Hydration ligands

Y = X = CIO4 X = halides, NCS, CIO4 NiXj 1318-1321 hydrate + ligand/EtOH NiL2(C104)2 + LiX... 

Substitution at a labile metal ion is generally discussed in terms of the mechanism Ila, which was proposed by Eigen as a result of his work with chemical relaxation techniques and discussed further in his review with Wilkins. The hydrated metal ion and the hydrated ligand diffuse together rapidly to form an outer-sphere complex (sometimes called, more specifically, an ion-pair) in which the lone-pairs on the ligand are separated... 

Metal-ion complexation reactions as they occur in strong solvating media like water consist in their most elementary form of the interaction of a hydrated metal cation with a hydrated ligand to form a hydrated complex, represented for a lanthanide and a simple mono anion as... 

Silicon, germanium, tin and lead can make use of unfilled d orbitals to expand their covalency beyond four and each of these elements is able (but only with a few ligands) to increase its covalency to six. Hence silicon in oxidation state -f-4 forms the octahedral hexafluorosilicate complex ion [SiFg] (but not [SiCl] ). Tin and lead in oxidation state -1-4 form the hexahydroxo complex ions, hexahydroxostannate(IV). [Sn(OH) ] and hexahydroxoplum-bate(IV) respectively when excess alkali is added to an aqueous solution containing hydrated tin(IV) and lead(IV) ions. 

These can be prepared by electrolytic oxidation of chlorates(V) or by neutralisation of the acid with metals. Many chlorates(VII) are very soluble in water and indeed barium and magnesium chlorates-(VII) form hydrates of such low vapour pressure that they can be used as desiccants. The chlorate(VII) ion shows the least tendency of any negative ion to behave as a ligand, i.e. to form complexes with cations, and hence solutions of chlorates (VII) are used when it is desired to avoid complex formation in solution. 

The anhydrous chloride is prepared by standard methods. It is readily soluble in water to give a blue-green solution from which the blue hydrated salt CuClj. 2H2O can be crystallised here, two water molecules replace two of the planar chlorine ligands in the structure given above. Addition of dilute hydrochloric acid to copper(II) hydroxide or carbonate also gives a blue-green solution of the chloride CuClj but addition of concentrated hydrochloric acid (or any source of chloride ion) produces a yellow solution due to formation of chloro-copper(ll) complexes (see below). 

In the presence of appropriate ligands, the values may be affected sufficiently to make Cu(l) stable but since the likely aquo-complex which Cu(I) would form is [Cu(H20)2], with only two water ligands, the (hypothetical) hydration energy of Cu is therefore much less than that of the higher charged, more strongly aquated [Cu(H20)e]. ... 

Also the arene-arene interactions, as encountered in Chapter 3, are partly due to hydrophobic effects, which can be ranked among enforced hydrophobic interactions. Simultaneous coordination of an aromatic oc amino acid ligand and the dienophile to the central copper(II) ion offers the possibility of a reduction of the number of water molecules involved in hydrophobic hydration, leading to a strengthening of the arene-arene interaction. Hence, hydrophobic effects can have a beneficial influence on the enantioselectivity of organic reactions. This effect is anticipated to extend well beyond the Diels-Alder reaction. 

These hydrated salts contain bidentate carbonate ligands and no water molecules are bound directly to the central metal atom. The only single-crystal x-ray diffraction studies available are those for salts of (4) (52—54) and the mineral tuliokite [128706 2-3], Na2BaTh(C03)2 -6H20], which contains the unusual Th(C02) 2 anion (5) (55). 

In the first step the hydrated ion and ligand form a solvent-separated complex this step is believed to be relatively fast. The second, slow, step involves the readjustment of the hydration sphere about the complex. The measured rate constants can be approximately related to the constants in Scheme IX by applying the fast preequilibrium assumption the result is k = Koko and k = k Q. However, the situation can be more complicated than this. - °... 

Hydrate isomerism of TiCl3.6H20, yielding [TiCl2(H20)4]" CU as one of the isomers, has already been referred to (p. 965) and analogous complexes are formed by a variety of alcohols. Neutral complexes, [T1L3X3] have been characterized for a variety of ligands such... 

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