Nitrates ligands

The coordination modes of the nitrate ligand in the complexes [TpBut]M(N03) (M = Cu, Ni, Co, Zn) are summarized in Fig. 46. (171, 184). Evidently, the coordination mode varies from unidentate for Zn to symmetric bidentate for Ni and Cu, with the cobalt derivative exhibiting an anisobidentate coordination mode. Moreover, the related cadmium derivative [TpBut,Me]Cd(N03) also exhibits bidentate coordination of the nitrate ligand, with Cd-0 bond lengths of2.272(6) A and 2.295(7) A (91). Such symmetric bidentate coordination contrasts with the significantly different Zn-0 interactions [1.978(3) A and 2.581(3) A] in unidentate [TpBut]Zn(N03). The coordination modes for a variety of [TpRR ]M(N03) complexes are summarized in Table VIII. 

The structural variations observed for [TpRR ]M(N03) (M = Co, Ni, Cu, Zn Cd) reveal that, for a given [Tp1 ] ligand, the preference for bidentate coordination increases across the series Zn < Co Cu, Ni, and Cd. Significantly, these structural preferences of the nitrate ligand correlate with the activity of the metal-substituted enzymes Zinc, the metal with the greatest tendency to exhibit unidentate coordination of the nitrate ligand, is the most active, while nickel, copper, and... 

The Zn-0 distance is 2.8 A, indicating a relatively weak association. Moreover, the nitrate ligand does not displace the coordinated water from the zinc center. See Mangani, S. H kansson, K. Eur. J. Biochem. 1992, 210, 867. 

The i.r. spectrum of K[Au(N03)4] indicated that all the nitrate ligands are unidentate, which is the first example of a square-planar complex containing only unidentate nitrate. ... 

A] (Figure 4.6b). In both compounds 10 and 11 the nitrate ligands are bound to mercury atoms in a bidentate or monodentate fashion, respectively. 

No compounds are known with ten or more distinct ligands (i.e., nonchelate structures) however, a few len-coordinate chelate structures have been described. One possible structure is a double trigonal bipyramid" with bidemate nitrate or carbonate ions at each of the TOP sites (Fig. 12.41a). Coordination numbers as high as 12 are also known with six bidentate nitrate ligands along the edges of a dodecahedron, but these complexes are rare (Fig. 12.41b). 

Eq. (17) and act in a bidentate fashion. The third charge is balanced by a bidentate nitrate ligand and four DMF ligands complete the 8-coordination. 

A model calculation on the basis of this structure is illustrated in Fig. 24 for one of the solutions. It leads to a symmetric or only slightly asymmetric bidentate bonding of about two nitrate ions to erbium (Fig. 24) with the Er—0(N03) bond lengths, 2.45 A, somewhat longer than the Er—0(H20) bonds, 2.32 A, in the first coordination sphere. The Er3 + ion and the nitrate ligand are coplanar as shown by the Er—03 distance of 4.1 A. This bonding of nitrate to the metal ion is very similar to that found in crystals, with the same orientation of the nitrate ion and the same difference between Er—0(H20) and Er—0(N03) bond lengths (36). 

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