Second sphere, coordination in the

The concept of coordination in the second sphere was introduced by Werner. All authors agree that such outer-sphere association exists in solution, hut they disagree about the kind and the extent of this association. Some advocate a second-sphere coordination which is closely analogous to the inner-sphere coordination. The data which support this hypothesis are not very convincing and can be criticized in various ways. The present author finds that the electrostatic theories of N. Bjerrum, Fuoss, and Kraus, according to which the formation of the ion-associates is a result of coulombic attraction, both qualitatively and quantitatively, give the most trustworthy picture of the outer-sphere association. However, this does not exclude the fact that some preferred mutual orientation exists in the ion pairs. 

Tn the third edition of his famous Inorganic Chemistry/ Alfred Werner introduces the concept of coordination in the second sphere and distinguishes between the following possibilities ... 

The interaction between the protein matrix and its nested coordination complexes is exemplary of outer sphere coordination (OSC). Recently, OSC has guided the design of both small molecules and metalloproteins to tune existing or imbue novel properties. While often cited to explain metalloprotein behavior, OSC is not typically treated in a descriptive manner. We will begin with Bjerrum s favored definition [1] of coordination in the second sphere (chosen from Werner s original postulation [2]) a complex with a fully occupied first sphere has residual affinity to attach groups. This mode of OSC is involved in supramo-lecular chemistry [3, 4], but does not quite suit our present discussion. 

Bjerrum J (1967) Coordination in the second sphere. In Wemer centennial. Advances in chemistry, vol 62. American Chemical Society, Washington, DC, pp 178-186... 

The study of the structure of the weak second coordination sphere around a metal complex in solution using NMR methods is now well established. (264) Second sphere effects are due primarily (but not solely) to dipolar interactions with unpaired electrons. Such interactions lead to an enhanced relaxation rate in the second sphere (usually measured by T m) and, in many cases, dipolar shifts [equation (13)]. 

A group in the second sphere is directly coordinated to a ligand in the first sphere. 

Interaction between the coordinatively bound acid and a labile epoxide molecule in the second sphere of the iron ion, which leads to the formation of the reaction product. When a propylene oxide molecule forms an oxygen bridge, it does not take part in the reaction. 

An additional relevant point is that the formation of bimetallic Ni-Fe or Co-Fe alloys cannot be inferred from the hyperfine field data. According to Jartych [23], the hyperfine field in body centered cubic Fe-Ni alloys increases by 0.94 T when one Ni atom substitutes an iron atom in the first coordination sphere and by 0.7 T, in the second sphere. A comparison of hyperfine field values for reduced FeFe204 (magnetite) and NiFe204 electrodes showed negligible differences (Fig. 28.5). 

Indirect involvement in the binding event via noncova-lent interactions with the gnest in the second-sphere of coordination. 

Figure 29 Ball-and-stick representation of the molecular mechanics (MM3)-derived model of the complex [Pd(25)(P(OAr)3)] with Cl ion. The six CH - Cl interactions in the second-sphere coordination are shown as dashed lines. The 2,4-rert-Bu groups on the aromatic rings have been removed for clarity.

For Gd(III) complexes there are several processes that can contribute to this correlation time. Electronic relaxation (l/Fi e) at the Gd(III) ion, rotational diffusion (1/tr) of the complex, and water exchange in and out of die first (l/tn,) or 2nd (1/Xni ) coordination sphere all create a fluctuating field that can serve to relax the hydrogen nucleus. It is die fastest rate (shortest time constant) that determines the extent of relaxation. For water in the second sphere, the relevant correlation time may be the lifetime of diis water, which may be on the order of tens of picoseconds. Water in the inner sphere typically has a much longer residency time (1-10,000 ns), so the relevant correlation time is usually rotational diffusion or electronic relaxation. 

In the relative coordinate system in which the first sphere is at rest, a collision can occur only if the center of the second sphere lies within the collision cylinder" as shown in figure 9.6. Now, the volume of the cylinder is equal to b d4> db u 5t. Prom figure 9.6 it should be clear that... 

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 specific structure of [(H20)5Ni(py)]2+ was observed in the complexes with the second-sphere coordination of calix[4]arene sulfonate.715 There are two different [(H20)5Ni(py)]2+ cations in the complex assembly. In one the hydrophobic pyridine ring is buried in the hydrophobic cavity of the calixarene with the depth of penetration into the calixarene cavity being 4.3 A (Figure 9). The second independent [(H20)5Ni(py)]2+ cation is intercalated into the calixarene bilayer. 

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