Hydration tricapped trigonal prismatic structure

Hydrated salts are readily prepared by reaction of the lanthanide oxide or carbonate with the acid. Salts of non coordinating anions most often crystallise as salts [Ln(OH2)9]X3 (X e g. bromate, triflate, ethylsulphate, tosylate). The tricapped trigonal prismatic structure was first established by X-ray diffraction in 1939 for the [Nd(H20)9] + ion in [Nd(H20)9] (6103)3 this was an important structure historically, as it was the first clear indication that lanthanides conld have high coordination numbers. It has since been verified for many other salts (Figure 10). 

For solid lanthanide salts in which the cation coordination sphere is saturated with nine water molecules, the oxygen atoms form a tricapped trigonal prismatic structure surrounding the cation. Within an isostructural series, the changes in bond length are directly proportional to the changes in ionic radius. No evidence has been found in solids for a change in the hydration number when the primary coordination sphere is... 

The tricapped trigonally prismatic coordination of europium in Eu(terpy)3(C104)3 is probably the first example of 9-coordination involving nitrogen donors [150], Tricapped trigonal prismatic geometry is present in some simple hydrated compounds [165,166] such as Nd(H20)9(Br03>3 and M OMC H SC Xq, where M = Pr, Er, Ho, V. The structure of [Yb(DPC)3]3 is shown in Fig. 5.26. 

It was only in 1995 that the first structure of a hydrated salt of scandium containing only water molecules in its coordination sphere was reported. Refluxing scandium oxide with triflic acid leads to the isolation of hydrated scandium triflate Sc(03SCF3)3 9H20. It is isomorphous with the hydrated lanthanide triflates, containing tricapped trigonally prismatic coordinate scandium in the [Sc(H20)g] ions, with Sc—O (vertices) = 2.171(9) A and Sc—O (face capped) 2.47(2) A. 

The structures of the primary hydration sphere in solution are similar to those reported for the hydrated crystals (Albertson and Elding 1977), i.e. for the lighter cations n = 9 with tricapped trigonal prism (TCTP) structure and D31, symmetry, and for the heavier cations n = 8 with square prismatic structure and D2h symmetry. The geometric parameters (Ln HjO and H2O H2O distances) were taken from the X-ray data on solutions from Habenschuss and Spedding (1979b, c, 1980). 

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