Asymmetry, metal ions

Coordination of ammonia or a substituted ammonia to a metal ion alters markedly the N — H dissociation rate (see See. 6.4.2). Since also proton dissoeiation of complexed ammines is base-catalyzed, then exchange can be made quite slow in an aeid medium. Thus, in a eoordinated system of the type 12, containing an asymmetric nitrogen atom (and this is the only potential souree of optical activity), there is every chance for a successful resolution in acid conditions, since inversion is expected only after deprotonation. It was not until 1966 that this was suc-eessfully performed, however, using the complex ion 12. A number of Co(III), Pt(II) and Pt(IV) complexes containing sarcosine or secondary amines have been resolved and their raeemizations studied.Asymmetrie nitrogen centers appear eonfined to d and d ... 

In a number of nonenzymatic reactions catalyzed by pyridoxal, a metal ion complex is formed—a combination of a multivalent metal ion such as cupric oi aluminum ion with the Schiff base formed from the combination of an amino acid and pyridoxal (I). The electrostatic effect of the metal ion, as well as the electron sink of the pyridinium ion, facilitates the removal of an a -hydrogen atom to form the tautomeric Schiff base, II. Schiff base II is capable of a number of reactions characteristic of pyridoxal systems. Since the former asymmetric center of the amino acid has lost its asymmetry, donation of a proton to that center followed by hydrolytic cleavage of the system will result in racemic amino acid. On the other hand, donation of a proton to the benzylic carbon atom followed by hydrolytic cleavage of the system will result in a transamination reaction—that is, the amino acid will be converted to a keto acid and pyridoxal will be converted to pyridoxamine. Decarboxylation of the original amino acid can occur instead of the initial loss of a proton. In either case, a pair of electrons must be absorbed by the pyridoxal system, and in each case, the electrostatic effect of the metal ion facilitates this electron movement, as well as the subsequent hydrolytic cleavage (40, 43). 

It is interesting to note that whereas four different unidentate ligands are required for asymmetry in a tetrahedral complex, only three are needed for the octahedron. An asymmetric or dissymmetric tetrahedral complex involving a transition metal ion has not yet been resolved (see, however, ref. ... 

Another factor contributing to the asymmetry and breadth of absorption bands in crystal field spectra of transition metal ions is the dynamic Jahn-Teller effect, particularly for dissolved hexahydrated ions such as [Fe(H20)6]2+ and [Ti(H20)6]3+, which are not subjected to static distortions of a crystal structure. The degeneracies of the excited 5Eg and 2Eg crystal field states of Fe2+ and Ti3+, respectively, are resolved into two levels during the lifetime of the electronic transition. This is too short to induce static distortion of the ligand environment even when the cations occupy regular octahedral sites as in the periclase structure. A dual electronic transition to the resolved energy levels of the Eg excited states causes asymmetry and contributes to the broadened absorption bands in spectra of most Ti(m) and Fe(II) compounds and minerals (cf. figs 3.1,3.2 and 5.2). 

The plot above has been produced with six identical distances from the centroid of the ligand (the metal center) to the amines. A non-spherical metal ion (e.g., a Jahn-Teller labile copper(II) ion) will induce different stresses to a symmetrical ligand. Alternatively, the ligand might be asymmetrical, i. e., the ligand itself might induce an asymmetry in the coordination sphere. Effects like these will be studied in Section 17.17. 

Another possible approach for stereospecific synthesis is the use of zeolites with appropriate transition metal fi amework ions to effect asymmetric chemistry. One basis of the prerequisite asymmetry may be the disruption of the local symmetry introduced by the size and the electronic structure of the chosen transition-metal ion impurity contained in the ciystal fi amework. additional chemical manipulations, such as the replacement of the oxide ligand or the addition of activating ligands, may result in desired catalytic activity and selectivity and the maintenance of uniformity among active sites. 

Counterions. 1. Sodium-23 Alkali metal MIR is a sensitive >robe of the immediate chemical environment and mobility of alkali metal ions in aqueous and nonaqueous solvents (7, 8). The chemical shifts of alkali metal nuclei will respond to" electronic changes only in the immediate environment of the cation since alkali metals rarely participate in covalent bonding (7). All alkali metal nuclei have spins greater than 1/2 and hence have quadrupole moments. The interaction of these moments with electric field gradients, produced by asymmetries in the electronic environment, is modulated by translation and rotational diffusive motions in the liquid. It is via this relaxation mechanism that the resonance line width is a sensitive probe of ionic mobility. 

It is interesting to note that whereas four different unidentate ligands are required for asymmetry in a tetrahedral complex, only three are needed for the octahedron. An asymmetric or dissymmetric tetrahedral complex involving a transition metal ion has not yet been resolved (see, however, ref. 246). Enantiomers of so-called pseudo-asymmetric organometallic compounds (20)-(22), in which a coordination site on a tetrahedron or square pyramid is occupied by an ij -cyclopentadienyl or 17 -arene ring (or similar w-donor ligands), have been well characterized however, due largely to the efforts of Brunner and coworkers. 

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