Coordination spheres solvento complexes

Because of ligand-field factors, certain transition metal ions, notably Cr + and Co +, almost exclusively exhibit a coordination number of six in their complexes. The kinetically inert nature of Cr and Co complexes, dramatically different from that of the extremely labile lanthanide solvento ions, facilitates the isolation of isomeric species and was crucial in enabling Alfred Werner to formulate the fundamental tenets of coordination chemistry. For simple lanthanide ion complexes, their lability and the lack of a marked sensitivity of their visually observed colors to the nature of the coordination sphere renders Wernerian procedures inapplicable, such that the establishment of high and variable coordination numbers as a characteristic of lanthanide ions has depended largely on modem spectroscopic and crystallographic measurements. 

In conclusion, the generation of solvento-complexes as reactive intermediates is extensively used in synthesis. Three distinct approaches have been developed to provide solvent-binding sites (i) conversion of ligands into products with low solubility or their transformation into volatile compounds, (ii) chemical transformations of coordinated ligands where the products of the reaction are poorer donors and are expelled from the coordination sphere of the complex, and (Hi) manipulation of the metal oxidation state to affect ligand-binding properties. 

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