O-Donor, for

The effect of alkyl substiments on the stabilities of carbenium ions provides the electronic basis of the textbook Markovnikov s rule. The stabilizing effect of positive hyperconjugation increases for stronger o-donors. For example, the stabihzing effect of a silyl substituent in p-silylethyl cation is calculated to be ca. 38kcal/mol stronger than a C-H donor of the ethyl cation in the gas phase (see Section 6.3). The effects of Ge, Sn, and Hg are also substantial. For example, hyperconjugative activation by a Sn-C bond can accelerate a reaction by a factor of >10 . ... 

The synclinal conformation (sc) is appropriate for overlap of an oxygen nonbonded pair with the a C—Cl orbital. The preferred ap relationship, requires an antiperiplanar alignment of a lone-pair orbital with the bond to the electronegative substituent. Because of the donor-acceptor nature of the interaction it is enhanced in the order F < O < N for the donor (D) atom and N < O < F for the acceptor (A) atom. 

Complexes of and The effect of complexation on the splitting of d orbitals is much greater in the case of second- and third-than for first-row transition elements, and the associated effects already noted for Ni are even more marked for Pd and Pi as a result, their complexes are, with rare exceptions, diamagnetic and the vast majority are planar also. Not many complexes are formed with O-donor ligands but, of the few that arc, [M(H20)4] ions, and the polymeric anhydrous acetates [Pd(02CMe)2l3 and [Pt(02CMc)2]4 (Fig. 27.10), are the most... 

Palladium forms clusters of these types far less readily than nickel and platinum, unless they are stabilized by o-donor ligands such as phosphines. This may be due to the lower energy of Pd-Pd bonds as reflected in the sublimation energies, 427, 354 and 565 kJ mol for Ni, Pd and Pt. 

Polymerization of alkynes by Ni" complexes produces a variety of products which depend on conditions and especially on the particular nickel complex used. If, for instance, O-donor ligands such as acetylacetone or salicaldehyde are employed in a solvent such as tetrahydrofuran or dioxan, 4 coordination sites are available and cyclotetramerization occurs to give mainly cyclo-octatetraene (cot). If a less-labile ligand such as PPhj is incorporated, the coordination sites required for tetramerization are not available and cyclic trimerization to benzene predominates (Fig. A). These syntheses are amenable to extensive variation and adaptation. Substituted ring systems can be obtained from the appropriately substituted alkynes while linear polymers can also be produced. 

A wide variety of complexes are formed by both metals in the +2 oxidation state indeed, it is the most important one for palladium. The complexes can be cationic, neutral or anionic. Both Pd2+ and Pt2+ are soft acids so that many stable complexes are formed with S or P as donor atoms but few with O-donors, though there are important ammines. There are pronounced similarities between corresponding palladium and platinum complexes the latter are more studied (and less labile). 

Complexes of O-donors are relatively rare, explicable by the soft nature of the divalent ions. A telling indication is that sulphoxide ligands will only bind through O if steric effects make S-bonding impractical. The most important complexes are diketonates and carboxylates (for the aqua ions see section 3.5). 

Apart from Au(N03)4, relatively few complexes of gold(III), and only those with O-donors, have been examined. Two that demonstrate the preference of gold(III) for square planar coordination are SrAu2(MeCOO)8 and SrAu2(OH)8 in the latter Au(OH)4 has Au-O 1.98 A [117]. 

The effect of the presence of alkali promoters on ethylene adsorption on single crystal metal surfaces has been studied in the case ofPt (111).74 77 The same effect has been also studied for C6H6 and C4H8 on K-covered Pt(l 11).78,79 As ethylene and other unsaturated hydrocarbon molecules show net n- or o-donor behavior it is expected that alkalis will inhibit their adsorption on metal surfaces. The requirement of two free neighboring Pt atoms for adsorption of ethylene in the di-o state is also expected to allow for geometric (steric) hindrance of ethylene adsorption at high alkali coverages. 

Coates JD, KA Cole, R Chakraborty, SM O Connor, LA Achenbach (2002) Diversity and ubiquity of bacteria capable of utilizing humic substances as electron donors for anaerobic respiration. Appl Environ Microbiol 68 2445-2452. 

Replacement of the phosphorous ligand with an NHC is a logical next step toward stabilising the D-type intermediate due to the o-donor strength of the NHC. Thus, choosing the correct NHC should allow for high selectivities without excess ligand. 

Fig. 5. Plot of enthalpies of complex formation in the gas phase for formation of Li+ complexes versus those for the corresponding Mn+ complexes. Ligands are segregated into correlations for hard ( ) and soft (O) donors. Energies in kcal mol 1. Redrawn after Ref. (14). 

FIG. 6. Plot of log Kx values for Ag+ complexes with unidentate ligands vs log Kx values for corresponding complexes with CH3Hg+. Data at 25°C and ionic strength zero in aqueous solution, from Ref. (11). Ligands are classified as soft ( ) intermediate (O) hard, N-donors (O) hard, O-donors ( ), or F ( ). Formation constant data from Ref. (11). 

Especially noteworthy is the relative large shielding for the 31P nucleus in the 31P NMR spectra of 15, which is quite unusual for two-coordinate phosphorus. Evidently, the latter is caused by the strong o--donor ability of the silyl and germyl groups, which is also reflected in the calcu-... 

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