Ionic radii, lanthanide-coordination chemistry

It is obvious from the table that the ionic radii of lanthanides are very similar to the ionic radius of the Ca2+ ion. Further, the ionic radii of lanthanides differ by about 0.3 A in changing the coordination environment from 6 to 12. The differences of 0.3 A in ionic radius between coordination number 6 and 12 makes the lanthanides(III) ions highly adaptable to many coordination environments. This has led to the development of a wide array of macrocyclic molecular complexes with exciting properties. From this point of view, lanthanides have been known as the chameleons of coordination chemistry [5]. 

The lanthanide or rare earth elements (atomic numbers 57 through 71) typically add electrons to the 4f orbitals as the atomic number increases, but lanthanum (4f°) is usually considered a lanthanide. Scandium and yttrium are also chemically similar to lanthanides. Lanthanide chemistry is typically that of + 3 cations, and as the atomic number increases, there is a decrease in radius for each lanthanide, known as the lanthanide contraction. Because bonding within the lanthanide series is usually predominantly ionic, the lanthanide contraction often determines the differences in properties of lanthanide compounds and ions. Lanthanide compounds often have high coordination numbers between 6 and 12. see also Cerium Dysprosium Erbium Europium Gadolinium Holmium Lanthanum Lutetium Praseodymium Promethium Samarium Terbium Thulium Ytterbium. 

Although scandium is formally a member of the rare earth family of elements, it differs from yttrium and the lanthanide elements in rather significant ways. Of primary importance is the fact that the ionic radius of scandium(III) which is 0.68 A is much smaller than that of yttrium(III) (0.88 A) or the lanthanide(III) ions (1.06 A to 0.85 A). On this basis the chemistry of yttrium should be very similar to that of the lanthanides, as it is. On the other hand, since the chemistry of these elements results from chemical bonding which is essentially ionic in nature, this smaller size for scandium has a marked influence on the nature and coordination number of the complexes which it forms. 

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