Direct bonding between centers coordination

The wide variety of compounds involving direct bonding between centers of coordination can be treated by one of the following practices ... 

Since the most direct evidence for specihc solvation of a carbene would be a spectroscopic signature distinct from that of the free carbene and also from that of a fully formed ylide, TRIR spectroscopy has been used to search for such car-bene-solvent interactions. Chlorophenylcarbene (32) and fluorophenylcarbene (33) were recently examined by TRIR spectroscopy in the absence and presence of tetrahydrofuran (THF) or benzene. These carbenes possess IR bands near 1225 cm that largely involve stretching of the partial double bond between the carbene carbon and the aromatic ring. It was anticipated that electron pair donation from a coordinating solvent such as THF or benzene into the empty carbene p-orbital might reduce the partial double bond character to the carbene center, shifting this vibrational frequency to a lower value. However, such shifts were not observed, perhaps because these halophenylcarbenes are so well stabilized that interactions with solvent are too weak to be observed. The bimolecular rate constant for the reaction of carbenes 32 and 33 with tetramethylethylene (TME) was also unaffected by THF or benzene, consistent with the lack of solvent coordination in these cases. °... 

Direct coordination of a metal ion to the non-bridging oxygen might render the phosphorus center more susceptible to nucleophilic attack (electrophilic catalysis Fig. 3e) or, alternatively, hydrogen bonding between a metal-... 

The activation of electrophiles by coordination is a direct result of file weakening of the bond between the donor atom and the rest of the. ligand molecule after the donor atom has become bonded to the coordination center. There is considerable evidence to support the claim that this bond need not be broken heteroly tically prior to reaction as an electrophile. When this does not occur the literal electrophile is that portion of the ligand which bears a partial positive charge. In such a case the activation process can be more accurately represented by ... 

The reaction control should be emphasized amongst the conditions of reactions of competitive complex formation [19,23], It is necessary to take into account that it is possible to determine, and frequently predict, the direction of the electrophilic attack to the donor center of di- and polyfunctional donors (ligands) only in the case when the thermodynamically stable products are formed under conditions of kinetic control. Thus, the thermodynamic stability of complexes is discussed, when the bond between the metal and di- and polydentate ligands is localized in the place of primary attack on one of any of the donor centers by the electrophilic reagent, without further change of coordination mode in the reaction of complex formation. 

A relatively common interaction in molecular coordination or organometallic compounds that nominally are coordinately unsaturated is the formation of a three-center two-electron bond between a metal center in the compound and a C-H bond of a hydrocarbon, a hydrocarbon fragment, or a hydrocarbon derivative that is a ligand in the complex. This interaction can be the prelude, the intermediate or transition state, to a subsequent reaction in which the CH hydrogen atom is transferred to the metal center and a direct a bond is formed between the carbon atom and the metal atom especially if the C-H bond is an activated bond. Internal oxidative addition of CH is a term often applied to this subsequent reaction step. The overall sequence is schematically outlined in 1. Factors that materially... 

Figure 24. Electron-transfer pathway between Cua and heme-a in P. denitrificans COX. The path consists of 14 covalent bonds and two hydrogen bonds. The direct distance between to two metal ion centers is 2.0 nm. The binuclear heme-03/CuB site is also shown. Calculations were based on the Beratan and Onuchic model (6, 7). Coordinates were taken from the PDB, code IQLE. (See eolor...

Addition and elimination are simply different aspects of the same transformation followed either in the forward or backward directions. In the process the coordination number of the reaction center is increased or decreased by one. The transition state of both reactions is characterized by a partial bond between the reactants. In Chapter 6 several examples of addition/elimination reactions have been discussed, primarily those involving carbonyl carbon and other three-coordinate atoms as reaction centers. The inorganic analogs given there illustrate the relationship between the organic transformations and the more general concept of addition and elimination of Lewis bases to and from Lewis acids. 

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