First coordination sphere

Fluorine ions form the first coordination sphere around the tantalum ion, which is the central atom of the complex. Potassium ions form the second coordination sphere, which significantly affects the geometry and force field of the first coordination sphere. The melting of K2TaF7 leads to the dissociation of the compound into ions, as follows ... 

This dissociation is in effect an extension of the diameter d2 of the second coordination sphere and the subsequent decrease in the intrinsic interaction potential of the outer sphere. Therewith, the inter-spherical interaction potential between the central atom and the first coordination sphere increases, leading to shortening of the distance di, which in turn leads to an increase in the frequency of the Ta-F bond vibration. 

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. 

The midpoint redox potentials were estimated to be +230 mV (pH = 8.6) or +281 mV (pH = 7.0) for the Rd-like centers, and +339 and +246 mV (pH = 7.0) for the diiron-oxo center 38, 43). This is a surprising observation, since the normal redox potential of Rd centers is about 0 mV. All spectroscopic evidence points to the fact that the monomeric iron centers present in Rr are virtually identical to the ones found in Rd. Hence, it is reasonable to assume that the first coordination sphere of these centers cannot be held responsible for the 250 mV difference in the midpoint redox potentials. 

Here the selectivity of NIS is particularly important since only the first coordination sphere of the iron atoms contribute appreciably to the NIS signal. 

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.

It is the mechanisms through which L1 and L interchange between the second and first coordination spheres which remain the subject of considerable debate (4, 6-9,15, 21-26), and which constitute the central facet of the discussion which follows. 

The definition of solvent exchange rates has sometimes led to misunderstandings in the literature. In this review kjs 1 (or fc2lsolvent]), sometimes also referred to as keJ s 1, is the rate constant for the exchange of a particular coordinated solvent molecule in the first coordination sphere (for example, solvent molecule number 2, if the solvent molecules are numbered from 1 to n, where n is the coordination number for the solvated metal ion, [MS ]m+). Thus, the equation for solvent exchange may be written ... 

The replacement of five waters in [Cr(H20)6]3+ by NH3 in [Cr(NH3)5H20]3+ causes a 20-fold increase in the lability of the single remaining water (which is quite small by comparison with such replacement in [Ni(H20)6]2 +, as seen from Table V) and appears to decrease the importance of the a-activation mode in the approach to the transition state (see Section III,G). Assuming that the free radii (222) of H20 (138 pm) and NH3 (169 pm) reflect their relative electrostricted coordinated radii, it is anticipated that the first coordination sphere of [Cr(H20)6]3+ will be less crowded than that of [Cr(NH3)5H20]3+ and that the extent of a-activation character in the substitution of water in the latter species will decrease. 

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). 

Michalowicz, A., Verdaguer, M., Mathey, Y., and Clement, R. Order and Disorder in Low Dimensional Materials Beyond the First Coordination Sphere with E.X.A.F.S., 145, 107-149 (1987). Montanari, F., Landini, D., and Rolla, F. Phase-Transfer Catalyzed Reactions. 101, 149-200 (1982). 

Fig. 21. Section of the solid-state structure of gallium(II). The first coordination sphere is marked and shown separately.

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