Structure second coordination sphere

Catalysis by transition metal complexes and metaUoenzymes involves a sequence of ligand exchange reactions. Such a reaction can he viewed as an interchange of an inner sphere and outer sphere ligand. It is argued therefore that the structure of the second coordination sphere is of direct relevance to catalysis. A method of studying second coordination sphere structure based on dipolar NMR shifts in paramagnetic complexes is discussed. Even in weakly bound complexes, there is a definite preferred structure for the complex which may or may not be favorable for a subsequent substitution reaction. 

What is known of second coordination sphere structure The term is an old one, first introduced by Werner (3) in the nineteenth century. Most of the examples which have been studied involve ion pairing of one kind or another. Work in this area has involved spectroscopic methods, polarography, the use of ion exchange resins, and a variety of other techniques (4). In spite of this, many of the details of the subject are not well understood. Turning to nonionic outer sphere complexes, the situation is much worse. Further, nonionic complexes are certainly most important in homogeneous catalysis since this usually occurs in non-aqueous and nonpolar solvents. In all probability, nonionic ligands are also of considerable significance in enzymatic reactions. 

Fig. 75. Schematic structure of the first and second coordination spheres...

It is helpful in the discussion to describe silicate structures using the Q nomenclature, where Q represents [SiOJ tetrahedra and the superscript n the number of Q units in the second coordination sphere. Thus, isolated [SiO ] " are represented as Q and those fully connected to other Q units as Q. In general, minerals based on Q , Q and units are decomposed by acids. Such minerals are those containing isolated silicate ions, the orthosilicates, SiO (Q ) the pyrosilicates, Si O " (Q ) ring and chain silicates, (SiOg) (Q ). Certain sheet and three-dimensional silicates can also yield gels with acids if they contain sites vulnerable to acid attack. This occurs with aluminosilicates provided the Al/Si ratio is at least 2 3 when attack occurs at A1 sites, with scission of the network (Murata, 1943). 

The closely related structures show completely different microbial uptake characteristics. The 3D structures described above show distinct different orientation of the backbone amide (tangental in type 1 versus radial in type 2), which can be explained by the interactions that take place between the FhuA receptor and the ferrichrome siderophore -As mentioned, the second coordination sphere of natural ferrichrome in FhuA receptor is very sensitive to the distance and orientation between a proton donor and the proton acceptor, therefore the orientation of the amide groups in the biomimetic siderophore plays a crucial role in receptor recognition. 

The second water molecule to be captured is placed in the position of Wat3, with no cost in free energy for the reaction involved, 20a + H2O—>21a (Fig. 28). During this step, the metal-cofactor bonds are broken and the oxidized cofactor enters the second coordination sphere where it hydrogen bonds with the water molecules. Rebinding instead the second water molecule in the position of Watl results in an almost lOkcalmol-1 less stable structure in which the cofactor still maintains bonds to the metal. 

A number of important structural aspects of zinc complexes as found in enzymes are introduced in this section to serve as background information for the subsequent sections. Aquated Zn(II) ions exist as octahedral [Zn(H20)6] + complexes in aqueous solution. The coordinated water molecules are loosely bound to the Zn + metal center and exchange rapidly with water molecules in the second coordination sphere (see Figure 1) with a rate constant of ca 10 s at 25 °C extrapolated from complex-formation rate constants of Zn + ions with a series of nucleophiles. The mechanism of the water exchange reaction on Zn(II) was studied theoretically, from which it was concluded that the reaction follows a dissociative mechanism as outlined in Figure 2. ... 

FIGURE 2. Transition state structure for water exchange on [Zn(H20)6] demonstrating the dissociation of an axially coordinated water molecule induced by an entering water molecule from the second coordination sphere... 

The symmetry of g-tensor reflects the symmetry of PC. From the data reported in Table 7.7 (axially symmetric g-tensor), it follows that the symmetry of paramagnetic center 2 is no lower than C3 (C3v according discussion in Section 6.1). For PCs 1 and 3, in which the central silicon atom is also coupled to three oxygen atoms, the g-tensor has three different principal values, suggesting that the symmetry of the defect lowers to second coordination sphere of paramagnetic center show up in its EPR spectrum. 

In the original RDFs (Figs 12 and 13) the next pronounced peak, beyond that of the first coordination sphere, occurs at about 4.5 A. It is most clearly seen in the reduced RDFs and it occurs in a region where a second coordination sphere would be expected. The large number of other types of interactions that also occur in this region prevent a quantitative analysis. In the RDFs obtained from the difference curves, however, the nonmetal interactions are eliminated and the structure beyond the first coordination sphere can be analyzed (Figs. 14 and 15). 

In the 1 M solution it shows a distinct and fairly well-separated peak at 4.5 A, corresponding to about 16 water molecules, which is roughly equal to twice the number of H20 molecules in the first coordination sphere. In the more concentrated perchlorate solution (Fig. 15) this peak has a more structured appearance, but is not resolved from longer distances, presumably due to the presence of perchlorate ions in the second coordination sphere, apparently bonded as bidentate ligands. The many overlapping interactions, however, prevent an unambiguous analysis. 

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