Tricapped trigonal prismatic species

In chloride solutions, the variation of coordination number from 9 to 8 has been observed in aquo lanthanide ions [204], This observation was negated by NMR studies. NMR studies show that La is nonacoordinated in chloride solutions while it is octacoordinated in LaBr3 solutions [211]. Thus the question of nonacoordination or octacoordination in La(III) still remains a puzzle and a plausible explanation is that an equilibrium mixture of both octa and nonacoordinated species is present in aqueous solutions with rapid interconversion of one into another. Both tricapped trigonal prismatic and dodecahedral structures may impart some stability to the two species with facile interconversion of one species into another. It will be useful to evaluate the energetics for the conversion of one structure into the other. 

Scandium is more like an element of the first transition series (or like aluminum) than like the rare earths. For example, it characteristically has a coordination number of 6 (6-coordinate radius, 0.89 A compared to 1.00-1.17 A for the lanthanides), although coordination numbers 8 and 9 are known. Examples of CN 8 are Na5[Sc(C03)4] -6H20 and the tropolonate, [SeT4] . There is but one example of CN 9, namely the tricapped trigonal prismatic [Sc(H20)9]3+ ion found in Sc(CF3S03)3-9H20.38 The aqua ion in solution, [Sc(H20)6]3+, is appreciably hydrolyzed and OH-bridged di- and trinuclear species are formed. 

FIGURE 78 Calculation of statistical factors by using the method of the symmetry numbers for equilibrium (83) and (84). (A) The metal ions are considered as nonsolvated species and (B) the metal ions are considered as nine-coordinate tricapped-trigonal prismatic solvates. 

The anions [ReH9] and [TcH9] (both d ) provide examples of 9-coordinate species in which the metal centre is in a tricapped trigonal prismatic environment (see Fig. 10.14c). A coordination number of 9 is most... 

Germanium has four valence electrons. Thus there are 38 valence electrons for Ge. After using 18 of them as nine lone pairs, there remain 20 electrons for the Ge9 skeleton. With n = 9, when have C = 20 = 2n + 2. Hence Gel is a closo species with a tricapped trigonal prismatic structure, as shown above. Similarly, for Ge , we have n = 9 and C = 22 = 2n + 4. So this anion has a nido structure derived from a bicapped square antiprism. This structure is also shown above. 

---