Dative binding

Figure 7.26 Dendrimers made with a disulfide-containing core can be reduced to produce dendrons having free thiol groups for surface modification. Dative binding of these thiol-dendrons to gold or metallic surfaces can provide a high density of amine groups for coupling proteins or other molecules.

The examples of dative binding of catalytically active POMs to acquire heterogeneous POM-based catalysts can be found in the literature quite rarely compared to the examples of electrostatic attachment which were discussed in the previous section. 

Figure 10. Dative binding of [PWnCo039] to NH2-modified silica [95].

Hemoproteins have also been extensively employed in the design of artificial metalloenzymes. In hemoproteins, the Fe ion is five- or hexacoordinated, the four equatorial positions being occupied by the tetradentate protoporphyrin IX ligand while at least one of the two axial positions is occupied by an amino acid of the apoprotein, either His, Cys, or Tyr. Thus, removal of the heme from hemoproteins should provide a vacant space for other metal complexes to insert together with one or two N-, S-, or O-ligands for dative binding of the nonnative metal. 

A second strategy in this area involves the use of multidentate ligands which can datively bind to metal centers. Thus, DME, TMEDA, and phosphines such as... 

Figure 9.60 Many different thiol-containing linkers can be used to prepare water-soluble QDs. The monothiol compounds suffer from the deficiency of being easily oxidized or displaced off the surface, thus creating holes for potential nonspecific binding. The dithiol linkers are superior in this regard, as they form highly stable dative bonds with the semiconductor metal surface that do not get displaced. The PEG-based linkers are especially effective at creating a biocompatible surface for conjugation with biomolecules.

As shown in Fig. 4.69, the HfFLi- alkene complex exhibits expected parallels with the HfFLi- H2 complex (Fig. 4.59), both in terms of molecular shape and in terms of valence interactions. The characteristic features of such weak dative bonding include long Hf—C distances (2.82 A), normal C=C bond length (1.34 A), planar alkene bond angles, and small binding energy (15.1 kcalmol-1)-... 

Resonance such as (5.28a)-(5.28c) is inherently a quantal phenomenon, with no classical counterpart. In NBO language, each of the resonance interactions (5.28a)-(5.28c) corresponds to a donor-acceptor interaction between a nominally filled (donor Lewis-type) and unfilled (acceptor non-Lewis-type) orbital, the orbital counterpart of G. N. Lewis s general acid-base concept. As mentioned above, Lewis and Werner (among others) had well recognized the presence of such valence-like forces in the dative or coordinative binding of free molecular species. Thus, the advent of quantum mechanics and Pauling s resonance theory served to secure and justify chemical concepts that had previously been established on the basis of compelling chemical evidence. 

Ab initio theoretical cluster calculations showed that all of the products, with the exception of the O-dative bonded product, have sufficient binding energies (from 40-60 kcal/mol) to be thermodynamically stable at room temperature within the timescale of the experiments [255]. Furthermore, most of the activation barriers... 

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... 

Such a bond, in which the donor molecule (or anion) provides both bonding electrons and the acceptor cation provides the empty orbital, is called a coordinate or dative bond. The resulting aggregation is called a complex. Actually, any molecule with an empty orbital in its valence shell, such as the gas boron trifluoride, can in principle act as an electron pair acceptor, and indeed BF3 reacts with ammonia (which has a lone pair, NH3) to form a complex H3N ->BF3. Our concern here, however, is with metal cations, and these usually form complexes with from 2 to 12 donor molecules at once, depending on the sizes and electronic structures of the cation and donor molecules. The bound donor molecules are called ligands (from the Latin ligare, to bind), and the acceptor and donor species may be regarded as Lewis acids and Lewis bases, respectively. 

Similar results were obtained with [(CO)2(PH3)2Fe(772-E=CH2)]. For E = S a substantially larger binding energy and a more pronounced deformation of the H2C=E ligand was calculated than for E = O. It was concluded that the 77-back-donation is the main bonding interaction between the metal and the H2C=E molecule in the complexes. In the case of the thioformalde-hyde complex, a nonnegligible ligand-to-metal interaction between thioformaldehyde and iron exists.41... 

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