Chelate rings combinations

One of the most extensively studied systems is that of [Co(dien)2]3+ (Table 7.1). The mer- isomer is chiral (C2) because of the two possible orientations of the proton at the secondary nitrogen atomst38,39], the unsym-fac-vsoiasi is also chiral (C2) while the sym-fac-isomer is achiral (Q). For the five-membered chelate rings all possible combinations of S and X conformations (see Fig. 6.5 for the SIX nomenclature) have to be considered, leading to a total of 40 isomers and conformers (some conformers are calculated to be unstable and may therefore be neglected144,451). The calculated distribution is based on partition functions (Eqs. 7.1, 7.2) ... 

Co(trab)2]3+ (trab = 1,2,4-triaminobutane) has five isomers and a total of 16 con-formers (all nondegenerate combinations of chair and skew-boat of the six-mem-bered chelate rings Table 7.3 for the structures see Fig. 7.3). Two of the chromatographic signals of the three isomers A, B, C with optically pure trab were overlap-... 

The parameters a), b) and c) determine the topography of the complex. One may consider a complex formed by a multidentate ligand as a combination of several sub-units comprizing three donor atoms and two condensed chelate rings. A complex with a hexadentate ligand of type EDTA, for example, gives six possible combinations of such sub-units. Such a tridentate unity offers two structural possibilities as shown in Fig. 4. Different chemical characteristics of the currently employed ligands are... 

Chelation the combination of a metallic ion with heterocyclic ring structures whereby the ion is held by bonds from each of the rings. 

As mentioned above (point 10) MOMEC does not allow you to compute negative torsion angles. Since, in the present case, it is necessary to drive the N-C-C-N torsion to -60° we have to adopt a somewhat clumsy procedure we have to setup the structures for both endpoints (N-C-C-N torsion of +60° and -60°) by reflecting the chelate ring in HyperChem (see e.g., Section 17.3), and then drive the N-C-C-N torsion from each side to 0°, while constraining the Co-N-C-C torsion at the specified value. The results indicate that you get an identical table and plot to that above, i.e., the two traces are, as expected, symmetrically related (note that this is not true for unsymmetrical structures). A combination of the two files leads to the plot shown in Fig. 17.15.7. 

In summary, bis(dithiolene) complexes are clearly distinct from traditional inorganic or organometallic complexes in which the chemical reactivity is dominated by the metal center. The unique properties of dithiolene ligands such as redox activity, aromaticity, and unsaturation of the metal-ligand chelate rings, in combination with the metal-centered reactivity paths, have generated many unusual reactivity patterns for this class of complexes. 

Theoretical and experimental studies on penta-coordinate silicon derivatives demonstrate that their existence is determined by a combination of factors electronegativity of the substituents and steric interactions between substituents. One should also emphasize the role and significance of polydentate ligand (chelate effect), the size and number of chelate rings involving the silicon atom, and strain reduction for five-membered ring system, wich can stabilize unusual structures, as well as to the role of medium effect. This is consistent with the results of NMR spectroscopic studies. 

Although formally an a-diketone, tropolone gives rise to a monoanion (equation 16) with coordination chemistry similar to that of the /3-diketonate anions. The tropolonate ion forms a five-membered chelate ring, which has a smaller Bite Angle (see Bite Angle) than /3-diketonate anions. This feature, in combination with its rigid planar nature, makes the tropolonate ion suitable for the formation of complexes... 

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