Donor sites

If an open-chain organic molecule contains an electron acceptor and an electron donor site, two carbon atoms may be combined intramolecularly. This corresponds to the synthesis of a monocyclic compound. 

Optical absorption measurements give band-gap data for cubic sihcon carbide as 2.2 eV and for the a-form as 2.86 eV at 300 K (55). In the region of low absorption coefficients, optical transitions are indirect whereas direct transitions predominate for quantum energies above 6 eV. The electron affinity is about 4 eV. The electronic bonding in sihcon carbide is considered to be predominantiy covalent in nature, but with some ionic character (55). In a Raman scattering study of vahey-orbit transitions in 6H-sihcon carbide, three electron transitions were observed, one for each of the inequivalent nitrogen donor sites in the sihcon carbide lattice (56). The donor ionization energy for the three sites had values of 0.105, 0.140, and 0.143 eV (57). 

Monomeric sulfur diimides have an extensive coordination chemistry as might be anticipated from the availability of three potential donor sites and two r-bonds. In addition, they are prone to fragmentation to produce thionitroso and, subsequently, sulfido and imido ligands. Under mild conditions with suitable coordinatively unsaturated metal... 

Ambidentate ligands possess more than 1 donor atom and can coordinate through either one or the other. This leads to the possibility of linkage isomerism (p. 920). The commonest examples are the ions NO2 (p. 463) and SCN (p. 325). Such ligands can also coordinate via both donor sites simultaneously, thereby acting as bridging ligands. 

Diphenyl-2-thienylphosphine is expected to serve as a bidentate ligand by coordination of either the phosphorus and sulfur atoms or the phosphorus atom and TT-electrons of the heteroring. Reaction of this phosphine with [Rc2(CO)lo] yields the species where both the sulfur and phosphorus atoms serve as the donor sites (960M786). 

Cases of the S-coordinated rhodium and iridium are quite scarce. To complete the picture, we next consider the possibilities of S-coordination using complicated derivatives of thiophene. 2,5-[Bis(2-diphenylphosphino)ethyl]thiophene is known to contain three potential donor sites, two phosphorus atoms and the sulfur heteroatom, the latter being a rather nucleophilic center (93IC5652). A more typical situation is coordination via the phosphorus sites. It is also observed in the product of the reaction of 2,5-bis[3-(diphenylphosphino)propyl]thiophene (L) with the species obtained after treatment of [(cod)Rh(acac)] with perchloric acid (95IC365). Carbonylation of [Rh(cod)L][C104]) thus prepared yields 237. Decarbonylation of 237 gives a mixture of 238 and the S-coordinated species 239. Complete decarbonylation gives 240, where the heterocycle is -coordinated. The cycle of carbonylation decarbonylation is reversible. 

The difference in the hydrogen bond acidities and basicities was far more marked. The a value is largely determined by the availability of hydrogen bond donor sites on the cation. Values range from 0.8-0.9 for the monoalkylammonium salts, and are slightly lower (0.3-0.8) for the imidazolium salts. In the absence of a... 

To date, only a few papers have been published on the structures of organotin(lV) complexes formed with carboxylate compounds containing O S donor sites. 

Condensation of dicesium 2-thioxo-l,3-dithiole-4,5-diselenolate with fo/s-alkylating polythioethers under high dilution conditions afforded the TTF-containing macrocycles possessing soft donor sites and 12-, 15-, and 18-membered rings <%JCS(P1)1995>. 

The introduction in the polymer of non-coordinative -0-(CH2)x-CH3 side chains (Table 13) leads to a decrease of the conductivity from 1.29x10 for x=2 to 1.9x10 S cm for x=9, which shows that a great number of donor sites is primordial for the ion transport [278]. 

There is an extensive number of nucleobase complexes that contain multiple metal ions (75). In some cases the binding sites are in close proximity and there is the possibility for metal-metal interactions. Figures 23 and 24 illustrate various binding modes involving the N,0-donor sites which can potentially give rise to such phenomena. 

Some Multi-Site Ligands Containing Nitrogen Donor Sites and Metal Complexes Derived from Them... 

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