Water, as ligand

Timasheff, S. N. 1992. Water as ligand preferential binding and exclusion of denaturants in protein unfolding. Biochemistry. 31 ... 

Type A and type C centers can be readily distinguished. Type B centers are usually readily identified, but it may be difficult to determine the respective occupancies of the two shells precisely, because the contributions are typically not well resolved in the Fourier transform. Also, it is unfortunate that EXAFS cannot distinguish between car-boxylate oxygens and water as ligands of zinc. However, coordination by imidazole is usually clearly evident because of the multiple-scattering pathways involving all atoms of the five-membered ring (23, 26). 

The fall in the at the use of water as ligand-modifier in this case... 

X TECHNOLOGICAL DEVELOPMENTS IN ORGANOPALLADIUM CHEMISTRY B.iv. Water as Ligand... 

When a salt like M(N03)2 dissolves in water, a complex ion forms, with water as ligands around the metal ion. In many cases, when we treat this hydrated cation with a solution of another ligand, the bound water molecules are replaced. For example, a hydrated ion, M(H20)/+, forms the ammoiuated ion, M(NH3)4 +, in aqueous NH3 ... 

The chemistry of the aqueous sol-gel process is quite complex due to metal and the double role of water as ligand and solvent as well as due to the large number of reaction parameters that have to be stricdy controlled (hydrolysis and condensation rate of the metal oxide precursors, pH, temperature, method of mixing, rate of oxidation, the nature and concentration of anions, etc.) in order to provide good reproducibility of the synthesis protocol. Another fundamental problem of aqueous sol-gel chemistry is that the as-synthesized precipitates are generally amorphous. The required postsynthetic annealing step to induce the crystallization process prevents any subtle control over crystal size and shape. For the preparation of bulk metal oxides, like the ceramic pigment case, these limitations play only a minor role however, in the case of nanoparticle synthesis, they constitute a major concern. 

---