Bond , 16-25 with carbon covalent

There are more complex examples of metal ion catalysis. Cobalt in vitamin B12 reactions forms covalent bonds with carbons of substrates.41,42 Metals can also act as electron conduits in redox reactions. For example, in cytochrome c the iron in the heme is reversibly oxidized and reduced. 

In general the role of the anions derived from Br nsted acids should be viewed in terms of their nucleophilicity, e.g. the facility with which they can recombine vritii the carbocation. Anions like Cl and CHsCOO have a pronounced tendency to form covalent bonds with carbon and therefore, except under very special circumstances such as extremely basic monomers or very polar media, the lifetime of ionic chain carriers will be very limited with such counterions. As we diall see, this situation is in fact much more general than commonly believed, and ester formation is quite important with practically all Br nsted acids. 

The percent ionicity (ionic character) is related to the difference between the EN values of the atoms of the C-Met bond (ENc-EN gJ. These are estimated values, which are affected by the nature of the substituents on carbon. Nevertheless, they indicate that the C-Li, C-Mg, C-Ti, and C-Al bonds are more ionic than C—Zn, C-Cu, C-Sn, and C-B, which form mainly covalent bonds with carbon. Manipulation of certain organometallic reagents requires special techniques. ... 

The Palacios technique was recently applied to modeling STM experiments of the Ceo molecule adsorbed on Al(lll) electrodes with different geometries. Contacts from A1 are not common in e)q)eriments, but in contrast to precious metals they create strong covalent bonds with carbon molecules. This is an important feature for practical implementations. 

Slow crystallization of 1-trifluoroacetoxybenziodoxaborole 188 from methanol afforded the tetrameric macrocyclic structure 192 resulting from self-assembly of the initially formed 4-fluoro-l,3-dimethoxy-l//-lX -benzoM[l,2,3]iodoxoborole (191) (Scheme 2.59). The structure of macrocycle 192 was established by a single-crystal X-ray analysis [243]. The driving force for formation of the eight-membered cyclic system 192 is the transformation of initial trigonal-planar sp hybrid boron atoms in 191 into tetrahedral sp hybridized atoms. Indeed, each boron atom in tetramer 192 forms one covalent bond with carbon and three covalent... 

A carbon atom has four unpaired electrons and can share four electrons wifli other atoms and form four covalent bonds. Such covalent bonds can be extremely strong (diamond), and the electrons can be locally and strongly bound. Therefore, sohd covalent crystals generally exhibit no electrical conductivity—neither electronic nor ionic. In biomaterials, covalent bonds with carbon are very important, and biomaterials usually have no molecular ionic or electronic conductivity. However, the charges in such a molecule may be far apart thus, very large dipole moments and strong electric polarization may occur (Table 2.3). 

Notice that when one pair of electrons from oxygen is moved to form a covalent bond with carbon, a pair of electrons in a covalent bond between carbon and the other oxygen atom is moved to oxygen. This is necessary to ensure that the carbon atom does not exceed its valence of 4. 

Since the IVA group elements. Si, Ge, and Sn, are expected to form stable covalent bonds with carbon, we have investigated possibility and stability of anionic living polymers of p-substituted styrenes with triaryl and trialkyl silicon, germanium, and tin. 

Chemical coordination of metal ions in living systems is a central question in metal ion homeostasis and is under intense study. The predominant atoms that coordinate with the metals are oxygen, nitrogen, and sulfur. However, sometimes metals can form covalent bonds with carbon (e.g., methylmercury). Oxygen is used to coordinate transition metals for transport in the case of siderophores (e.g., enterochelin and aerobactin), but examples of proteins using oxygen atoms on amino acid side chains to coordinate metals are rare. Sulfur on cysteine or methionine residues is a common coordinating atom. Examples of cysteine coordination with... 

Lithium and magnesium cations exhibit a small ionic radius which explains the partial covalent character of their bond with carbon atoms in nonpolar solvents, provided that the carbanion is not too delocalized. 

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