from an aqueous to an organic phase. With the /wa-/-butylcalix[ ]arenes in alkaline solution, a value of n = 6 gives the best carrier for lithium(I), sodium(I), and potassium(I), with a value of n 8 giving the best carrier for rubidium(I) and caesium(I).15,16 Titanium(IV) complexes have been characterized,17-19 as well as those of niobium(V) and tantalum(V).20-22 These complexes are classified as... 

Coordination of a hard metal ion, Cr111, to soft S- or Se-donors is regarded as unusual.646 Typical structures of crystallographically characterized compounds ((138)-(142)) are illustrated.647-651 By far the most numerous known mononuclear complexes of this type are those with l,l-dithiolato(l—) ligands, including dialkyldithiocarbamates, xanthates, dithiocarboxylates,... 

In an important development of these generalizations by Pearson, cations (Lewis acids) and ligands (Lewis bases) were classed as being either hard or soft . The principle of hard and soft acids and bases (HSAB) is used to rationalize observed patterns in complex stability. In aqueous solution, complexes formed between class (a), or hard, metal ions and ligands containing particular donor atoms exhibit trends in stabilities as follows ... 

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