Coordination number above nine

There are few structures known with coordination numbers larger than 8. The existence of coordination number 12 in some crystal lattices was mentioned above. Discrete nine-coordinate structures are known for complexes such as (LnlH.O) )1 and for the hydnde complexes [MHJ2- (where M = Tc or Re). These structures are formed by adding a ligand to each of the rectangular feces of a trigonal prism (Fig. 12.40). 

Hydrates of rare earth chlorides also have two different crystal systems a triclinic system for lanthanum, cerium, and praseodymium, as well as a monoclinic system for neodymium to lutetium and yttrium. CeCl3-7H20, as an example of the former system, is different from the above infinite polymer as two cerium atoms are connected by two [i2-bridges to form a dimer. The formula for this dimer is [(H20)7Ce([i2-Cl)2Ce(H20)7]Cl4 as shown in Figure 1.18. Therefore, the coordination number of cerium is nine and the polyhedron takes on a destroyed mono-capped square antiprism configuration. 

Brusset et al. (1971b) reported the coordination number to be eight, but Rice and Robinson (1976) later showed that it should be nine. The coordination polyhedron of La can be visualized as an irregular pentagon with two additional oxygen atoms above and below the plane of the pentagon. 

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