The dative bond

Ammonia gas (NH3) reacts with boron trifluoride gas (BF3) to give a colourless molecular solid, H3NBF3. This can be explained by adding a further postulate to 1-9 above  

A bond can be formed between an atom with a lone pair and an atom with an incomplete shell. 

The boron atom now has an octet, and there is a new kind of bond between the nitrogen atom and the boron. This is similar to a covalent bond, but instead of one electron being supplied by each atom, both electrons come from the same atom, in this case the nitrogen. 

The bond just described has been variously written and variously named. One way of writing it is 

From this, it is sometimes called a semipolar double bond - a combination of a single ionic bond and a single covalent bond. 

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

The formation of a strong intramolecular dative P—B bond in the hydro-borylation reaction of diallylphenylphosphine with triethylaminophenyl-borane results in a bicyclic product, l-borata-5-phosphoniabi-cyclo[3,3,0]octane (204) [Eq. (145)] (64JA5045). The presence of the dative bond in this compound is indicated by an NMR study and by its stability to hydrolysis and oxidation. 

A related ligand, MPL (3-methoxypropanol), was combined with Al(f-Bu)3 and Al(Me)3 [28]. The resulting Bll compound was similar to those formed by MEL, with a dative interaction in the methyl compound but not the f-butyl one. However, the dative bond is much longer (2.39 A vs. 2.27 A) for the propyl compound. Another related ligand is MlP(l-methoxy-2-propanol), which was combined with Ga(Me)3 [33]. This complex is nearly identical to that formed by MEL with Ga(Me)3. 

MMPH (2-mercaptomethylphenol) is an example of an aromatic S,0 ligand [59]. This can be combined with Al(i-Bu)3 and Al(Me)3 to form two five-coordinate BII compounds. The two complexes were very similar structurally and quite close to the aliphatic S,0 compounds. There was, however, a noticeable difference between the bridging and covalent O—Al bonds (covalent approximately 1.86-1.87 A, bridging 1.96-1.98 A). Also, the dative bond was slightly shorter (2.7-2.8 A) than that seen in Barron s compound, this is probably due to resonance from the aromatic ring giving sulfur increased sp character. 

Figure 5.17. Illustration of the dative bonded product formed when trimethylamine adsorbs on Si(100)-2 x 1.

In order to understand why the activation energies differ between the two pathways, Mui et al. examined the transition state geometries [279]. They found that as electron density is donated from the amine lone pair to the down silicon atom upon adsorption into the precursor state, the up Si atom in the dimer becomes electron rich. At this stage, the dative bonded precursor can be described as a quaternary ammonium ion. The N—H dissociation pathway can thus be interpreted as the transfer of a proton from the ammonium ion to the electron-rich up Si atom through a Lewis acid-base reaction. In the transition state for this proton transfer, the N—H and Si—H... 

The participation of the germanium dimers in nucleophilic/electrophilic or Lewis acid/base reactions has been the subject of several investigations on the Ge(100)-2x1 surface [16,49,255,288,294,313-318]. As for the case of silicon, adsorption of amines has provided an excellent system for probing such reactions. Amines contain nitrogen lone pair electrons that can interact with the electrophilic down atom of a tilted Ge dimer to form a dative bond via a Lewis acid/base interaction (illustrated for trimethylamine at the Si(100)-2 x 1 surface in Ligure 5.17). In the dative bond, the lone pair electrons on nitrogen donate charge to the Ge down atom [49]. 

Pyridine, a six-membered cyclic aromatic amine, has also been studied on Ge(100)-2 x 1 both theoretically [315,316] and experimentally by STM [314]. It adsorbs selectively through a Ge—N dative bond on the surface. Theoretical calculations showed that the dative-bonded adduct is more stable than other possible reaction products (e.g., cycloaddition products) on Ge [315,316]. Furthermore, STM images show formation of a highly ordered monolayer at the surface with a coverage of 0.25 ML. The pyridine overlayer forms a c(4 x 2) structure in which the molecules bind to the down atoms of every other dimer to minimize repulsive interactions between pyridine molecules. 

A number of other studies have now shown that dative bonding is a phenomenon common to many organic reactions on Ge(100)-2 x 1, as it is for Si(100)-2 x 1. In some cases, e.g., with the methylamines and pyridine, the dative-bonded state is the final surface species. This dative-bonded state can be quite stable. For example, the nitrogen dative bonds formed via exposure of methylamines to Ge(100)-2 x 1 have binding energies near 25 kcal/mol [49]. The STM study of pyridine on Ge(100)-2x1 revealed that 90% of the dative-bonded surface adducts remain after one... 

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]. 

Figure 5.22. Structure of the O-dative bonded adduct formed by adsorption of N-methylformamide on Ge(100)-2 x 1, showing the presence of a Ge—H interaction, which stabilizes the dative bonded state [321].

Pentacoordinate silicon forms two types of bonds with tricoordinate nitrogen atoms, a pure covalent bond and a N - Si dative bond. The first is significantly shorter than the second. The average covalent Si—N bond length in compounds where pentacoordinate silicon atom is bonded to tricoordinate nitrogen atom was calculated from 48 XRD experimental values to be 1.761 A (s.d. 0.06 A, s.m. 0.009 A). An example of the difference in bond length is shown in 127141 where the covalent Si—N bond lengths are 1.766 and 1.770 A and the dative bond is 2.333 A. 

An interesting example of an intermolecular complex is the trisilicon complex 194, in which only the central silicon is coordinated to the bidentate donor molecule225. The structure is a regular octahedron, with two tetrahedral termini. The silicon nitrogen bonds are rather short (2.012 and 1.991 A), and are comparable to those of octahedral intramolecular complexes (Table 23). 194 permits a comparison of Si—Cl bonds in a tetrahedral silicon moiety (2.03 to 2.07 A) with Si—Cl bonds trans to the dative bond in a hexacoordinate silicon (2.39 and 2.21 A). As expected, the latter are substatntially longer than the regular covalent bonds. 

Since in this work the most studied molecular systems involve the 7r-donor and 7r-acceptor subunits, we will consider the relation between a nonbonding donor-acceptor combination, on the one hand, and the dative bond, on the other hand. 

On the other hand, since biradical excited states are in principle easy polarizable,39 105 they will have something in common with the charge-separated states of a broken dative bond. In order to find out the essentials about the excited states of the dative bonds, especially about the geometries corresponding to minima from which emission might take place, we will proceed in four steps. 

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