Coordination complexes, from Lewis acid-base

We should note that the formation of this bond confers formal charges on the B and N atoms. In this bond and many similar Lewis acid-base complexes both the electrons forming the bond come from the same atom rather than from different atoms, as in the formation of a bond between two chlorine atoms. This type of bond is often called a donor-acceptor bond, a dative bond, or a coordinate bond, and is sometimes given a special symbol—an arrow denoting the direction in which the electron pair is donated ... 

The chemistry of coordination compounds is a broad area of inorganic chemistry that has as its central theme the formation of coordinate bonds. A coordinate bond is one in which both of the electrons used to form the bond come from one of the atoms, rather than each atom contributing an electron to the bonding pair, particularly between metal atoms or ions and electron pair donors. Electron pair donation and acceptance result in the formation of a coordinate bond according to the Lewis acid-base theory (see Chapter 5). However, compounds such as H3N BC13 will not be considered as coordination compounds, even though a coordinate bond is present. The term molecular compound or adduct is appropriately used to describe these complexes that are formed by interaction of molecular Lewis acids and bases. The generally accepted use of the term coordination compound or coordination complex refers to the assembly that results when a metal ion or atom accepts pairs of electrons from a certain number of molecules or ions. Such assemblies commonly involve a transition metal, but there is no reason to restrict the term in that way because nontransition metals (Al3+, Be2+, etc.) also form coordination compounds. 

The first step of the mechanism is the coordination of BFI3 (Lewis acid) to the tertiary nitrogen atom (Lewis base) of the CBS catalyst from the -face. This coordination enhances the Lewis acidity of the endocyclic boron atom and activates the BH3 to become a strong hydride donor. The CBS catalyst-borane complex then binds to the ketone at the sterically more accessible lone pair (the lone pair closer to the smaller substituent) via the endocyclic boron atom. At this point the ketone and the coordinated borane in the vicinal position are cis to each other and the unfavorable steric interactions between the ketone and the CBS catalyst are minimal. The face-selective hydride transfer takes... 

As for the chiral ytterbium and scandium catalysts, the following structures were postulated. The unique structure shown in scheme 13 was indicated by 13C NMR and IR spectra. The most characteristic point of the catalysts was the existence of hydrogen bonds between the phenolic hydrogens of (R)-binaphthol and the nitrogens of the tertiary amines. The 13 C NMR spectra indicated these interactions, and the existence of the hydrogen bonds was confirmed by the IR spectra (Fritsch and Zundel 1981). The coordination form of these catalysts may be similar to that of the lanthanide(III)-water or -alcohol complex (for a review see Hart 1987). It is noted that the structure is quite different from those of conventional chiral Lewis acids based on aluminum (Maruoka and Yamamoto 1989, Bao et al. 1993), boron (Hattori and Yamamoto 1992), or titanium... 

A second type of oxygen-chelated complex that can be formed with acetylacetone is the simple Lewis acid-base adduct. In these complexes acetylacetone does not lose its acidic proton to form an enolate anion, but rather as the neutral molecule in the keto tautomer donates electrons from the oxygens of each carbonyl to an acceptor or acidic species. Examples of this type of complex are the six-coordinate adducts formed between typically strong Lewis adds as tin tetrachloride or titanium... 

As noted earlier in this section lithiation of oxazoles with -butyllithium can be plagued by the formation of products derived from the tautomeric isocyanovinyl-lithium alkoxide species. Vedejs and Monahan conceived an elegant solution to avoid this side reaction (Scheme 1.245). The authors precomplexed an oxazole 896 with THF-borane to form the stable, isolable Lewis acid-base complex 914. This complex effectively coordinated the lone pair of electrons on nitrogen that is essential for the undesired electocyclic ring opening reaction. In addition, the authors anticipated that the coordination would enhance the acidity of the C(2)-H. Deprotonation of the complex with LiTMP (n-BuLi and i-BuLi are also effective)... 

The mechanisms of ionic and coordination polymerizations are more complex and are not as clearly understood as those of free radical polymerization. Here, we will briefly highlight the essential features of these mechanisms, and more details will be given in Chapter 7. Initiation of ionic polymerization usually involves the transfer of an ion or an electron to or from the monomer. Many monomers can polymerize by more than one mechanism, but the most appropriate polymerization mechanism for each monomer is related to the polarity of the monomers and the Lewis acid-base strength of the ion formed. 

Coordination of the water molecules to a metal center is a Lewis acid/base reaction in which charge is transferred from the ligands (water) to the metal. This makes a coordinated water molecule more acidic than a noncoordinated one, i.e., dissociation of a proton from a coordinated water is easier than from a noncoordinated water. Depending on the magnitude of the charge transfer, the equilibrium concentrations of aquo complexes, hydroxo species, and oxo species (Equation (3)) are different. The formation of metal hydroxide species from aquo complexes is called hydrolysis. 

The M /nH exchange is initially fast, given the enlargement of the d caused by the presence of die base. Complex species are subsequently formed between the layers of the solid, if the basic groups of the ligand prefer to deprotonate from the Bronsted PO -OH/H O sites in order to coordinate with the Lewis acids, i.e., the transition metal ions. 

The most common activator is known as MAO (methylaluminoxane). MAO is a complex mixture of chemical species, but has the rough C A1 0 stoichiometry of 1 1 1. MAO is prepared from the careful reaction of trimethylaluminum with water. As described earlier, the Rappe group has developed a MAO model based on structural studies and analogy to AIR(NR ) clusters. The model consists of a (AIMeO)9 cluster that has abstracted a Me and has had the resulting two-coordinate oxygen coordinated by the Lewis acidic AlMes. In the following discussion, we refer to this counteranion model as MA09. 

Transition Metal Complexes Related to the Simon test is a family of color-producing reactions based on transition metal complexes (coordination complexes) and tightly associated ion pairs. Coordination complexes arise from a Lewis acid-base interaction between a metal cation, such as cobalt, and an atom with unshared electrons, such as water or, in the case of drugs, basic nitrogen found in alkaloids and amines. Metals that have been used in these reagents include copper, vanadium, bismuth, and cobalt Cobalt, as part of two common reagents (cobalt thiocyanate and Dilli-Koppanyi) is perhaps the most versatile. Cobalt has an electron structure of 3d 4s, while ttie cation has a 3d (2 ) or 3d (3 ") structure. 

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