Coordination chemistry hydrates

Steric Selectivity. In addition to the normal regularities that can be rationalized by electronic considerations, steric factors are important in coordination chemistry. To illustrate, 8-hydroxyquinoline, or 8-quinolinol (Hq) [148-24-3J, at 100°C precipitates both Mg " and AE" from aqueous solution as hydrated Mg(q)2 (formulated as Mg(q)2(H20)2 [56531 -18-1]) and as Al(q)3 [2085-33-8] respectively. 2-Meth54-8-hydroxyquinohne [826-81-3] (6),... 

The coordination chemistry of the large, electropositive Ln ions is complicated, especially in solution, by ill-defined stereochemistries and uncertain coordination numbers. This is well illustrated by the aquo ions themselves.These are known for all the lanthanides, providing the solutions are moderately acidic to prevent hydrolysis, with hydration numbers probably about 8 or 9 but with reported values depending on the methods used to measure them. It is likely that the primary hydration number decreases as the cationic radius falls across the series. However, confusion arises because the polarization of the H2O molecules attached directly to the cation facilitates hydrogen bonding to other H2O molecules. As this tendency will be the greater, the smaller the cation, it is quite reasonable that the secondary hydration number increases across the series. 

Chemistry of ion coordination and hydration revealed by a K+ channel-Fab complex at 2.0 A resolution. Nature 414, 43-48. 

In coordination chemistry two types of complex can occur between metals and complexant ligands. Outer-sphere complexes are relatively weak electrostatic associations between a hydrated metal ion and a complexant ligand, and in which both of the charged species retain a hydration shell. In contrast, inner-sphere complexes are stronger interactions in which a covalent bond is formed between a metal ion and a ligand. 

Zhou YF, Morais-Cahral JH, Kaufman A, MacKinnon R. Chemistry of ion coordination and hydration revealed hy a K- -... 

As mentioned above, solution chemistry was bom at the end of the 19th century and developed on the basis of thermodynamics and statistical thermodynamics. Main fields supporting the solution chemistry were physical chemistry and coordination chemistry, which also absorbed some other parts of chemistry in order to establish an interdisciplinary field of chemistry in 1950-1960. Even in this period, however, solvent, e.g, water, was recognized as a continuum in most studies except for some works on ionic hydration, and thus, molecular picture of water was not clearly recognized. In modem solution chemistry, ion-solvent and ion-ion interactions should be depicted more clearly at the molecular level. 

Chemistry of ion coordination and hydration revealed by a K+ channel-Fab complex at 2.0 angstrom resolution, Y. F. Zhou, J. H. Morais-Cabral, A. Kaufman and R. MacKinnon, Nature, 2001, 414, 43. 

Thus in the laboratory we tend to meet almost all metals in a pure form as synthetic cationic salts of common anions. These tend to be halides or sulfates, and it is these metal salts, hydrated or anhydrous, that form the entry point to almost all of metal coordination chemistry. In nature, it is no accident that metal ions that are relatively common tend to find roles, mediated of course by their chemical and electrochemical properties. Thus iron is heavily used not only because it is common, but also because it forms strong complexes with available biomolecules and has an Fe(II)/(III) redox couple that is accessible by biological oxidants and reductants and thus useful to drive some biochemical processes. 

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