First Coordination Sphere Ligands

The appearance of the seventh ligand (Xn ) predominantly in the first coordination sphere of the complex or outside of the complex depends on the polarization potential of the alkali metal cation, M+, and on the polarity of the seventh anion, Xn". Increased polarity of the anion favors its entering into the first coordination sphere of the complex ion. 

Fig. 1. Mean lifetimes of a single water molecule in the first coordination sphere of a given metal ion, th2o> and the corresponding water exchange rate constants, h2o- The tall bars indicate directly determined values, and the short bars indicate values deduced from ligand substitution studies. References to the plotted values appear in the text.

The first coordination sphere is a special case. It can be generated from the trigonal planar arrangement by adding a further ligand, resulting a tin atom which simultaneously acts as an acid and a base. An illustrative example for this kind of bonding is compound 733) in which the tin atom receives electrons from pyridine and transfers electrons to the chromium atom (see also Chapter 6). 

Figure 2. Dy(P30io)2 is a lanthanide shift reagent commonly used in biological 7Li NMR experiments. The Dy3+ ion has a coordination number of nine with two P3O10 moieties, acting as tetradentate ligands, and one molecule of H2O coordinated in the first coordination sphere up to seven Li+ ions can bind in the second coordination sphere.

Quantitative information about the first coordination sphere structure depends on analysis of EXAFS data. From analytical data or knowledge of common ligands in metalloenzymes (N, O, S, Se), one can decide which ligands are likely to be present in the coordination sphere. An example discussed by Scott4 tests the hypothesis of a Cu(II)-S bond being present in the compound shown in Figure 3.3. 

The hfs tensors of ligand nuclei in the first coordination sphere of a metal complex are usually dominated by the isotropic interaction, i.e. the transition probabilities may be approximated by the formulae given in Table 2.1 for aiso > 2v . 

Since the unpaired electron in transition metal complexes is generally localized near the central ion and the ligand atoms in the first coordination sphere, summation in (5.5) over these nuclei is often sufficient. In this approximated form, the point-dipole model has frequently been applied in ENDOR studies of transition metal complexes to determine the proton positions from their hfs tensors (Sect. 6). In some cases the accuracy of this method has turned out to be significantly higher than that of an X-ray diffraction analysis62,130 131). 

The key assumption to be made in interpreting the pressure data in terms of solvational change is that neither the ligands in the first coordination sphere of the complex nor the solvating solvent molecules are significantly compressible relative to bulk solvent (volume Vs), the compression of which is described by the modified Tait equation... 

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