Heteropolynuclear system

A final example concerns the formation of heteropolynuclear hydroxide complexes.116 The complexes [(OH)Fe(OH)2Cr]3+, [(OH)Fe(OH)2Cr(OH)]2+ and [(OH)2Fe(OH)2Cr(OH)]+, or polymers such as Fe(OH)2M "+ (M = V, Cr, Mn, Co, Ni, Cu n = 2-4) have been studied with a view to an understanding of the inclusion of transition metals in iron ores as mixed oxides rather than their occurrence as discrete mineral phases. Many other examples might have been chosen in this section. Reference should be made to the general reviews given above. However it should be clear that simple inorganic coordination complexes play a major role in the chemistry of natural aqueous systems at low temperatures. 

Balzani et al. (70) employed a strategy focusing on complexes as metals and as ligands (71,72), for developing a number of interesting supramolecular systems. Ditopic polypyridyl ligands, for example, 2,3- and 2,5-dipyridylpyrazine (2,3-dpp and 2,5-dpp), 2,2 -biquinoline (biq), tetrapyrido[3,2-fl 2, 3 -c 3",2"-/z 2", 3 "-/]-phenazine (73,74) (tppz), and 2,3,5,6-tetrakis(2-pyridyl)pyrazine (75-77) (tpypz), in combination with monotopic ligands (e.g., bpy and phen) were utilized in the preparation of a number of homo- and heteropolynuclear complexes of well-defined structures (Fig. 2). 

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