Coordination sphere The metal ion and

Primary Coordination Sphere The metal ion and the ligands attached to it is called primary coordination sphere. In K4[Fe(CN)6], [Fe(CN)6]4- represents coordination sphere. 

Coordination sphere The metal ion and its coordinated ligands, but not any uncoordinated counterions. 

The slow acetylation of the hydroxyl group is difficult to explain. One is inclined to suggest that this group is coordinated to the metal ion and consequently rendered inactive. Such a possibility requires either a coordination number of 7 for chromium (III) or displacement of carboxylate from the coordination sphere by hydroxyl. In the latter instance an uncoordinated functional group would still be present to react with ketene (COO or COOH) and anhydride should be detected in the crude reaction product. This is not the case. 

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). 

Although there are aqua ions still to be identified, many have been characterized as already described. On the other hand, information about the number of water molecules in the second-coordination sphere of metal ions and their residence times is scarce, and the only experimentally determined lifetime of a water molecule exchanging between the 12 H20 of the second-coordination sphere and bulk water is 1.28 x 10-los (Ih o = 7.8x 109s 1) at 25°C for [Cr(H20)6]3+whieh compares with 1.44 x 10-los from molecular dynamics calculations.24 63 Similar calculations show Nd3+, Sm3+, and Yb3+ to have 17.61, 17.13, and 16.74 water molecules in the second-coordination sphere, with residence times of 1.3 x 10-11 s, 1.2 x 10—11 s, and 1.8 x 10 11 s, respectively.211 These studies are consistent with the exchange of water between the second-coordination sphere and the bulk solvent being close to diffusion controlled, as has generally been assumed in mechanistic models for the substitution of water in the first-coordination sphere. 

In 1994, Riordan and coworkers1 have prepared monoanionic, acyclic borates of the form B(CH2SR)4, potentially C3l,-symmetric facecapping ligands. These molecules can be considered sulfur analogs of the versatile poly(pyrazolyl)borates. The thioether ligands described in this chapter provide a softer coordination sphere for metal ions and, sometimes, a different metal atom environment. 

Ammonia also forms clusters in the gas phase and the reactions of ammonia clusters with bare metal ions have been studied (61). The ammonia clusters probed by electron impact as [(NH3) H]+ showed a monotonic decrease in intensity with increasing value of n, but the metal complex ions [M(NH3) ]+ showed intensity gaps. Thus for most of the metals the [M(NH3)2]+ ion was much more intense than the [M(NH3) ]+ ions, where n 2, and so the coordination number 2 was reported to be the favored coordination number in the first coordination sphere. The favored ions M(NH3)m]+ were n = 2 for Cr+, Mn+, Fe+, Co+, Ni+, and Cu+, and n = 4 for V+. The non-transition metal Mg+ and Al+ had the favored coordination number of 3. 

Water molecules in the so-called coordination sphere of a metal ion complex can exchange with water molecules in the medium, and the rates depend largely on the nature of the metal ion and its electric charge, and to a lesser extent on the other coordinated substituents. The rate constants for substitutions of inner sphere water of various aqua ions were determined largely by Eigen s... 

In recent years there has been considerable interest in the reactions of nitriles in the coordination sphere of metal ions. Breslow et al.312 first reported that the hydrolysis of 2-cyano-l,10-phenanthro-line to the corresponding carboxamide is strongly promoted by metal ions such as copper(II), nickel(II) and zinc(II). Base hydrolysis of the 1 1 nickel complex is 107 times faster than that of the uncomplexed substrate. The entire rate acceleration arises from a more positive value of AS. Somewhat similar effects have been observed for base hydrolysis of 2-cyanopyridine to the corresponding carboxamide. In this case rate accelerations of 109 occurred with the nickel(II) complex.313... 

On a more sophisticated level, functional metallobiosite models may also, in principle, be prepared in the same conceptual fashion as applied to purely organic systems. Such systems are much more challenging and their success depends on the extent to which the enzyme activity is related to the nature of the metal first coordination sphere. It becomes, in general, almost exponentially more difficult to control the metallobiosite environment, the further the departure from the metal ion and its primary coordination sphere. 

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