Bidentate species

In the corresponding experimental evaluation, a bidentate (=AIO)2Pb complex was formed in addition to =AIOPb+. To keep our argumentation simple, we neglect the bidentate species (Hohl and Stumm, 1976). 

ToF-SIMS, and AFM results, the formation of ordered monolayers of octade-cylphosphoric acid on a Ta205 surface involves both monodentate and bi-dentate phosphate species [135]. In the case of goethite, (y-AlOOH), it was found that methylphosphonic acid bound to the surface as a monodentate or a bidentate species depending on the pH and the concentration [163]. 

In Eq.(8), the monodentate nitrate species is written as an example of surface nitrate species which does not exclude possible bridging and bidentate species. 

Figure 3 shows that room temperature stirring of MCM-48 with DMDCS, using NEt3 as a catalyst, removes all silanols and leaves the surface covered with silyl groups. Chemical analysis established that surface is covered with 20% of bidentate species ((Si-0)2Si(CH3)2, which are completely inert towards further reaction) and 80 % of... 

Figure 7 presents the overall, idealized reaction mechanism. The surface of MCM-48 contains 0.9 OH / nmJ, which react completely with DMDCS in the liquid phase, if NEt3 is used as a catalyst. The majority of the silanols react monofunctionally but a small fraction also reacts further, according to reaction (3) to yield inert, bidentate species. All chlorine functions on the surface are converted towards hydroxyls upon hydrolysis. The VO(acac)2 is reacted in a gas-phase reactor with this silylated, hydrolyzed surface. All recreated silanols react with the VO(acac)2 in a 1 1 stoichiometry, following a ligand-exchange mechanism. Upon calcination at 450°C, the acac ligands are decomposed but the methylsilyl functions remain intact. Most of the V-species are converted into isolated, tetrahedral VvOx species, as indicated in Figure 4. However, a small fraction clusters to form surface oligomers, hereby recreating a fraction of the silanols.

Figure 8 Schematic representation of the relationship of octahedral [M(AA)2B2] or [M(AA)2(BB)] structures to left-and right-handed helices. Note that the line aa that represents the helix axis must be placed behind the tangential line bb in order to define the helicity. It should also be noted that it is immaterial which chelate-spanned edge is taken as the axis or the tangent and that in tris(bidentate) species it is only necessary to consider one pair of edges...

Table 10 Rate Constants for the Metal Ion-promoted Base Hydrolysis of some Bidentate Species of Cysteinate Esters at 25 °C...

Thus, Worsfold 133) measured the association behavior, via light scattering, UV-visible spectroscopy and viscosity, of polystyrene chaips capped at one end with the dimethylamine group where the associating group was the bis-(2,6-dinitrohydro-quinol). This bidentate species ... 

The rather difficult question how many alkoxygroups of one trialkoxysilane molecule participate in the chemical bonding has been studied by high resolution NMR on 29Si and 13C nuclei.92 93,94 Sindorf and Maciel92 concluded that in the case of dehydrated silicas, mainly monodentate and some bidentate species are formed. Reactions involving all three alkoxy groups do no occur on the surface of dry silica. 

The formation of the bidentate species can occur in two ways as shown in reactions (B) and (D).1,11,14-15,19 Reaction (B) is the direct modification reaction of a BX3 with two surface hydroxyls, reaction (D) is a consecutive reaction of a monodentate specie... 

The reaction of B with the hydroxyl groups resulting in the formation of hydrogen gas and a bidentate specie (reaction (J), R=4). This in agreement with the results obtained by Shapiro et al.36... 

A variety of Lewis bases have been used to control microstructure in anionic polymerization, the main requirement being that the Lewis base is sufficiently stable in the presence of the propagating anion to allow living polymerization. The most commonly used modifiers are ethers and tertiary amines. Since amines are poisons for many hydrogenation catalysts, ethers are used more frequently in the production of hydrogenated polymers. A further distinction can be made between monobasic species such as dialkyl ethers and bidentate species that have the potential to coordinate with lithium, such as glyme ethers and TMEDA (N, V, N V -tetramethylethylenediamine). The former must... 

Later, a number of studies attempted to determine the nature of acid sites in the catalyst. Also applying XPS and IR spectroscopy, Tanabe and others proposed a structure of chelating bidentate complexes, in which the sulfate species chelates to a single Zr atom [102, 146]. This model, a chelating bidentate species, was also proposed by Ward and Ko [61]. 

The fact that the stability constants of monodentate ligand complexes of polyion systems are comparable in magnitude with those of the simple molecule resembled, once corrected for the electrostatic effect, is consistent with expectation based on the statistical arguments discussed earlier. The absence of multidentate complex formation is also predictable. Whereas the formation of MA is not affected by the low accessibility of the polyion species, MA and A, their nonideality, and canceling in the mass action expression for the formation of MA (f +/f - = 1), this is not the case for the bidentate species, because f /(f -) = l/f -, the nonideality term for 1/A" remaining uncanceled. The tendency for bidentate complex formation is, on this basis alone, a factor of f - less likely. 

All of this thinking does not explain CAMP, unless we argue that, during the hydrogenation step, this monodentate ligand prefers to occupy adjacent sites on the metal center and acts as a bidentate species. 

In allylation reactions of secondary iodide substrates, silyloxy and methoxy derivatives reacted with comparable speed [6], suggesting that the monodentate and bidentate complexes had reacted at similar rates. Conversely, within the tertiary bromide series, the silyloxy substrate reacted eight times faster than the methoxy derivative in hydrogen transfer reactions [3]. This result suggested that the monodentate complex had reacted faster than the bidentate. However, the consistent attacks that occurred on the same face of the radical in the allylation and reduction reactions implied that the bidentate complex formations had been stable and the exchange rates between the mono- and bidentate species had been slow. 

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