Nine-coordinate species

Overall, then, the nature of the hydrated lanthanide ion in solution now appears to be well understood. In summary, the coordination number of [Tn(H20) ] + (aq) is believed to be 9 for the early lanthanides (La-Nd or Sm) and 8 for the later metals (Dy-Lu, Y), with the intermediate metals exhibiting a mixture of eight- and nine-coordinate species. The nine-coordinate species are assigned TTP structures and the eight-coordinate species square antiprismatic coordination. 

As in aqueous solution, the lanthanide contraction favors a change from nine-coordination for the light lanthanides to eight-coordination for the light lanthanides such that [Ln(DMF)8]3+ is the major species when Ln3+ = Ce3+-Nd3+, and that this becomes the only detected species when Ln3+ = Tb3+-Lu3+ in dimethylformamide perchlorate solution (11, 92, 93, 321-323). Thus, Nd3+ is characterized by AH° = -14.9 kJ mol-1, AS0 = -69.1 J K"1 mol-1, and AV° = - 9.8 cm3 mol-1 for the equilibrium shown in Eq. (25) (93). The molar volume of DMF is 72 cm3 mol- and it therefore appears that the substantially smaller magnitude of AV° is a consequence of significant... 

In the trivalent state the stereochemistry of the actinides is similar to the lanthanides, that is, eight coordinate. However, higher coordination states are known for some trivalent actinides, for instance, UC13 exists in the nine coordinate state. In the tetravalent state the species normally encountered are eight coordinate. 

A novel ten-vertex bimetallocarborane, (CsHo C C BeHs, is produced upon the polyhedral expansion of 1,7-C2B6H8 (16). The expected monometallic nine-vertex species, C5H5CoC2B6H8, is also produced in this reaction (see Fig. 4). An X-ray crystal structure of the bimetallic product (61) showed the 2 cobalt atoms to occupy adjacent positions on the two equatorial belts of the bicapped square antiprism. The carbon atoms occupy the low-coordinate caps. Thermal rearrangement (30) produced an isomeric complex in which the metals are separated from each other, but the carbon atoms remain in the low-coordinate sites. 

The only complexes of lanthanum or cerium to be described are [La(terpy)3][C104]3 175) and Ce(terpy)Cl3 H20 411). The lanthanum compound is a 1 3 electrolyte in MeCN or MeN02, and is almost certainly a nine-coordinate mononuclear species the structure of the cerium compound is not known with any certainty. A number of workers have reported hydrated 1 1 complexes of terpy with praseodymium chloride 376,411,438), and the complex PrCl3(terpy)-8H20 has been structurally characterized 376). The metal is in nine-coordinate monocapped square-antiprismatic [Pr(terpy)Cl(H20)5] cations (Fig. 24). Complexes with a 1 1 stoichiometry have also been described for neodymium 33, 409, 411, 413, 417), samarium 33, 411, 412), europium 33, 316, 411, 414, 417), gadolinium 33, 411), terbium 316, 410, 414), dysprosium 33, 410, 412), holmium 33, 410), erbium 33, 410, 417), thulium 410, 412), and ytterbium 410). The 1 2 stoichiometry has only been observed with the later lanthanides, europium 33, 411, 414), gadolinium, dysprosium, and erbium 33). 

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