Bond, chemical coordinate

In cases of complexes bearing an exocyclic double bond directly coordinated to the metal center, the carbons of the double bond usually exhibit coupling with NMR-active metal centers and/or auxiliary ligands.6 14 18 19 The chemical shifts of the quaternary carbon atom vary from 66.976 to 82.2818 ppm, while the methylene group gives rise to signals at 29.16,14 41.91,6 or 51.3418 ppm in the 13C 1H NMR spectra. As one can see, the chemical shift variation is relatively broad and significantly affected by the nature of the metal center. 

Lenhert and Hodgkin (15) revealed with X-ray diffraction techniques that 5 -deoxyadenosylcobalamin (Bi2-coenzyme) contained a cobalt-carbon o-bond (Fig. 3). The discovery of this stable Co—C-tr-bond interested coordination chemists, and the search for methods of synthesizing coen-zyme-Bi2 together with analogous alkyl-cobalt corrinoids from Vitamin B12 was started. In short order the partial chemical synthesis of 5 -de-oxyadenosylcobalamin was worked out in Smith s laboratory (22), and the chemical synthesis of methylcobalamin provided a second B 12-coenzyme which was found to be active in methyl-transfer enzymes (23). A general reaction for the synthesis of alkylcorrinoids is shown in Fig. 4. 

DeKock, R. L., and Gray, H. B. (1980). Chemical Structure and Bonding. Benjamin/Cummings, Menlo Park, CA. Chapter 6 presents a good introduction to bonding in coordination compounds. 

The complete topographic description of a chiral n-point figure in three-dimensional space requires 3n coordinates relative to an external coordinate system. However, these coordinates are impractical because they contain information on the position of the figure, i.e., its translational and rotational state. A more convenient description for the chemist is obtained by incorporating chemical bonds, and the use of 3n — 5 (linear models) or 3n —6 internal ( chemical ) coordinates which are bond lengths bond angles torsion (dihedral) angles chirality sense of tripods. 

Derivatization with achiral reagents is required for nonvolatile or highly polar substrates such as a-amino acids. Appropriate derivatization leads to improved resolvability by the introduction of suitable functional groups capable of hydrogen bonding or coordination. If derivatization is necessary, it has to be ensured that the chemical reaction does not lead to any racemization of the sample. 

Scheme 19 shows a general mechanism for C—H bond activation. In principle, any donor groups, including olefinic bonds, carbanions, heteroatom anions, neutral heteroatoms, for example, can activate their adjacent C—H bonds through coordination with appropriate transition metal centers. The metal hydride complexes formed by oxidative addition or /3-elimination, undergo unique chemical transformations. 

See also Bond (Chemical) Chemical Elements Coordination Number Molecule Oxidation and Oxidizing Agents and Oxidation Number,... 

Other mechanisms for sorption of cationic pesticides include hydrogen bonding and coordination between sorbate and the exchangeable cation. When the surface acidity of the clay is one or two pH units lower than the dissociation constant, chemical sorption could occur. 

A coordination chemist might prefer a formula like B and consider the X—Y metal fragment. However, someone engaged in the chemistry of either element X or Y might look at it in a different way. Formula B stresses the chemical origin of most of these compounds, which are usually obtained by addition of X—Y to a coordina-tively unsaturated metal fragment [Eq. (1)]. 

More generally, the chemical shifts are due to a combination of valence, electronegativity of the bonding ligands, coordination number, and other structural features. These factors are expressed by Batsanov s (19, 224, 225) concept of coordination charge, t, as... 

The synthetic chemist can see beauty in an approach to a chemical object, be it in the form of the perceived elegance, efficiency, directness, or combination of approaches. Covalent bond forming, coordination chemistry and purely non-covalent possibilities are all important of course, but each one on its own is weak. Take the statistical approach to link formation A flexible linear molecule in random motion is not likely to form a knot, and it is even less likely that we could separate and characterise it even if it did. There is no direction in the reactivity. Covalent bonds can be used to hold fragments together, and then those bonds used to orient the fragments are removed to leave the linked molecule. With today s control over molecular conformation and covalent bond making and breaking, this has to be considered a viable approach. But, those approaches based on coordination chemistry and non-covalent bonds are more direct and efficient for the moment. 

The immobilization of a large volume of liquid by a small quantity of gelator is achieved efficiently if the elementary assemblies are rodlike and have large aspect ratios. Such linear structures are determined by specific binding forces associated with the chemical constitution of the gelators. In nonaqueous liquids, the attractive forces are mainly the van der Waals type and can be supplemented by dipolar interactions, intermolecular hydrogen bonds, metal-coordination bonds, or electron transfers, etc. 

Molecules of N-substituted 2,2-diaryl-l,3-dioxa-6-aza-2-silacyclooctanes exist in solutions in two equilibrium forms non-coordinate (without a Si<-N bond) and coordinate An increase in the donor ability of the substituents at the silicon atom and in size of the substituents at the nitrogen atom shifts the equilibrium to a conformation lacking the Si<-N bond. The activation energy of < 11 kcal mol" for this process is much less than that of similar tin derivatives A calculated change in the Si chemical shift for a completely coordinated boat-boat shape as compared with a model molecule of diphenyldiethoxysilane depends linearly on the shift of the... 

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