Bond, dative additivity

Steps 1-3 of the catalytic cycle correspond to various steps of other catalytic cycles already discussed in Chapter 13. Step 1 tt-complex formation by combination of the aryl triflate and a sufficiently valence-unsaturated and thus sufficiently reactive Pd(0) species. Step 2 oxi-dative addition of the aryl triflate to Pd with formation of a Cspi—Pd(II) bond. Steps 3a and 3b exchange of a PPh3 ligand by an acrylic acid methyl ester via a dissociation/addition mechanism. The newly entered acrylic acid ester is bound as a 77 complex. 

VSEPR model. The axial positions of the donor molecules are also in accordance with the VSEPR model, particularly if it is assumed that an accepted bond pair requires less space at the acceptor atom than a covalent bond pair. Addition of a sectmd donor molecule increases the covalent Al-Cl distance by 6 pm and the dative N—>A1 bond distance by 21 pm. 

Additional computational studies [16] provided detailed information about the nature of the dative bond, the strength of the acid-base interaction and the... 

So, this fact is strong evidence in favor of the intra- and inter-molecular dative P—B bond in 126. In solution, 126 is easily oxidized and undergoes addition reactions with sulfur and selenium, with the formation of the corresponding oxide (166), sulfide (167), or selenide (168) [Eq. (129)]. The 3,P chemical shifts of other compounds (166) (27 ppm), (167) (47 ppm), and (168) (46 ppm) are close to that observed for their structural analogues. These data and the values of dipole moments of 4,3 and 4,5 D for 166 and 167, respectively, make it possible to exclude the formation of intramolecular dative P—B bonds similar to those observed for borylphosphine ethene... 

B) In this case, a lone electron pair on oxygen forms a bond to Ff. A coordinate covalent bond (also known as a dative covalent bond) is a special type of covalent bond in which the shared electrons come from one of the atoms only. After the bond has been formed, its strength is no different from that of a covalent bond. Coordinate covalent bonds are formed when a Lewis base (an electron donor) donates a pair of electrons to a Lewis acid (an electron accepter) the resultant componnd is then called an addnct (a compound formed by the addition reaction between two molecules). The process of forming a dative bond is called coordination. 

In other cases, the dative-bonded state acts as a precursor for formation of more thermodynamically stable products. Whether the reaction progresses toward other products depends on the stability of the product and the size of any activation barrier between the dative-bonded precursor state and the other product(s). Cases where the dative-bonded state is a precursor to the final product include the heterocycloadditions (discussed in Section 6.2.2) in addition to pyrrole. Another case is that of secondary amides. A new study has shown that in the reaction of N-methylformamide, the oxygen-dative-bonded state is a precursor to N—H dissociation, via a cychc species shown in Figure 5.22. At room temperature, a mixture of the dative-bonded product and the N—H dissociation product are observed. However, the product distribution can be tuned using thermal control at low temperatures (240 K), only the dative bonded species was observed, whereas upon annealing to higher temperatures (450 K), only the N—H dissociation product was found [321]. 

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