Hard metal cations

These phenomena can be explained by the (hard-soft) acid-base principal as follows C=N-OH is a soft base, hence has stronger affinity towards soft basic metal cations than hard metal cations. The strong participation of the N-OH group in complex formation was further confirmed by the results shown for extraction experiments with 5 and 6. 

The rational design of chelating agents as antidotes requires a careful consideration of acid-base chemistry. Metal ions are Lewis acids, while the chelating agents or ligands are Lewis bases. The concepts of hardness and softness may be used to describe systematically the interaction between them. A hard metal cation is one that retains its... 

Recently, the effect of two proximate cations on anion-binding was investigated [53], Toward this end, the basic calix[4]arene framework was functionalized on the lower rim with two methyl ester units and in two separate positions with two urea-linked bipyridine-functionalized polyether units to give 13. This receptor was designed to allow for the coordination of hard metal cations via the ester groups, as... 

Alkylation of the lower rim substitution with carbonyl-containing groups (101) has produced probably the most widely studied of the calixarene-based ionophores. These derivatives have been studied predominantly as ionophores for hard metal cations, and can form complexes of comparable stability to those of the cryptands in some cases. Such calixarenes have found applications as ionophores in ion selective electrodes. ... 

Hard acids (hard metal cations) form more stable complexes with hard bases (hard ligands), while soft acids (soft metal cations) show a preference for soft bases (soft ligands). 

Various other aza crowns were also shown to keep the affinity for amines but not for hard metal cations. The bis-pyrazole macrocycle [94] extracts /-butylammonium better than potassium, and benzylamine better... 

The knowledge of coordinative binding preference of metal ions is important relative to the capacity of metal ions to form stable and irreversible coordination complexes with various donor atoms of the macromolecules, and therefore notionally, metal toxicity. In biological systems, hard metal cations prefer oxygen donor atoms that are present in groups such as -OH, -COOH, -PO/" or nitrogen donor atoms... 

Compounds containing the P(0)(N)n(0)3-n (n = 1, 2, 3), P(0)(N)m(0)2-mX (m = 1, 2, X = C, Cl, F, S etc.) and P(0)(0)3 moieties are among the weU-studied inorganic compounds [an interested reader may find many examples of compoimds with the mentioned skeletons through a CSD search, [1]]. N,N,N, N, N",N"-hexamethyl phosphoric triamide (HMPA, Scheme 1) is an important polar aprotic solvent with a high-dielectric constant [2] and an excellent ligand for interaction with hard metal-cations [3]. 

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