Surface dative bonds with

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.

Thiol-containing molecules can interact with metal ions and metal surfaces to form dative bonds. Dative bonds also are known as coordinate covalent bonds. They differ from normal... 

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... 

Cao, X. P. and Hamers, R. J. Silicon surfaces as electron acceptors Dative bonding of amines with Si(001) and Si(lll) surfaces. Journal of the American Chemical Society 123, 10988... 

The calculated results confirm that the 1-propanol molecule initially interacts with the Si(001)-(2 x 1) surface via the formation of a dative bond between the oxygen atom and the electrophilic down Si atom of the surface dimer. Specifically, the O-Si bond may be characterized as a covalent connection arising from the lone pair of the O atom. The 1-propanol molecule remains essentially intact (this motivates the nomenclature 1-1,1-2 and 1-3 for the physisorbed configurations) at the physisorbed sites, and the O-H bond assumes various orientations with respect to the Si surface. The obtained structures are shown in Figure 14-3 which illustrates that the direction of the O-H bond can be parallel (1-1), antiparallel (1-2) or perpendicular (1-3) to the Si dimer. However, the energies of the three configurations are very close to each other, i.e., the rotation of the 1-propanol molecule around the Si-O bond is quite facile. 

The toxicity of an element such as sulfur is dependent on the presence, in the valency shell of the toxic element, of free electron pairs which are evidently necessary for the formation of the link with the catalyst. The toxicity—i.e., the power of forming a relatively strong chemisorptive bond—disappears if the structure of the molecule is of a shielded type in which this element is already associated with a completely shared electron octet. Thus, it appears (Maxted, 8) that the chemical bond by means of which the poison is linked to the metallic surface resembles the ordinary dative bond in which the poison is the donor. In the case of methyl sulhde adsorbed on palladium, indications have been obtained (Dilke, Eley, and Maxted, 9) by means of magnetic susceptibility measurements that electrons from the methyl sulfide enter the d-band of the adsorbing metal to give a coordinate link, the process being probably accompanied (Maxted, 10) by a filling up of the fractional deficiencies or holes in the d-band of the metal due to d- -band overlap which seem to be responmble for the catalytic activity of the transition metals (11). 

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