Oxyanions polynuclear

The vanadate (1, 2), molybdate (1-5), and tungstate (1-3) systems have been described in previous reviews. Although the focus in this chapter is on more recent developments, earlier well-established knowledge is included where needed for perspective and also to present a coherent picture of the hydrolysis behavior of these oxyanions. Equilibria of mono- and polynuclear species are described and information about known structures are given. Some recent work about mixed polyoxoanions is briefly reviewed. 

Only a brief mention will be made of the role of polynuclear oxyanions as ligands [X,OJ" as these tend to involve infinite rigid lattices (one-, two- or three-dimensional) and thus lose their ability to act as independent ligands. In these rigid oxyanion structures the oxygens still act as ligands, as... 

Various Ca(II) complexes with carboxylates have been synthesized and crys-tallographically characterized [75]. Three modes are most commonly observed for these complexes the unidentate mode, the bidentate mode, and the anti-mode. In the unidentate mode, the Ca ion interacts with only one of the two 0 atoms of the carboxylate group. The coordinating atom is a carboxylate oxyanion rather than the carbonyl 0 atom. In the bidentate mode, the two 0 atoms of the carboxylate chelates to one Ca ion. In the anti-mode, the Ca complex is chelated between one of the carboxylate 0 atoms and a neighboring atom. Furthermore, the anti-mode is involved in a polynuclear mode with the 0 atom of the carboxylate forming a bridge between two Ca ions. 

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