Transition metal ions coordination sphere

The coordination chemistry of oxalate (ox, C2042-) compounds provides a series of very interesting compounds from the stereochemical and magnetic points of view [197]. Most frequently the compounds form honeycomb layers in the presence of transition metal ions, in which the stereochemistry of the metal ion coordination sphere alternates between A and A. However, a three-dimensional homochiral structure is also possible. On the other hand, the negative charge of the oxalates necessitates the incorporation of cations between them, which provides the opportunity to introduce chirality and additional functionality in materials. The compound formed between homochiral manganese II oxalate and iron II tris bipyridinc (bpy) with formula [Mn oxls]2 " [Fcn(bpy)3]2+ crystallises in the space group fJ4 32. 

These studies indicate that the sequence of duplexes and the accessibility of bases are the dominant factors that determine the binding pattern of the labile metal ions to DNA, and that the molecular electrostatic potential further affects the metal binding. Of importance for this chapter is that in all of the reported solution and solid-state structures, the 3d transition metal ions coordinate to the N7 of a G nucleobase and complete their octahedral coordination sphere with water molecules or with phosphate groups from adjacent nucleotides within the duplexes. Thus they do not form direct interstrand bridges. 

Estimated lifetime of the transition state in covalent reactions Change in metal ion coordination sphere in metalloenzymes Enzyme-substrate local conformational motion Covalent enzyme-substrate intermediate lifetime Enzyme-substrate complex conformation isomerization Enzyme-substrate complex unfolding transition... 

The specific feature of polymerization as a catalytic reaction is that the composition and structure of the polymer molecule formed show traces of the mechanism of the processes proceeding in the coordination sphere of the transition metal ion to which a growing polymer chain is bound. It offers additional possibilities for studying the intimate mechanism of this heterogeneous catalytic reaction. 

Coordination Numbers and Radii. In the transition metal ions, the interaction of the ligand orbitals with the d orbitals of the metal ions generally determines the coordination number and geometry of the oordination sphere about the metal. The... 

Unlike the other alkaline earth and transition metal ions, essentially on account of its small ionic radius and consequent high electron density, Mg2+ tends to bind the smaller water molecules rather than bulkier ligands in the inner coordination sphere. Many Mg2+-binding sites in proteins have only 3, 4 or even less direct binding contacts to the protein, leaving several sites in the inner coordination sphere occupied by water, or in the phosphoryl transferases, by nucleoside di- or triphosphates. 

Coordinative Environment. The coordinative environment of transition metal ions affects the thermodynamic driving force and reaction rate of ligand substitution and electron transfer reactions. FeIIIoH2+(aq) and hematite (a-Fe203) surface structures are shown in Figure 3 for the sake of comparison. Within the lattice of oxide/hydroxide minerals, the inner coordination spheres of metal centers are fully occupied by a regular array of O3- and/or 0H donor groups. At the mineral surface, however, one or more coordinative positions of each metal center are vacant (15). When oxide surfaces are introduced into aqueous solution, H2O and 0H molecules... 

Reaction (63) is an example of 0 acting as an oxidant and it probably proceeds via an inner-sphere electron transfer mechanism in which incompletely coordinated Cu binds O2 prior to electron transfer [87]. HO2 and 0 also react readily with a number of other transition metal ions, either by electron transfer or through the formation of a complex [83], for example ... 

In aqueous solutions, in which the most probable ligand is the water molecule, most of the lower oxid ation states (i.e. + 2, + 3 and some of the + 4 states) of transition metal ions are best regarded as hexaaqua complex ions, e.g. [Feu(H20)6]2 +. In these ions the six coordinated water molecules are those that constitute the first hydration sphere, and it is normally accepted that such ions would have a secondary hydration sphere of water molecules that would be electrostatically attracted to the positive central ion. The following discussion includes only the aqua cations that do not, at pH = 0, undergo hydrolysis. For example, the iron(III) ion is considered quite correctly as [Fe(H20)6]3 +, but at pH values higher than 1.8 the ion participates in several hydrolysis reactions, which lead to the formation of polymers and the eventual precipitation of the iron(III) as an insoluble compound as the pH value increases, e.g. ... 

Somewhat better data are available for the enthalpies of hydration of transition metal ions. Although this enthalpy is measured at (or more property, extrapolated to) infinite dilution, only six water molecules enter the coordination sphere of the metal ion lo form an octahedral aqua complex. The enthalpy of hydration is thus closely related to the enthalpy of formation of the hexaaqua complex. If the values of for the +2 and +3 ions of the first transition elements (except Sc2, which is unstable) are plotted as a function of atomic number, curves much like those in Fig. 11.14 are obtained. If one subtracts the predicted CFSE from the experimental enthalpies, the resulting points lie very nearly on a straight line from Ca2 lo Zn2 and from Sc to Fe3 (the +3 oxidation state is unstable in water for Ihe remainder of the first transition series). Many thermodynamic data for coordination compounds follow this pattern of a douUe-hunped curve, which can be accounted for by variations in CFSE with d orbital configuration. 

A prototypical example of a molecular probe used extensively to study the mineral adsorbent-solution interface is the ESR spin-probe, Cu2+ (Sposito, 1993), whose spectroscopic properties are sensitive to changes in coordination environment. Since water does not interfere significantly with Cu11 ESR spectra, they may be recorded in situ for colloidal suspensions. Detailed, molecular-level information about coordination and orientation of both inner- and outer-sphere Cu2+ surface complexes has resulted from ESR studies of both phyllosilicates and metal oxyhydroxides. In addition, ESR techniques have been combined with closely related spectroscopic methods, like electron-spin-echo envelope modulation (ESEEM) and electron-nuclear double resonance (ENDOR), to provide complementary information about transition metal ion behaviour at mineral surfaces (Sposito, 1993). The level of sophistication and sensitivity of these kinds of surface speciation studies is increasing continually, such that the heterogeneous colloidal particles in soils can be investigated ever more accurately. 

In the field of molecular knots templated by transition metal ions, another remarkable achievement deserves to be mentioned. Hunter and co-workers recently prepared a trefoil knot in a very satisfactory fashion by first wrapping a string-like molecule around a single transition metal ion with an octahedral coordination sphere (Zn2+) so as to obtain an open knotted structure and, subsequently, they could link the two ends of the string to form a real trefoil knot [38]. This nice piece of work again confirms the power of transition metals when it comes to preparing topologically novel species. 

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